Decoupling Capacitor Calculator

Decoupling capacitors (also called bypass capacitors) are small capacitors placed close to integrated circuits (ICs) on a printed circuit board (PCB). Their job is to filter out power supply noise — rapid voltage spikes and dips caused by sudden changes in current demand as the IC switches states.

Why Decoupling Capacitors Matter

Digital ICs switch millions or billions of times per second. Each switch causes a sudden current surge that creates a voltage glitch on the power supply line. Without decoupling capacitors, this noise can cause data errors, timing problems, or spurious resets. Decoupling caps act as a local energy reservoir — providing the burst of current needed by the IC without disturbing the main power rail.

The Formula

The key relationship is: C = ΔQ / ΔV = I × Δt / ΔV

Where:

• C = Required capacitance (Farads)
• I = Peak current demand of the IC (Amps)
• Δt = how long the capacitor has to hold the rail up. This calculator uses one full period, 1/f, which is the conservative choice: the real demand is usually a fraction of a cycle, so the answer errs on the large side.
• ΔV = Maximum acceptable voltage ripple (Volts)

For frequency-domain filtering, the capacitor’s impedance (Z) at the frequency to be filtered must be much lower than the source impedance. The capacitor impedance is:

Z = 1 / (2π × f × C)

Rearranging: C = 1 / (2π × f × Z_target)

Standard Decoupling Practice

For digital ICs, use two capacitors in parallel:

1. A large bulk capacitor (10–100 μF electrolytic or tantalum) to supply slow current transients
2. A small ceramic capacitor (0.1 μF or 100 nF) to handle high-frequency noise

Place them as close as possible to the IC’s power pins. The 100 nF ceramic is the most common decoupling capacitor in electronics.

Capacitor Types for Decoupling

Important: ESR

Real capacitors have equivalent series resistance (ESR). For high-frequency decoupling, choose capacitors with very low ESR (ceramic caps are excellent). High-ESR capacitors like older electrolytics are less effective at high frequencies.

More important: ESL, and why big values stop helping

Every capacitor also has equivalent series inductance (ESL) from its own body, its solder pads, and the via down to the ground plane. That inductance and the capacitance form a series resonant circuit, and at the resonant frequency the part’s impedance hits a minimum. Above it, the inductance wins and the capacitor behaves as an inductor: impedance climbs with frequency and the thing stops decoupling.

With about 1.5 nH of mounting inductance, which is realistic for an 0603 part with a short via, the numbers fall out like this:

Read that table again, because it inverts the intuition. The 10 μF capacitor is useless at 100 MHz and the 1 nF is the useful one. This is why a datasheet tells you to fit 10 μF and 100 nF and 10 pF rather than one big capacitor: each covers a different band, and the small ones cover the fast stuff.

It also explains why layout beats value past about 50 MHz. You cannot reduce the inductance of a via by choosing a different capacitance, but you can shorten it, use a smaller package, or fit three capacitors in parallel so their inductances divide.

Typical Values by Application