⚡ Power Rail Sizing Tool

Power Supply Capacitor Calculator — Bulk Input, Output Smoothing, Hold-Up Time & Ripple Voltage for Rectifiers & Converters

Three capacitors, three different jobs: the bulk reservoir after the rectifier, the ripple filter at a converter output, and the ride-through energy store for mains dropouts. Pick your symptom below or jump straight into the numbers.

Power Supply Capacitor Calculator

Three modes — rectifier bulk, buck-converter output, and hold-up time. Live on every keystroke.

Which capacitor do I need?

Load Current I (A)

Mains frequency

Rectifier type

Allowable Ripple ΔV (V)

Quick presets

Capacitance required

8 333µF

Ripple frequency: 120 Hz (2 × 60 Hz mains)

Next standard value: 10 000 µF electrolytic

Voltage rating hint: ≥ 1.3 × peak secondary voltage (e.g. 12 VAC secondary → ≈ 17 V peak → 25 V cap)

📌 Key formula. Bulk capacitance formula for rectifiers: C ≥ I_load / (2 × f_line × ΔV). The 2 accounts for full-wave rectification doubling the ripple frequency. For half-wave, drop the factor of 2.

The three roles

What Bulk, Output, and Hold-Up Capacitors Actually Do

A power supply is a chain of energy conversions, and every stage asks its capacitor for something different. Right after the rectifier sits the bulk capacitor — usually the largest part on the board — whose job is to catch the pulsing humps of rectified AC and turn them into usable DC. Every search for how to calculate capacitor for power supply values is really a question about this part: how big must the reservoir be so the voltage between charging peaks never sags outside the load’s tolerance band?

Downstream of the bulk stage the picture changes. A switching regulator — a buck converter being the most common — chops the DC at tens to hundreds of kilohertz and reassembles it with an inductor and its own output capacitor. Here the question is not reservoir sizing but output capacitor calculation: absorbing the inductor’s triangular ripple current and holding the rail flat through load steps that happen in microseconds.

The third job is the quietest: the hold-up capacitor. It sits fully charged, doing nothing, until the input power disappears — and then it becomes the supply. Its size is set not by ripple but by a simple energy budget: how many watts for how many milliseconds. That is why any serious power supply capacitor calculator needs three distinct modes — the three formulas share almost nothing.

Electrolytic and ceramic capacitors on an electronics workbench beside a bench power supply

Three capacitors, three different jobsPhoto: Unsplash

Rectifier stage

Full Wave Rectifier Capacitor Size Calculation — Ripple Physics from the Sawtooth

Between charging peaks, the bulk capacitor discharges into the load. If the load draws a roughly constant current I, the capacitor’s voltage falls by ΔV = I·Δt/C. Strictly the decay is exponential — the RC time constant calculator shows the exact curve — but over a single ripple period the exponential is so flat that treating it as a straight line is accurate to a fraction of a percent for any practical supply. One full-wave ripple period lasts Δt ≈ 1/(2 × f_line): 8.33 ms on 60 Hz mains, 10 ms on 50 Hz. Set your ΔV budget — how much sag the load tolerates — and the capacitance follows directly.

C ≥ I_load / (f_ripple × ΔV) · f_ripple = 2 × f_line (full-wave) or f_line (half-wave)

The factor of 2 is where half-wave and full-wave part ways, and it is the first question a good rectifier capacitor calculator asks. A full wave rectifier capacitor size calculator multiplies the mains frequency by two before dividing; a half-wave supply recharges only once per cycle, so it needs exactly double the capacitance for the same ripple — and draws its charging current in one tall pulse per cycle besides, which stresses the transformer and raises the output impedance. Half-wave survives in tiny auxiliary supplies; for anything that powers real circuitry, full-wave wins.

Worked example: a 12 V 1 A linear supply with a 1 V ripple budget on 60 Hz mains. The ripple frequency is 120 Hz, so C = 1/(120 × 1) farads = 8 333 µF. No factory makes that value, so a dc power filter capacitor calculator rounds up to the next stocked part — 10 000 µF. Rounding up is always safe for ripple (more capacitance only flattens the sawtooth further); what it costs you is inrush current at power-up, physical volume, and charging-pulse stress on the rectifier diodes.

From Formula to Shelf: Standard Bulk Capacitor Sizes

Every row below is a finished bulk capacitor calculation for a classic linear supply on 60 Hz mains — the same arithmetic Mode 1 of the calculator above performs, plus the voltage-rating suggestion that keeps the electrolytic inside the 80 % derating rule discussed further down.

