The Capacitor Calculator Guide Library

Capacitor Guides That End at a Number You Can Use

20 topics across 5 categories — fundamentals, codes and units, circuit design, plate physics, and field service. Every guide is written to be read with the calculator open in the other tab: the reading explains why a number moves, and the tool shows how far it moves.

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Assorted capacitors — ceramic discs, electrolytics and film — on an electronics workbench

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About This Library

What the Capacitor Calculator Guides Cover

Ask what a capacitor marking means and the internet hands you three contradictory answers before lunch. Forum threads argue in circles — one swears 104 means 104 µF, another says multiply by something — and neither shows the arithmetic. Manufacturer charts are accurate but narrow: they cover that maker’s markings and stop at the first nonstandard code. YouTube walkthroughs show the result but rarely the tolerance math that makes the value trustworthy.

This library is the fourth thing. It is a reference written to stand on its own, from the first question about what capacitance measures to the last discharge resistor on a service call. Every topic follows one design rule: it must end at a number you can enter somewhere. Code reading connects to the decoder on the home page. Unit habits connect to the capacitance converter. Combination rules connect to the bank calculator. Dielectric constants connect to the plate calculator. Health thresholds connect to the under-load test. If a concept cannot change a number you can enter, it does not get a page.

Two promises apply to every page. First, no product pitches: the site sells nothing and is affiliated with no manufacturer, so recommendations stop where the component comparison begins. Second, checkability: wherever a number appears, the arithmetic behind it is either shown in the text or one click away in a calculator. A guide you cannot verify is just a longer advertisement.

How to Use the Library

Pick Your Question

Find the topic that matches the decision in front of you — code reading, unit conversion, bank sizing, dielectric choice, part testing. Every card states its practical rule in the first sentence, so the answer is never buried.

Read the Rule, Then the Reason

Each topic is short by design: the rule first, the why underneath it. If you only have thirty seconds at the bench, the opening paragraph is the answer; if you want the reasoning, keep reading.

Run the Paired Tool

Every card ends at a calculator input. Enter your real numbers there and the concept becomes a decoded value, a converted unit, a bank total or a replace-or-keep verdict — the idea stops being theory and becomes a decision.

Electronics workbench with a breadboard project and multimeter during a guide walkthrough

Read the why, run the howPhoto: Unsplash

Reading Paths

Same Library, Four Ways Through

The categories below follow a project from part to field call. These paths follow a reader instead: three stops in order, each naming the tool where the idea becomes a number. Start where you recognize yourself.

The Hobbyist Reading a Parts Reel

You ordered capacitors from three suppliers and the markings disagree: 104, 100n, 0.1µ — and one just says 10. You want to know what you actually received.

The Engineer Sizing a Power Rail

A supply design needs bulk capacitance, high-voltage headroom, and stable decoupling — and the BOM only has parts you already stock.

The Student Checking the Physics

A homework set asks for plate capacitance, stored charge and code decoding — and you want to reproduce every number, not just copy an answer.

The HVAC Tech on a No-Cool Call

The condenser runs but the compressor trips on overload. You have a clamp meter, a multimeter, and ten minutes before the customer asks the obvious question.

The Library

Browse by Category

Five categories, ordered the way a real project unfolds — fundamentals first, then codes and units, then circuit design, then physics, then field service. Each topic card links to the calculator page or home-page section where the idea becomes a number.

Capacitor Fundamentals

Every capacitor decision starts with knowing what the part actually is: two plates, a dielectric, and a field between them. This category covers the ground the datasheets assume you already know — what capacitance measures, why the farad is so large that everything is quoted in pF and µF, what a voltage rating really promises, and how the main families (ceramic, electrolytic, film, supercapacitor) divide the work between them. Master these topics and nothing on a parts reel surprises you again.

