What Is Capacitance? Definition, Formula and Units
Capacitance is the ratio of stored charge to voltage, C = Q/V, measured in farads: a capacitor with one farad of capacitance holds one coulomb of charge for every volt across its plates. Because one farad is enormous by electronics standards, real components are quoted in picofarads (pF), nanofarads (nF) and microfarads (µF). Everything else on this page — the unit ladder, the conversion shortcuts, the dielectric, the capacitor families — follows from that single ratio.
The defining equation
C = Q / V
C — capacitance, in farads (F)
Q — stored charge, in coulombs (C)
V — voltage across the plates, in volts (V)
1 farad = 1 coulomb per volt: one coulomb of charge stored on the plates for every volt applied.
Two plates, one insulator, one ratioPhoto: Unsplash1 F
one coulomb stored per volt
×1 000
every step of the unit ladder
5
units in daily use, pF to F
E = ½CV²
energy held in the field
What Is Capacitance?
From two plates to a definition
The honest answer starts with two pieces of metal. Take any two conductors — plates, foils, circuit-board traces, even two wires running side by side — and separate them with an insulator that keeps charge from crossing the gap. Connect a battery across the pair: electrons pile onto one conductor and are pulled off the other, and charge accumulates in proportion to the voltage applied. Measure the charge Q that has moved and the voltage V that pushed it, and their ratio comes out the same every time, whatever the voltage you chose.
That fixed ratio is the capacitance of the pair, and it is a purely physical property. It is set by the size of the conductors, the distance between them and the material in the gap — not by the battery. Double the voltage and twice the charge lands on the plates, so Q/V stays put. This is why capacitance can be quoted on a part before anyone decides what circuit it will go into: it describes the part, not the circuit.
A worked example: 100 µF at 12 V
Put numbers on it. A 100 µF capacitor connected to a 12 V supply stores Q = C × V = 100 µF × 12 V = 1 200 µC, which is 1.2 mC of charge — on the order of 7.5 × 10¹⁵ electrons shifted from one plate to the other. Run the definition the other way and it still holds: 1.2 mC at 12 V divides out to exactly 100 µF, and the same capacitor on a 6 V rail would hold only half as much charge, 0.6 mC. That is the whole content of C = Q/V: capacitance is the conversion factor between the voltage you apply and the charge that lands on the plates.
The main calculator on the home page performs exactly this arithmetic for any capacitor and voltage pair, and it also reports the stored energy — a number that grows with the square of the voltage rather than linearly, which is why a capacitor rated for twice the volts does four times the work.
Why one farad is enormous
By the definition, a one-farad capacitor holds one coulomb per volt — five coulombs at 5 V, an enormous amount of charge for a small component. That is why a true 1 F supercapacitor is the size of a thumb, while the ceramic chips scattered across a circuit board hold picofarads: a billionth of a farad is already a useful part. Nobody quotes ordinary components in farads. Radio, crystals and digital work live in pF; decoupling and timing live in nF; power supplies, audio and motors live in µF; and the farad itself belongs almost entirely to supercapacitors.
Keep the ladder in mind — pF, nF, µF, mF, F — and every value you meet lands in a familiar neighbourhood. The next two sections walk that ladder rung by rung, then untangle the symbol soup that decades of regional habits have printed on datasheets and motor labels.
Units and Symbols of Capacitance
Capacitance is measured in farads, but the farad is a unit you will almost never write on its own. This section walks the ladder from picofarads to farads, then sorts out the unit symbols and their real-world spelling variants — µF, uF, mfd, MFD, MF, nf, pf and µµF — that datasheets, schematics and motor labels have accumulated over decades.
The farad: one coulomb per volt
The farad is defined straight from the formula: 1 F = 1 C/V. One coulomb is the charge carried by about 6.24 × 10¹⁸ electrons, so a one-farad capacitor at one volt has moved serious charge — which is precisely why the unit is so large. It is named after Michael Faraday, and it anchors every other unit on this page: each of the prefixes below is just the farad divided by a power of ten.