Linear Power Supply — Standard Bulk Capacitor Sizes by Current & Ripple

Output Current	Peak Ripple @ 120Hz	Capacitance Required (µF)	Voltage Rating Suggestion	Cap Type
1 A	1 Vpp	8 333	25 V	Electrolytic
2 A	1 Vpp	16 667	25 V	Electrolytic
5 A	2 Vpp	20 833	35 V	Electrolytic
10 A	1 Vpp	83 333	50 V	Electrolytic

Read the table as a set of design trade-offs, not just outputs. The 1 A and 2 A rows keep a tight 1 Vpp budget, which is what audio and analog circuits want; the 5 A row deliberately relaxes to 2 Vpp so the requirement stays near 20 833 µF instead of doubling toward 42 000 µF — a reminder that ripple budget and capacitor volume trade against each other. The 10 A row shows the limit of the linear approach: 83 333 µF at 1 Vpp is a physically enormous capacitor, which is exactly why high-current supplies either accept more ripple or move to switch-mode pre-regulation.

💡 Tip. Always round up to the next standard value and then check the capacitor’s ripple-current rating, not just its microfarads. The charging pulses through a bulk cap run at 2–4× the DC load current, and ripple current — not capacitance — is what cooks electrolytics. A cooler-running cap lasts roughly twice as long for every 10 °C you shave off.

Converter stage

Buck Converter Capacitor Calculator — Output Capacitor Calculation Step by Step

A buck converter’s inductor carries a triangular current that swings above and below the load current once per switching cycle. The peak-to-peak swing is ΔI_L = (Vin − Vout) × D/(L × f_sw), where D = Vout/Vin is the duty cycle. The output capacitor must swallow that triangle: the charge it absorbs and releases each cycle translates into ΔV = ΔI_L/(8 × f_sw × C), and rearranging gives the working form C = ΔI_L/(8 × f_sw × ΔV). A buck converter capacitor calculator performs exactly this chain — duty cycle, inductor ripple, then capacitance — which is what Mode 2 above does on every keystroke.

Run the default numbers: Vin = 12 V, Vout = 5 V, L = 10 µH, f_sw = 100 kHz, ΔV = 50 mV. Duty cycle D = 0.417, inductor ripple ΔI_L = (7 × 0.417)/(0.00001 × 100 000) = 2.92 A, and the output capacitor calculation gives C = 2.92/(8 × 100 000 × 0.05) = 72.9 µF — call it 100 µF in a standard low-ESR value. Notice the 1/f scaling: double the switching frequency to 200 kHz and the same circuit needs only half the capacitance, which is why modern converters running at 500 kHz to 2 MHz manage with tiny ceramic output caps.

D = Vout/Vin · ΔI_L = (Vin − Vout) × D / (L × f_sw) · C = ΔI_L / (8 × f_sw × ΔV)

Capacitance is only half the story, because real capacitors have series resistance. The ripple current through that ESR produces a square-wave step of ΔV_ESR = ΔI_L × ESR riding on top of the capacitive ripple. With ΔI_L = 2.92 A, a generic aluminum electrolytic at 100 mΩ contributes 292 mV — nearly six times the 50 mV target — no matter how many extra microfarads you bolt on. This is why low-ESR polymer capacitors and MLCC ceramics dominate converter outputs: at a few milliohms, their ESR term effectively vanishes. Ceramic dielectrics lose capacitance under DC bias, so derate them by 30–50 % from the rated value at working voltage.

❌ Don’t mix formulas. Never size a buck converter’s output with the rectifier bulk formula, and never drop a big general-purpose electrolytic onto a converter rail just because the microfarads add up. Its high ESR injects the 292 mV step described above, and its ESR-versus-frequency behavior can even destabilize the converter’s control loop. Use parts specified for switching service — low-ESR polymer, solid, or ceramic — and reserve bulk electrolytics for the rectifier stage where they belong.

Ride-through stage

Capacitor Hold-Up Time Calculation — the Voltage-Squared Effect

Hold-up sizing starts from energy, not from ripple. A capacitor charged to V₁ and allowed to fall to V₂ releases exactly E = ½C(V₁² − V₂²) joules on the way down. Set that equal to the load’s demand P × t and you get C = 2Pt/(V₁² − V₂²). A capacitor hold up time calculator runs this directly — and runs it backwards too, since t = C(V₁² − V₂²)/(2P) tells you how long an existing capacitor will coast before the load browns out.

C = 2Pt / (V₁² − V₂²) · t = C(V₁² − V₂²) / (2P)

📌 Key formula. Hold-up time is the classic formula C = 2Pt/(V1² - V2²) — remember the voltage is squared, so operating at higher bus voltages dramatically reduces the capacitor size for the same hold-up energy.

That squared voltage is the single most useful insight in hold-up design. Compare a 12 V bus allowed to sag from 12 V to 8 V with a 48 V bus allowed to sag proportionally, from 48 V to 32 V. The denominators are (144 − 64) = 80 versus (2 304 − 1 024) = 1 280 — exactly 16× — so the 48 V bus needs sixteen times less capacitance for the same load power and ride-through time. It is the reason server and industrial hold-up banks live on 48 V or 400 V buses rather than 12 V rails. The same ½CV² relationship drives our capacitor energy calculator, if you want to explore the energy side directly.