What Capacitance Actually Measures

Capacitance is the ratio of stored charge to applied voltage: one farad holds one coulomb at one volt. That definition explains everything downstream — why bigger plates hold more charge, why thinner dielectrics hold more still, and why the energy stored grows with the square of voltage (E = ½CV²), so a capacitor at twice the voltage holds four times the energy. The main calculator computes Q and E for any C and V pair.

Compute Q and E

The Five Families: Ceramic, Electrolytic, Film, Tantalum, Supercap

Ceramics are small, cheap and non-polar — the decoupling workhorse from 1 pF to 100 µF. Aluminium electrolytics own the bulk-storage range from 1 µF to tens of thousands, at the cost of polarity, tolerance and lifespan. Film caps trade size for precision and stability; tantalums pack high µF into small polarized packages; supercapacitors reach whole farads for backup power. Family choice fixes the tolerance, lifespan and failure mode long before the value does.

Compare the families

Voltage Ratings: Headroom Is Not Optional

A capacitor's voltage rating is a ceiling, not a suggestion — exceed it and the dielectric breaks down, often explosively for electrolytics. Working practice keeps the applied voltage at 50–80% of rating, derating further in hot environments. The rating may be exceeded upward when substituting (a 440 V part replaces a 370 V one happily; never the reverse), and the HVAC under-load page shows why measured terminal voltages legitimately exceed the supply.

See real terminal voltages

Reading a Datasheet: The Five Numbers That Matter

Beyond the value and tolerance, five numbers decide whether a part survives your circuit: rated voltage, temperature coefficient or series (X7R, C0G, Y5V), equivalent series resistance (ESR), ripple current rating, and lifetime hours at rated temperature. A 10 µF X7R can lose half its capacitance at rated voltage and temperature — the datasheet's derating curves, not the headline value, are the real specification.

Learn the numbers

Codes & Units

Capacitors are the last mass-market component still hiding their value behind a code. These guides cover the reading skills: the 3-digit EIA system and its pF anchor, the tolerance letters that follow it, the unit ladder from picofarads to farads and the regional habits that make European datasheets look different from American schematics.

The 3-Digit Code: Two Digits and a Power of Ten

The first two digits are the significant figures in picofarads; the third digit is how many zeros follow. Code 104 means 10 followed by four zeros: 100 000 pF, i.e. 100 nF or 0.1 µF. Codes of one or two digits read directly in pF, and values below 10 pF use decimal R-notation (4R7 = 4.7 pF). The code converter decodes any marking and runs the reverse, so you can pick the code a value should carry.

Decode any marking

Tolerance Letters: From ±0.1 pF to +80/−20%

The letter after the code is the EIA tolerance class: B, C and D pin precision parts to fractions of a picofarad; F (±1%) and G (±2%) serve timing circuits; J (±5%) and K (±10%) are the everyday grades; M (±20%) covers general-purpose ceramics; Z (+80%/−20%) belongs to large electrolytics. A 104K is 100 nF guaranteed between 90 and 110 nF — the converter shows that range for every code.

See the tolerance ranges

The Unit Ladder: pF, nF, µF, mF, F

Each step of the ladder differs by exactly 1 000, so every conversion is three decimal places per hop: 22 000 pF = 22 nF = 0.022 µF. The traps are regional and historical — American schematics skip nanofarads entirely, and old motor labels write mF when they mean µF. The capacitance converter keeps all five units on screen at once, updated on every keystroke.

Convert any value

SMD Codes and the 10 µF Ceiling

Above roughly 10 µF, ceramic parts abandon the 3-digit code and print the value directly (10, 22, 47 with a µF marker), because the multiplier digits grow unwieldy. SMD tantalums print the value plus a voltage letter. Reading habits from through-hole days mislead here: a ceramic marked '10' is 10 µF, while a mica marked '10' is 10 pF — package family disambiguates what digits alone cannot.