The unit ladder: pF, nF, µF, mF, F
Each step of the ladder differs from its neighbours by exactly 1 000, so every hop moves the decimal point three places. Reading down the ladder — toward smaller units — you multiply by 1 000 at each step; reading up it, you divide. Because the factor is always a clean thousand, every conversion is three decimal places per hop, and nothing else is ever involved.
Unit symbols and their spelling variants
Two different things get called the symbol of capacitance. The quantity — the property itself — is written C in every equation, as in C = Q/V and C = K × ε₀ × A/d. The unit symbols attach to numbers: F for farad, mF for millifarad, µF for microfarad, nF for nanofarad and pF for picofarad. The trouble is that real labels are written by humans and machines with limited keyboards, and each region papered over the gap differently.
Microfarad suffers most. The proper symbol µF uses the Greek letter mu, but plain-text parts lists, PCB silkscreens and older test equipment write uF instead — same meaning, ASCII keyboard. American motor and HVAC labels add mfd and MFD, both abbreviating microfarad, and occasionally MF as well: on those parts the capital M means micro, not mega. Nanofarad is sometimes written lowercase as nf on European parts. Picofarad appears as pf on old schematics, and on very old US drawings as µµF — the micromicrofarad, spoken aloud exactly that way before the pF prefix was standardized. All of these are spelling variants, not different quantities: a 10 mfd capacitor, a 10 µF capacitor and a 10 uF capacitor are the same part.
Watch the m on old motor labels. A run capacitor marked 30 mF is a 30 µF part — the manufacturer means microfarads and has for decades, long before millifarad supercapacitors existed. Reading the label literally puts you off by a factor of 1 000 000 and sends you hunting for a part nobody makes. When a label is ambiguous, measure rather than guess: the under-load calculator turns a live reading into the true microfarad value.
| Unit | Symbol | In farads | Typical use |
|---|---|---|---|
| picofarad | pF | 0.000 000 000 001 F | Crystal loading, RF tuning, stray capacitance |
| nanofarad | nF | 0.000 000 001 F | Decoupling, timing, snubbers, audio coupling |
| microfarad | µF | 0.000 001 F | Supply smoothing, motor run capacitors, speaker crossovers |
| millifarad | mF | 0.001 F | Large electrolytics — usually written as thousands of µF |
| farad | F | 1 F | Supercapacitors, memory backup, energy harvesting |
Conversion Shortcuts: The ×1000 Ladder
Because every neighbouring pair of units differs by exactly 1 000, there is only one rule to remember: moving up the ladder (toward larger units) divides by 1 000, and moving down it (toward smaller units) multiplies by 1 000. Each hop is three decimal places, so a two-hop conversion is six places and a three-hop conversion is nine. There are no odd factors anywhere in the system — the difficulty is never the arithmetic, only remembering which direction the decimal moves.
22 000 pF = 22 nF = 0.022 µF
Divide by 1 000 to reach 22 nF, then by 1 000 again to reach 0.022 µF. One value, three costumes — and the marking 223 decodes to exactly this part.
470 nF = 0.47 µF
One hop up the ladder: divide by 1 000. European datasheets write 470n, American bills of materials write 0.47 µF, and both refer to the same decoupling capacitor.
1 F = 1 000 000 µF
Multiply by 1 000 to reach 1 000 mF, then by 1 000 again to reach a million microfarads. A supercapacitor is a million µF — which is why the µF habit breaks down at the top of the ladder.
The ladder also explains the classic regional trap. American schematics historically skipped nanofarads entirely, jumping from 0.001 µF straight to 1 000 pF, while European parts were labelled in nF throughout — so the same circuit can carry two notations for the same value. When a datasheet, a parts list and the label on the old part all disagree about units, the capacitance converter keeps all five units on screen at once, updating on every keystroke: type the value once, read it in every unit, and check the closest 3-digit code while you are there.
What Is a Dielectric?
