Time Constant Calculator

The time constant (τ) of a circuit tells you how quickly it charges or responds to a signal — a fundamental value in electronics for designing filters, timers, and amplifiers. Select your Circuit Type (RC or RL), then enter your Resistance along with either Capacitance or Inductance to get the Time Constant (τ) in seconds. Secondary outputs include Cutoff Frequency, 63.2% Charge Time, and 95% Charge Time.

Ω
µF
H

Results

Time Constant (τ)

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Cutoff Frequency

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63.2% Charge Time

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95% Charge Time

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Results Table

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Frequently Asked Questions

What is an RC circuit time constant?

The RC time constant (τ) is the product of resistance (R) and capacitance (C). It represents the time required for a capacitor to charge to 63.2% of its final voltage or discharge to 36.8% of its initial voltage.

How do I calculate the time constant for an RL circuit?

For an RL circuit, the time constant τ = L/R, where L is inductance in henries and R is resistance in ohms. This represents the time for current to reach 63.2% of its final value.

What is the relationship between time constant and cutoff frequency?

The cutoff frequency fc = 1/(2πτ), where τ is the time constant. This frequency represents the -3dB point where the circuit's response is 70.7% of its maximum value.

How long does it take for a capacitor to fully charge?

Theoretically, a capacitor never fully charges, but it reaches 95% charge in 3τ, 99% in 5τ, and is considered fully charged after 5 time constants (99.3% charged).

Is RC time constant the same as capacitor charging time?

No, the time constant τ is specifically the time to reach 63.2% charge. Full charging time is typically considered to be 5τ, when the capacitor reaches 99.3% of its final voltage.

What units should I use for the time constant calculation?

For RC circuits: resistance in ohms (Ω) and capacitance in farads (F) gives τ in seconds. For RL circuits: inductance in henries (H) and resistance in ohms gives τ in seconds.

Can I use this calculator for both low-pass and high-pass filters?

Yes, the time constant is the same for both filter types using the same R and C values. The difference is in the circuit configuration and which signal you measure, not the time constant itself.