Inrush Current Calculator
When a power supply switches on, a discharged capacitor briefly behaves like a short circuit, drawing a current surge limited only by the total series resistance. This inrush current can blow fuses, weld relay contacts, or stress rectifiers. This calculator estimates the worst-case peak inrush current as the source voltage divided by the limiting series resistance, plus the RC time constant that governs how quickly the surge decays. Enter your supply voltage, series resistance, and bulk capacitance.
Inrush current formula
Peak current I = V / R
Time constant tau = R * C
Settle time = 5 * tau (about 99 percent charged)
At the instant of switch-on the capacitor voltage is zero, so the full source voltage appears across R and the peak current is V / R (Ohm's law). The current then falls exponentially with time constant tau = R times C, reaching about 99 percent of steady state after five time constants.
Worked example
A 120 V supply, a 10 ohm inrush limiter, and 470 microfarads of bulk capacitance: peak current = 120 / 10 = 12.00 A. Tau = 10 * 470e-6 = 0.0047 s = 4.70 ms. Settle time = 5 * 4.70 = 23.50 ms. The surge peaks at 12 A and largely subsides within 24 ms.
Inrush current: frequently asked questions
What is inrush current?
Inrush current is the brief, high surge of current drawn the instant a load is connected to a supply, before the circuit reaches steady state. A discharged capacitor looks like a short circuit at the first moment, so the only thing limiting the peak is the total series resistance.
How is peak inrush current calculated?
For a capacitor charged through a series resistance R from a source voltage V, the worst-case peak current at the instant of switch-on is V divided by R (Ohm's law with the capacitor at zero volts). The current then decays exponentially with time constant tau = R times C.
How do I limit inrush current?
Add series resistance: an NTC thermistor, a fixed resistor, or an active soft-start circuit raises R and lowers the V/R peak. Larger R reduces peak current but increases the charging time constant, so there is a trade-off between surge protection and start-up speed.
Sources
- NIST: Physical Measurement Laboratory (electrical units).
- Peak inrush (V/R) and RC charging (tau = RC) follow directly from Ohm's law and the capacitor charging equation.
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. See our methodology.