The insulator between the plates
The dielectric is the insulating material that fills the gap between a capacitor’s plates — the ceramic in a disc cap, the oxide film in an electrolytic, the polyester in a film box, or simply air around two traces. It has to be an insulator, because charge crossing the gap would discharge the capacitor; but it does something more useful than just staying out of the way. Its molecules are neutral overall yet slightly polar, and in the electric field between the plates they twist into line.
Polarization: why K multiplies capacitance
That alignment is called polarization, and it partially cancels the field inside the gap. With the field weakened, a given battery voltage can pile more charge onto the plates before the plate voltage balances the supply — so the charge-per-volt ratio, the capacitance, goes up. The multiplier is the material’s relative permittivity, always written K or εr, and it is defined against vacuum: K = 1 means no help at all, K = 8.4 means the same geometry holds 8.4 times the charge.
| Material | Typical K | Where you meet it |
|---|---|---|
| Air (and vacuum) | ≈ 1.0 | The baseline every other K is measured against; variable tuning capacitors |
| PTFE (Teflon) | 2.1 | Precision film capacitors and RF work where stability matters |
| FR-4 PCB substrate | ≈ 4.4 | Every trace pair and pad on a circuit board — the source of stray pF |
| Aluminium oxide | 8.4 | The dielectric grown in place inside electrolytic capacitors |
| Barium titanate ceramics | 1 000s | Class 2 ceramic chips — huge K per volume, but the value drifts with voltage and temperature |
These are typical values for common grades, not guarantees — ceramic compositions vary widely between manufacturers. The pattern is what matters: every manufacturing trick in the industry is an attack on one term of the plate formula C = K × ε₀ × A/d. Electrolytics grow an oxide dielectric just fractions of a micron thick, ceramics push K into the thousands, and film capacitors roll metres of foil into a small can to maximize area A.
Dielectric strength is a different number
K tells you how much charge a dielectric helps store; dielectric strength tells you how many volts per unit of thickness it survives before it breaks down and conducts — often destructively, and explosively so for electrolytics. The two properties are not correlated: barium titanate wins on K by three orders of magnitude over air but does not win on strength, and a thin high-K layer is exactly the combination that makes a capacitor small and voltage-limited at the same time. This is why the voltage rating on a part is chosen by the dielectric, not by the capacitance value.
To see the trade-off with your own numbers, the parallel plate capacitor calculator runs C = K × ε₀ × A/d on any geometry, with 25 dielectric presets from air to barium titanate — change the material and watch the capacitance move by the K ratio alone.
Every board is full of accidental dielectricsPhoto: UnsplashCommon Capacitor Types at a Glance
The definition is one equation, but the industry builds it five different ways, and the family a value comes in decides its tolerance, lifespan and failure mode long before the number does. Here is the one-paragraph version of each family, with the ranges you will actually meet on reels and in catalogues.
Ceramic capacitors
Small, cheap and non-polarized, spanning roughly 1 pF to 100 µF. C0G/NP0 grades are stable enough for timing and resonant circuits; the high-K X7R and Y5V grades pack more capacitance into less space but lose value with applied voltage and temperature — a 10 µF X7R can deliver half its label at rated conditions. They own the decoupling job on every digital board made.
Aluminium electrolytic capacitors
The bulk-storage workhorses, from about 1 µF to tens of thousands of µF. Polarized — reverse voltage destroys the oxide dielectric — with looser tolerances (±20% is normal) and a liquid electrolyte that dries out over years, especially in heat. Power supplies, audio rails and motor circuits live on them.
Film capacitors
Polyester, polypropylene and PTFE dielectrics rolled into boxes, typically from 1 nF to tens of µF. Non-polarized, tight-tolerance and stable, with excellent behaviour at audio and mains frequencies. They trade size for precision, which is why they show up in timing circuits, snubbers, crossovers and X/Y mains-safety positions.
Tantalum capacitors
Polarized parts packing high capacitance — roughly 0.1 µF to 1 000 µF — into a small solid package. Stable over temperature and long-lived when respected, but unforgiving of voltage stress: good practice derates them to half the labelled rating, and failure tends to be a short circuit rather than a quiet fade.