Real targets make the numbers concrete. The ATX specification requires a PC power supply to hold its power-good signal for at least 16 ms after mains loss — one full cycle plus margin — so a 10 W auxiliary load coasting from 12 V to 8 V needs C = 2 × 10 × 0.016/80 = 4 000 µF. A microcontroller riding out a brown-out typically needs only 10–50 ms of 200 mW–2 W logic, which small capacitors cover comfortably. “Keep my router alive through a brief blink” stops being guesswork once a hold up capacitor calculator turns the requirement into a single microfarad number.

Wind turbines holding rotational energy in reserve — the grid-scale cousin of a hold-up capacitor

Hold-up capacitors coast through the gapsPhoto: Unsplash

Safety margin

Voltage Rating and Derating — the 80% Rule

Capacitance gets the attention, but the voltage rating decides whether the part survives its first week. The rating must exceed the worst-case peak the capacitor ever sees — not the nominal rail. That worst case stacks a high mains day (+10 %), the secondary voltage rising at light load, and the very first charging peak after switch-on. A linear supply from a 12 VAC secondary whose peak is ≈ 17 V should use a capacitor rated at least 1.3 × 17 ≈ 22 V, which lands on the 25 V standard value — the same suggestion printed in the table above and shown live in Mode 1’s voltage-rating hint.

The working habit behind this is the 80 % rule: run electrolytics at no more than 80 % of their rated voltage in continuous service. A “12 V” rail gets a 16 V or 25 V part, a 400 V DC bus gets 450 V — never 400 V — and a 5 V rail is comfortable on anything from 6.3 V up. Electrolytic life roughly doubles for every 10 °C you remove and every point of voltage stress you shed, so generous ratings are cheap insurance. When the bus exceeds what single parts handle, series strings divide the voltage but divide the capacitance too, and each capacitor needs a balancing resistor — the series-parallel capacitor calculator works out both directions of that trade.

⚠️ Warning. Mains-side bulk capacitors hold a lethal charge long after the plug is pulled — a 400 V bus capacitor can store tens of joules at a voltage that stops hearts. Always discharge through a power-rated bleeder resistor and verify with a meter before touching anything. The same ripple currents that warm the capacitors also make transformer laminations hum: a supply that buzzes louder over the years is usually telling you its electrolytics are drying out.

Power Supply Capacitor Questions — Answered

The ten questions that come up most often when sizing power rail capacitors, answered with exact formulas and worked numbers. If you arrived with a symptom, the three-button router at the top of the page already pointed you at the right mode — the answers below go deeper.

FAQ topic mix

Bulk & ripple — 40%DC-DC output — 30%Hold-up & rating — 30%

Top QuestionHow do I calculate the capacitor for a full wave rectifier?▼

Bulk & ripple

Use C ≥ I_load / (2 × f_line × ΔV) with a full-wave rectifier: the ripple frequency is 2 × f_line — 120 Hz on 60 Hz mains, 100 Hz on 50 Hz. Worked example: a 1 A load with 1 V of allowable ripple on 60 Hz mains needs C = 1/(120 × 1) = 8 333 µF, so you buy the next standard value up — 10 000 µF — rated at least 1.3 × the peak secondary voltage. Enter the same three numbers in Mode 1 of the calculator above and it reproduces every figure, including the standard-value rounding.

Top QuestionWhat size capacitor for a 12 V 1 A power supply?▼

Bulk & ripple

For a 12 V 1 A linear supply with 1 V peak-to-peak ripple: on 60 Hz mains, C = 1 A/(120 Hz × 1 V) = 8 333 µF → use 10 000 µF; on 50 Hz mains, C = 1 A/(100 Hz × 1 V) = 10 000 µF exactly. Voltage rating: a 12 V-class secondary peaks near 17 V, and 1.3 × 17 ≈ 22 V, so a 25 V electrolytic is the standard pick. Also check the ripple-current rating — the capacitor delivers charging pulses well above 1 A, so a ≥ 2 A ripple rating is a sensible floor.

Top QuestionHow does a buck converter output capacitor calculation work?▼

DC-DC output

Three steps. First, duty cycle D = Vout/Vin (12 V → 5 V gives D = 0.417). Second, peak-to-peak inductor ripple ΔI_L = (Vin − Vout) × D/(L × f_sw): with 10 µH and 100 kHz that is (7 × 0.417)/(0.00001 × 100 000) = 2.92 A. Third, the output capacitor absorbs that triangle: C = ΔI_L/(8 × f_sw × ΔV) = 2.92/(8 × 100 000 × 0.05) = 72.9 µF → 100 µF standard. On top of the capacitive ripple, ESR adds ΔV_ESR = ΔI_L × ESR, which is why low-ESR parts are mandatory.