Check the chart

Combining & Circuit Design

Real designs rarely use one capacitor where three will do. These guides cover the combination rules and the habits that keep them honest: when to parallel for bulk, when to series for voltage, why balancing resistors exist, and how the same arithmetic runs on decoupling networks and timing circuits.

Parallel: Adding Capacitance and Spreading Ripple

Capacitors in parallel add directly — C_total = C₁ + C₂ + … — the mirror of the resistor rule. Power supplies parallel several smaller electrolytics instead of one giant can because it spreads ripple current, lowers ESR, and fits board space. Decoupling networks stack 10 µF, 1 µF and 100 nF in parallel because each size covers a different frequency band. The bank calculator totals any parallel combination live.

Total a parallel bank

Series: Reaching Voltages One Part Cannot

Series capacitors combine by reciprocals — 1/C_total = 1/C₁ + 1/C₂ + … — and the voltage divides across the string inversely to capacitance. Four 100 V capacitors make a roughly 400 V bank, which is how high-voltage supplies and power electronics reach ratings no single part offers. The catch: leakage differences bias the split, so power circuits add balancing resistors across each capacitor.

Size a series string

Why Two Equal Caps in Series Halve the Value

Series stacking effectively widens the dielectric: two identical capacitors in series behave like one capacitor with double the gap, hence half the capacitance and double the voltage rating. Three make a third of the value, and so on — C/n for n equal parts. It is the same physics the plate calculator shows geometrically: separation d in the denominator.

See the geometry

Timing Circuits: Where Tolerance Actually Matters

RC timing sets oscillator periods and reset delays, and the timing error is the sum of the resistor and capacitor tolerances — a ±10% capacitor plus a ±5% resistor can swing a 'one-second' delay from 0.85 to 1.15 seconds. Timing circuits deserve F (±1%) or G (±2%) parts with stable dielectrics (C0G, film), not M-grade ceramics whose value also drifts with voltage and temperature.

Check your tolerances

Physics & Geometry

Behind every part number is the same equation: C = K × ε₀ × A / d. These guides run the physics from the plate up — how dielectric constants differ across materials, why PCB copper makes stray picofarads everywhere, and what fringing does to the ideal formula at the edges.

The Plate Formula, Term by Term

Capacitance rises with dielectric constant K, with plate area A, and falls as separation d grows: C = K × ε₀ × A / d, with ε₀ = 8.854 × 10⁻¹² F/m. Every manufacturing trick is one of these three variables — electrolytics thin the dielectric to a sub-micron oxide, ceramics push K into the thousands, and film capacitors roll enormous area into a small can. The plate calculator runs the formula on any geometry.

Run the formula

Dielectric Constants From Air to Barium Titanate

Vacuum and air sit at K ≈ 1; PTFE and polyethylene near 2; paper and FR-4 around 3.5–4.5; aluminium oxide 8.4; ceramics from 6 (C0G) to 12 000 (Class 2 barium titanate). High K buys size, at the cost of stability — Class 2 ceramics lose capacitance with voltage and temperature, which is why precision circuits prefer C0G or film despite the size penalty. The calculator presets 25 materials.

Browse the table

Stray Capacitance: The Picofarads You Didn't Order

Two 1 cm² pads on opposite sides of 1.6 mm FR-4 make about 2.3 pF — harmless until it sits across a crystal, a feedback node or a high-impedance input. Parallel traces, connector pins and cable runs all add stray pF that shifts filter corners and timing. Knowing the plate formula turns 'mysterious analog behavior' into arithmetic you can run and guard-ring away.

Estimate your strays

Fringe Fields and When the Formula Lies

The ideal formula assumes infinite plates and a uniform field. Real plates have edges where field lines bulge — fringing — adding a few percent when the gap is small against the plate width, and tens of percent when the gap approaches the plate size. The plate calculator warns when you enter the fringing regime, and the honest fix is more plate area, not tighter tolerances.

