RC Time Constant Calculator
⚡ Electricity & magnetism. Use the RC time constant formula to find time constant τ, resistance, capacitance: pick the value you want, type the others, and the answer updates as you go.
After one time constant a discharging capacitor has 37% of its charge left.
Pick the ○ next to the value you want to find, fill in the others, and it solves as you type.
What each letter means
| Quantity | Unit |
|---|---|
| Time constant τ | s |
| Resistance | Ω |
| Capacitance | F |
Worked example
Find the time constant τ when Resistance = 1000 Ω, Capacitance = 0.001 F.
Using τ = RC: Time constant τ = 1 s.
Frequently asked questions
What is the formula for RC time constant?
τ = RC. After one time constant a discharging capacitor has 37% of its charge left.
What do the letters stand for, and what units are used?
Time constant τ in s, Resistance in Ω, Capacitance in F.
How do you calculate time constant τ?
Put the values into τ = RC. For example, with Resistance = 1000 Ω, Capacitance = 0.001 F, time constant τ = 1 s.
Can I solve for a different value?
Yes. In the calculator above, pick the circle next to any value (time constant τ, resistance, capacitance) and fill in the others: it rearranges the formula for you.