What is hold-up time and how do I calculate it?▼

Hold-up & rating

Hold-up time is how long a supply keeps its output in regulation after the input power disappears — the capacitor coasts, discharging from a start voltage V₁ down to the minimum voltage the converter still works at, V₂. Energy sets it: ½C(V₁² − V₂²) = P × t, rearranged to C = 2Pt/(V₁² − V₂²). Example: 10 W for 20 ms from 12 V down to 8 V needs C = 2 × 10 × 0.02/(144 − 64) = 5 000 µF. Run the same equation backwards (t = C(V₁² − V₂²)/(2P)) to find how long an existing capacitor will coast.

Why does my supply buzz or have ripple?▼

Bulk & ripple

Audible buzz at exactly 100 Hz (50 Hz mains) or 120 Hz (60 Hz mains) is ripple you can hear. The usual causes: bulk capacitance too small for the load (recompute C = I/(f_ripple × ΔV)); an aged electrolytic — years of heat dry the electrolyte, cutting capacitance and raising ESR, so ripple grows even though the circuit is unchanged; or a failed rectifier diode that silently turns your full-wave supply into half-wave, doubling the sag per cycle. Scope the rail first: measure Vpp, compare it with your ΔV budget, then re-size.

What voltage rating should my capacitor have?▼

Hold-up & rating

Worst-case peak, not nominal. That includes a high mains day (+10 %), the secondary voltage rising at light load, and the first charging peak. The working rule: run electrolytics at no more than 80 % of their rating in continuous service. A '12 V' rail fed from a 12 VAC secondary really sees ~17 V peaks → 1.3 × 17 ≈ 22 V → the 25 V standard value. A 400 V DC bus gets a 450 V capacitor, never 400 V. Above roughly 100 V, series strings divide the stress but need balancing resistors across each part.

Half-wave vs full-wave — does it double the capacitor?▼

Bulk & ripple

Yes — exactly double. Half-wave rectification recharges the capacitor once per mains cycle (f_ripple = f_line); full-wave recharges twice (f_ripple = 2 × f_line); and required capacitance scales inversely with ripple frequency: C_half = I/(f_line × ΔV) = 2 × C_full. The pain compounds: half-wave also delivers its charging pulse in one big hit per cycle, raising RMS winding current and output impedance. Toggle Full-wave → Half-wave in Mode 1 above with the same numbers and watch 8 333 µF become 16 667 µF.

Does ESR affect output ripple?▼

DC-DC output

Strongly — and at switching frequencies the ESR term often dominates the capacitive term. Total ripple ≈ ΔV_capacitive + ΔV_ESR, where ΔV_ESR = ΔI_L × ESR. With ΔI_L = 2.92 A (the 12→5 V, 100 kHz example), a generic electrolytic at 100 mΩ adds 292 mV — nearly six times a 50 mV target — no matter how many microfarads you add. Low-ESR polymer or MLCC ceramics (a few milliohms) collapse that term, and paralleling parts divides ESR further. Check the datasheet's ESR-vs-frequency curve, and derate ceramics for DC bias.

Can I parallel capacitors to get more bulk capacitance?▼

DC-DC output

Yes — capacitances simply add, with bonus benefits: ripple current splits between the parts, effective ESR falls, and heat per capacitor drops. Two 4 700 µF caps behave like 9 400 µF with roughly half the ESR. Adding a small ceramic (1–10 µF) alongside a big electrolytic lets the ceramic handle high-frequency content the electrolytic cannot see. Two cautions: parts with unequal ESR share current unequally (the low-ESR part works harder), and power-up inrush scales with total capacitance. Our series-parallel capacitor calculator handles mixed combinations.

How long will my device run during a power glitch?▼

Hold-up & rating

Run the hold-up equation backwards: t = C × (V₁² − V₂²)/(2 × P). Example: a 4 700 µF capacitor charged to 12 V, device brown-out at 8 V, 10 W load: t = 0.0047 × 80/20 = 18.8 ms — enough to ride through one full 50 Hz cycle with margin, not enough for a real outage. Narrow the voltage window and it collapses fast: the same part from 12 V down to 10 V lasts only 4.7 ms. For hundreds of milliseconds, raise the bus voltage or move to a supercapacitor bank rather than stacking electrolytics.

This power supply capacitor calculator bundles the three rail-sizing problems most people meet in one project: the rectifier’s bulk reservoir, the buck converter’s output filter, and the hold-up bank for mains dropouts. The home page of Capacitor Calculator decodes capacitor markings and converts units, and the calculators hub links every tool on the site — from reactance to motor-run sizing — so the part you buy matches the number you computed.

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