Supercapacitors
The only family quoted in whole farads, from about 0.1 F to thousands of farads, built on electrochemical charge storage rather than a classic dielectric. Individual cells sit around 2.7 V, so higher voltages need series strings with balancing. They bridge power gaps and back up clocks and memory — not fast decoupling, and not mains-frequency filtering.
Notice the polarity split. Ceramics and film are non-polarized: they can sit in the circuit either way round, which is what coupling and AC applications require. Electrolytics and tantalums are polarized — the positive terminal must always sit at the higher potential, or the dielectric chemically degrades — which restricts them to DC and properly-biased positions. Supercapacitors are polarized too. Getting this wrong is not a tolerance issue; it is a lifespan and safety issue, and it is the first thing to check when an unfamiliar part goes into a board.
This is the sixty-second version. For temperature coefficients, ESR, failure modes and a full side-by-side comparison, read the complete guide to capacitor types.
How Does Capacitance Work — and Where Is It Used?
Everything a capacitor does in a circuit follows from the one behaviour the definition describes: it stores charge in an electric field, and it takes time — and current — to change that charge. Six applications cover almost every capacitor you will ever place, and each one has a tool on this site that computes it.
Charge storage
The direct use of C = Q/V: hold charge at a voltage and deliver it on demand. Every value of C and V also fixes the stored energy E = ½CV² and the number of electrons on the plates.
Compute Q and E for your partFiltering and decoupling
A capacitor resists voltage changes, so it smooths ripple after a rectifier and supplies the fast current bursts a chip’s pins demand. How well it does this depends on reactance, which falls as frequency rises.
Run the reactance numbersTiming and delays
Charge a capacitor through a resistor and the voltage climbs on a fixed schedule: τ = R × C sets the pace, reaching 63% in one τ and settling in about 5τ. Oscillators, resets and dimmers all rent time from this product.
Calculate your time constantCoupling and DC blocking
Because X꜀ = 1/(2πfC) shrinks at high frequency, a series capacitor passes the audio signal between two amplifier stages while blocking each stage’s DC operating voltage — the cleanest way to join circuits that live at different potentials.
Check coupling at any frequencyEnergy storage
E = ½CV² is the working engineer’s version of the definition: doubling voltage quadruples energy. Camera flashes, ride-through modules and supercapacitor backup all bank energy in the field and spend it later.
Compute stored energy liveMotor start and run
A phase-shifted current through a capacitor gives a single-phase motor the rotating field it cannot make alone. Start caps deliver a short kick; run caps stay in circuit for life, which is why their health decides the compressor’s.
Size a motor capacitorWhich Calculator Should I Use?
Understanding the definition is step one; step two is pointing it at a real part. Each row below is a question this page equips you to ask, with the tool that answers it in numbers.
Most people arrive here by typing what is capacitance into a search bar while a real capacitor sits on the desk in front of them — so this page is built to end at the bench, not at more reading. The homepage calculator decodes the marking on the part you are holding, and the guides hub collects every article in the series, from code reading to under-load testing, if you want the full picture first.
Frequently Asked Questions About Capacitance
What is capacitance in simple words?▼
Capacitance is a measure of how much electric charge a capacitor can hold for every volt you apply to it. The ratio is written C = Q/V: stored charge divided by voltage. A 100 µF capacitor connected to a 12 V supply, for example, stores 100 µF × 12 V = 1.2 mC of charge. Bigger capacitance means more charge stored at the same voltage — the same way a wider bucket holds more water at the same depth.
What is the unit of capacitance?▼
The standard unit of capacitance is the farad, abbreviated F. One farad means one coulomb of charge is stored per volt of potential difference (1 F = 1 C/V). Because one farad is enormous for everyday electronics — a whole-farad capacitor is the size of a thumb and is called a supercapacitor — practical values are quoted in picofarads (pF), nanofarads (nF) and microfarads (µF), each step of that ladder being 1 000 times the previous one.
