Decoupling Capacitor Calculator

Decoupling that's undersized shows up as brownouts and glitches the moment a board sees a real current transient; oversized decoupling just wastes board space and money. This selector works from a target power-rail impedance and the bulk charge reservoir a load actually needs, so bypass and bulk capacitance get sized with a number behind them instead of a rule of thumb copied from the last design.

Decoupling Capacitor Selector

Target impedance and bulk charge reservoir

Show your work

Target frequency

Target impedance

Loop inductance (ESL)

Required capacitance

15.9 nF

Nearest E12 value

15.0 nF

Self-resonant frequency

39.9 MHz

Z from ESL alone at f

628 mΩ

Bulk / transient reservoir

Transient current

Rise time

Allowed droop

Bulk capacitance (C = I·Δt / ΔV)

20.0 nF

Formula

C = \frac{1}{2 π \cdot f \cdot Z}

SRF = \frac{1}{2 π \cdot \sqrt{L \cdot C}}

Z_{ESL} = 2 π \cdot f \cdot L

C_{bulk} = \frac{I \cdot Δ t}{Δ V}

The first three size the high-frequency decoupling capacitor; the last sizes the bulk reservoir for load transients. They are separate problems with very different answers.

f = target frequency (Hz)
Z = target impedance (Ω)
C = capacitance (F)
L = loop inductance (ESL) of the capacitor and its pads and vias (H)
SRF = self-resonant frequency (Hz)
Z_ESL = impedance of the loop inductance alone at f (Ω)
I = transient current (A)
Δt = rise time of the transient (s)
ΔV = allowed droop (V)

Above the self-resonant frequency the part is inductive and stops decoupling — that impedance figure is set by loop inductance, not by capacitance. Shorten the via-to-pad loop before adding more capacitors.