Capacitor Size Calculator

Written by Great Medico

Updated September 25, 2026 •2 min read

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When you need a quick capacitor size check, the useful number is the one you can reproduce. Circuit sizing becomes much clearer when the ideal equation is separated from practical constraints such as ripple, temperature, tolerances, surge, and component ratings. The goal is to make the equation, inputs, and result easy to sanity-check rather than hide them behind a black box.

What this calculator does

This calculator focuses on required capacitance from Start-up energy, Voltage. It is designed for quick estimation and hand-checking, with the visible fields defining the scope of the model rather than implying a larger simulation.

How to use it

Start with Start-up energy, Voltage. Let the unit controls handle supported conversions instead of converting values mentally. If the page offers both inputs and derived fields, fill the quantities you know and leave result fields for the calculator.

How the calculation works

Rearranging E = ½CV² gives required capacitance C = 2E/V². For the same stored-energy target, a higher allowable voltage reduces the required capacitance by the square of voltage.

Example

With the default setup (Start-up energy = 100 J; Voltage = 12 V), the page reports required capacitance of 1.388889 F. Reproducing this default result is a quick way to verify units before replacing the values with your own case.

How to interpret the result

The required capacitance belongs to the ideal relationship shown above. Keep voltage, current, frequency, impedance, duty cycle, and component units consistent, then compare the result with real component limits rather than using it as the limit itself.

Limitations and notes

Real circuits include switch and diode losses, ESR/ESL, ripple, tolerances, thermal limits, control-loop behavior, transients, electromagnetic interference, and manufacturer derating that ideal equations may not capture. The most important values to verify are Start-up energy, Voltage. The calculator is a compact model of the visible fields, not a replacement for measurement uncertainty, component data, governing standards, or a full numerical analysis when those are required.