What is the symbol of capacitance, and how do you write the microfarad symbol?▼
Capacitance as a quantity is symbolized by the letter C, in equations such as C = Q/V. The unit symbol is F for the farad. Microfarad is written µF (the Greek letter mu), and you will also meet the spellings uF, mfd, MFD and MF on older parts and motor labels — all of them mean microfarads. Nanofarad is nF (sometimes written nf), and picofarad is pF (older US schematics wrote pf or µµF, spoken as micromicrofarad).
What is a picofarad and what is a nanofarad?▼
A picofarad (pF) is one trillionth of a farad: 0.000 000 000 001 F, or 10⁻¹² F. A nanofarad (nF) is one billionth of a farad: 0.000 000 001 F, or 10⁻⁹ F — so 1 nF = 1 000 pF. Picofarads are the currency of crystal loading capacitors, RF tuned circuits and stray capacitance; nanofarads dominate decoupling, snubbers and timing. The marking 104 means 100 000 pF, which is 100 nF or 0.1 µF.
How do you calculate capacitance?▼
Two routes. From charge and voltage, use the definition: C = Q/V — the charge stored divided by the voltage across the plates. From geometry, use the parallel-plate formula C = K × ε₀ × A/d, where K is the dielectric constant of the insulator, ε₀ = 8.854 × 10⁻¹² F/m, A is the plate area and d is the separation. A 1 cm² pair of plates 1 mm apart in air, for instance, gives roughly 0.885 pF.
What does a dielectric do?▼
The dielectric is the insulating material between a capacitor's plates. Its molecules polarize in the electric field, which lets the capacitor store more charge for the same voltage — capacitance is multiplied by the material's dielectric constant K. Air is about 1, PTFE about 2.1, FR-4 PCB substrate about 4.4, aluminium oxide 8.4, and barium titanate ceramics reach into the thousands. The dielectric also sets the voltage limit: its dielectric strength, in volts per unit of thickness, determines where breakdown begins.
What is capacitance measured in?▼
In farads — but in practice, in fractions of a farad. Handheld multimeters typically offer nF and µF ranges; bench LCR meters reach down to picofarads; HVAC technicians compute µF directly from clamp-meter readings. The everyday ladder runs pF (0.000 000 000 001 F), nF (0.000 000 001 F), µF (0.000 001 F), mF (0.001 F) and F, with each step exactly 1 000 times the one before it.
How does capacitance work in a circuit?▼
A capacitor stores energy in the electric field between its plates and opposes sudden changes in voltage. On DC it charges up to the supply voltage and then blocks further current; on AC its reactance X꜀ = 1/(2πfC) falls as frequency rises, so it passes AC while blocking DC. That one behaviour powers most applications: decoupling supplies fast current bursts, the RC product τ = R × C sets timing, series capacitors couple audio stages, and the stored energy E = ½CV² does real work in flashes, backup supplies and motor start circuits.
Sources & Notes
Every definition, formula and unit relationship on this page — C = Q/V, the farad and its prefixes, the plate formula C = K × ε₀ × A/d, and the energy relation E = ½CV² — is classical textbook physics, reproduced from standard references rather than any proprietary dataset, and verifiable in any introductory electromagnetics text. Values quoted for materials, such as the dielectric constants in the table above, are typical ranges for common commercial grades; the exact figure for a specific product belongs to that product’s datasheet, since ceramic compositions in particular vary between manufacturers.
To keep search results honest: this is the concept page for capacitance — the definition, the units and the physics. Numeric unit conversion lives on the capacitance converter, and capacitance-from-geometry lives on the parallel plate calculator, so neither page has to blur its purpose to cover the other’s job.
The page exists for one reason: the question it answers is asked millions of times a year, and the useful answer is shorter than the forums make it. A reader who leaves here able to state the definition, place a value on the unit ladder and pick the right calculator has everything the topic actually requires — and every claim made above can be checked against the linked tools, which run the same arithmetic in the open.
Maintained by the CapacitorCalculator.com editorial team · Last updated: November 2, 2026
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