capacitor discharge time calculator

What this capacitor discharge time calculator does

This calculator estimates the time required for a capacitor to decay from one voltage to another in a simple RC circuit. It is useful for electronics design, power-down behavior, delay timing, sensor filtering, and safety checks for stored energy.

If you have ever asked, “How long until this capacitor reaches a safe voltage?”, this is exactly the calculation you need.

The formula behind capacitor voltage decay

For a discharging capacitor in a resistor-capacitor (RC) network, voltage follows an exponential curve:

V(t) = V0 × e^(-t / (R × C))

Solving for time:

t = -R × C × ln(Vt / V0)

• V0 = initial voltage
• Vt = target voltage
• R = resistance in ohms
• C = capacitance in farads
• t = discharge time in seconds

The product R × C is called the time constant (tau, τ).

How to use the calculator

Step-by-step

• Enter the initial capacitor voltage.
• Enter the target voltage you care about.
• Enter resistor value and select the proper unit (Ω, kΩ, or MΩ).
• Enter capacitor value and select its unit (F, mF, µF, nF, pF).
• Click Calculate Discharge Time.

You’ll get discharge time, time constant τ, and the number of time constants elapsed.

Quick interpretation of time constants

In many designs, engineers approximate capacitor discharge using multiples of τ:

Elapsed Time	Remaining Voltage	Discharged
1τ	36.8%	63.2%
2τ	13.5%	86.5%
3τ	5.0%	95.0%
5τ	0.67%	99.33%

Worked example

Example: 12 V capacitor to 1 V

Suppose you have:

• Initial voltage: 12 V
• Target voltage: 1 V
• Resistor: 10 kΩ
• Capacitor: 100 µF

Then:

• R = 10,000 Ω
• C = 0.0001 F
• τ = R × C = 1 s
• t = -1 × ln(1/12) ≈ 2.48 s

So it takes about 2.48 seconds to drop from 12 V to 1 V.

Design tips and practical notes

• Higher resistance means slower discharge.
• Higher capacitance also means slower discharge.
• Electrolytic capacitors can have leakage and tolerance that shift real results.
• Real circuits may include extra paths (IC pins, pull-ups, protection diodes).
• Use a bleeder resistor for predictable power-down behavior.

Safety reminder

Large capacitors can store dangerous energy even after power is removed. Always verify voltage with a meter before handling circuitry. Do not rely purely on an estimated time in high-voltage applications.

FAQ

Can target voltage be zero?

Mathematically, reaching exactly 0 V takes infinite time in an ideal exponential model. In practice, choose a small threshold such as 1 V, 0.5 V, or 5% of initial voltage.

What is an RC time constant calculator?

It is a calculator centered around the time constant τ = R × C, helping you estimate charging or discharging behavior in RC circuits.

Does this work for capacitor charging too?

This page is for discharge timing. Charging uses a related equation with different voltage conditions, but still depends on the same time constant τ.