What formula does this calculator use?

C = (delta I times delta t) / (delta V minus delta I times ESR), where delta t is one switching period (1/f). This is the standard charge-balance sizing method used in power-supply and PCB decoupling app notes.

Why does switching frequency matter for sizing the capacitor?

The capacitor alone has to hold the rail steady for the time between replenishment events. A higher switching frequency means a shorter hold time, so a smaller capacitor can meet the same ripple target.

What if I do not know my capacitor ESR?

Leave it at 0 for an idealized best-case result. Ceramic MLCC capacitors typically have ESR in the low single-digit milliohms and add little ripple, but electrolytic or tantalum bulk caps can have tens of milliohms that matter at high current steps.

Should I use one large capacitor or several smaller ones in parallel?

Multiple smaller capacitors in parallel are common practice. They lower the combined ESR and equivalent series inductance compared to a single larger part, and let you place capacitance physically closer to the load pins.

Why does the calculator show a "nearest standard value"?

Capacitors are manufactured in E12 preferred values (1.0, 1.2, 1.5, 1.8, 2.2, 2.7, 3.3, 3.9, 4.7, 5.6, 6.8, 8.2 and their decades). The suggestion rounds your minimum requirement up to the next value you can actually buy.

Decoupling Capacitor Calculator

Enter the transient current step, switching frequency, and ripple tolerance to get the minimum decoupling capacitor value and the nearest standard part.

Rail & Load Parameters

Transient current step (delta I)

Switching / refresh frequency

Target ripple tolerance

Capacitor ESR (optional)

mOhm

Minimum decoupling capacitance

10.0 uF

Switching period (delta t)

1.00 us

Ripple used for sizing

50.0 mV

ESR-contributed ripple

0.0 mV

Nearest standard value

10 uF

Formula used

C = (delta I x delta t) / (delta V - delta I x ESR)

Summary

This calculator sizes a bypass or decoupling capacitor from a charge-balance model: the capacitor must supply a load's transient current step for one full switching period before the regulator or supply can replenish it, while keeping the rail voltage inside a target ripple tolerance. Add the capacitor's ESR to see its resistive contribution to ripple on top of the capacitive term, and get a suggested standard E12 capacitor value to specify on your BOM.

How it works

Enter the transient current step (delta I) that the load can suddenly draw or release.
Enter the switching or refresh frequency of the supply feeding the rail.
Set a target ripple tolerance, either an absolute millivolt limit or a percent of the supply voltage.
Optionally enter the capacitor ESR to include its resistive contribution to ripple.
Read the minimum capacitance and the nearest standard E12 value to specify.

Use cases

Sizing bypass capacitors for a microcontroller I/O bank that switches simultaneously
Sizing a bulk decoupling capacitor at a voltage regulator output for a known load step
Checking an existing decoupling capacitor value against a tighter ripple spec
PCB power-stage design checklist item for a switching regulator output
Sizing FPGA or ASIC core rail decoupling for digital switching noise
Sizing gate-driver or motor-driver supply decoupling for current spikes

Frequently Asked Questions

Last updated: 2026-09-09 · Reviewed by Nham Vu