Last updated October 10, 2026 · Report an error
When current through an inductor and resistor changes (e.g. a switch closes), it doesn't change instantly — it ramps toward its new value at a rate set by the circuit's time constant. This calculator solves for whichever value — inductance, resistance, or time constant — is missing.
How it works
Enter any 2 of the 3 values (inductance in millihenries, resistance in ohms, or time constant in milliseconds) and leave the one you want to solve for blank. The calculator applies τ = L/R.
- Enter inductance (mH) — leave blank to solve for this.
- Enter resistance (Ω) — leave blank to solve for this.
- Enter time constant (ms) — leave blank to solve for this.
- Click Calculate to see your results.
Examples
A small RL circuit
A 10 mH inductor in series with a 100 Ω resistor has a time constant of 0.1 ms — after about 5 time constants (0.5 ms), the current is considered essentially fully settled.
Who should use it
- Electronics coursework on transient circuit behavior.
- Estimating switching/settling time in inductor-based circuits.
Industry applications
- Electronics and circuit design
- Power electronics and motor-drive circuits
Advantages
- Solves for any of the three variables, not just forward time-constant calculation.
- Simple, exact formula for ideal RL circuit behavior.
Limitations
- Assumes an ideal inductor and resistor — real components have some additional parasitic effects not captured here.
Common mistakes to avoid
- Confusing the RL formula (τ = L/R, resistance divides) with the RC formula (τ = RC, resistance multiplies) — they behave oppositely with respect to resistance.
- Mixing units (e.g. inductance in henries with a result expected in milliseconds) without converting consistently.
Best practices
- Remember "5 time constants" as a practical rule of thumb for when a circuit is essentially fully settled, rather than assuming it happens instantly at 1τ.
- Double check whether your circuit is genuinely RL or RC before picking the matching formula — the resistance term behaves oppositely in each.
Tips
- Pair this with the Inductor Energy Calculator to understand both the timing and the energy-storage behavior of the same RL circuit.
Frequently asked questions
Yes, with no signup and no limit on how many calculations you run.
It's the time for current to rise (or fall) to about 63.2% of the way toward its final value after a change — not the total settling time. A common rule of thumb is that a circuit is essentially fully settled after about 5 time constants.
They're structurally analogous but describe different circuits — an RC circuit's time constant is τ = RC (resistor and capacitor), while an RL circuit's is τ = L/R (inductor and resistor) — note the RL formula divides rather than multiplies.
A larger resistance dissipates energy faster relative to the inductor's stored magnetic energy, so the current reaches its new steady state more quickly — the opposite intuition from an RC circuit, where a bigger resistance slows things down.