Check your geometry

Field Service & HVAC

The most-consumed capacitors in the world are the run capacitors inside air conditioners, heat pumps and motors — and the most common question is not what value they are but whether they are still good. These guides cover the field reality: the under-load test, the health thresholds, substitution rules and the safety protocol around several hundred stored volts.

The Under-Load Test: Why Bench Meters Mislead

A dying run capacitor can read its label value on a bench meter — cold, at low voltage, out of circuit — and still starve a compressor at two in the afternoon. The under-load method measures while the system runs: clamp amps on the start-winding lead, measure volts across the terminals, compute µF = 2652 × A ÷ V. It is a performance measurement, not a specification.

Run the test

Reading the Label: 35/440 and What ±6% Promises

A dual-run capacitor label like 35/5/440 means 35 µF for the compressor (HERM), 5 µF for the fan, 440 V rating. The tolerance on the label is the new-part guarantee; field practice grades against ±5% healthy, 5–10% marginal, beyond ±10% replace. The under-load calculator applies those bands automatically to your measured value.

Grade your part

Substitution Rules: Voltage Up, Capacitance Faithful

Replacement parts may exceed the original voltage rating without limit — 440 V replaces 370 V safely — but capacitance must stay at the label value within a few percent. Oversized capacitors push excess current through the start winding and cook it; undersized ones lose torque and the compressor runs hot. Frequency matters too: the 2652 constant is 60 Hz only, and 50 Hz systems compute with 3183.

Check the thresholds

Discharge and Safety: The 20 kΩ Rule

HVAC capacitors hold a dangerous charge after power-off, and terminals can sit above 300 V while the unit runs. Discharge through a 20 kΩ 5 W resistor — never a screwdriver, which welds and splatters — verify with a meter before touching, and clamp meter leads instead of probing where possible. The under-load page carries the full protocol; this is the one habit that keeps a service call from becoming an ambulance call.

Review the protocol

Questions About the Guide Library

Who are these guides written for?▼

Anyone who reads, specifies, combines or tests capacitors: hobbyists decoding a parts reel, engineers sizing banks and rails, students checking plate-formula homework, and HVAC technicians grading a run capacitor on a live call. Each topic opens with the practical rule and then explains where the rule comes from, so the same page serves a thirty-second lookup and a slower read.

Do the guides replace the calculator?▼

No — they feed it. A guide explains where the 3-digit code system comes from, why series strings need balancing resistors, or what the ±5% threshold really guarantees; the calculators turn that understanding into a decoded value, a bank total or a health verdict. Every topic card links to the exact tool where the idea becomes a number.

Why do topic cards link to calculator pages and home-page sections instead of full articles?▼

The library is deliberately paired with the tools rather than duplicating them: a topic card is a focused walkthrough that lands on the input it discusses. Longer, fully illustrated articles are being added to the published-guides index on this page as they are finished, and anything released there appears automatically — the section is built from the guide registry, so it fills in as publishing proceeds.

How do I suggest a new topic?▼

Send it through the contact page: the question you were trying to answer, the part or circuit you were working on, and what you wish had been written down. Requests that describe a real, repeated problem — an RC timing walkthrough, a supercapacitor sizing guide, a step-by-step ESR explanation — go to the top of the content queue.

Are the numbers in the examples real?▼

The worked figures use the same arithmetic the calculators run — code decoding, unit conversion, the plate formula, 2652 × A ÷ V — so every number is reproducible with the stated inputs. They are illustrative rather than design-final: real circuits depend on datasheet derating curves and real measurements, which the tools make visible instead of hiding.

Do the guides cover both 60 Hz and 50 Hz systems?▼

Yes. The under-load guides and calculator carry both constants — 2652 for 60 Hz North American mains and 3183 for 50 Hz systems in Europe, Asia and Australia. Everything else in the library is frequency-independent.

Turn the Reading Into a Number

Open the calculator, type the marking you just read about, and watch the code decode, the tolerance range appear and the charge compute in real time — free, no account, entirely in your browser.

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