Electronics
RC Time Constant
Supply a resistor and capacitor value (in any popular unit) to calculate the RC time constant and common charging milestones.
Compute τ = RC along with the time to reach 63%, 95%, and 99% charge in a simple RC circuit.
Exponential charge
τ = R × C
V(t) = V₀ (1 − e^{−t/τ})Once τ is known, 63% charge is reached at one time constant, ~95% at 3τ, and ~99% near 5τ. The calculator reports each checkpoint automatically.
How to use
- Select the units for R (Ω, kΩ, MΩ) and C (F → nF).
- Enter the numeric values for resistor and capacitor.
- Read τ in seconds plus the time to reach 63%, 95%, and 99% of the final voltage.
Example
Input: R = 4.7 kΩ, C = 10 µF
Output: τ ≈ 0.047 s, 95% ≈ 0.141 s, 99% ≈ 0.235 s
Student-friendly breakdown
This walkthrough emphasizes the most searched ideas for RC Time Constant: RC Time Constant. Start with the formula above, then follow the guided steps to double-check your work. For quick revision, highlight the givens, plug into the equation, and finish by verifying your units.
Need more support? Use the links below to open the long-form guide, browse additional examples, or hop into adjacent calculators within the same topic — each one is a quick way to double-check your work or handle a related question without starting from scratch.
Deep dive & study plan
RC Time Constant: Finds τ = R·C plus the time to hit 63%, 95%, and 99% charge levels. It's built around rc circuit, time constant, tau, so you can go from a raw question to a checked answer without switching tools.
The math behind it: Once τ is known, 63% charge is reached at one time constant, ~95% at 3τ, and ~99% near 5τ. The calculator reports each checkpoint automatically. The core relationship is τ = R × C V(t) = V₀ (1 − e^{−t/τ}), shown above the calculator so you can see exactly how your inputs turn into the result.
To use it well: (1) Select the units for R (Ω, kΩ, MΩ) and C (F → nF). (2) Enter the numeric values for resistor and capacitor. (3) Read τ in seconds plus the time to reach 63%, 95%, and 99% of the final voltage. Keep your units consistent as you go, and re-run a case you already know the answer to — it's the fastest way to catch a typo before it throws off a result you're relying on.
Worked example: entering R = 4.7 kΩ, C = 10 µF returns τ ≈ 0.047 s, 95% ≈ 0.141 s, 99% ≈ 0.235 s. Try swapping in your own numbers next, especially a case you're unsure about, before you use this for something that matters.
Quick retention checklist
- Speak the formula aloud (or annotate it) so the relationships stick.
- Write each step in your own words and compare with the numbered list above.
- Swap in new numbers for the Example to make sure the calculator (and your logic) handles edge cases.
- Check at least one related calculator below — it's the fastest way to confirm your numbers still line up from a different angle.
FAQ & notes
Does this apply to discharge as well?
Yes. RC discharge follows the same time constant; the milestones tell you when voltage decays to 37%, 5%, and 1%.
Can I include load resistance?
The base formula assumes a single resistor. Combine equivalent resistances first, then plug the result into the calculator.
What formula does the RC Time Constant use?
Once τ is known, 63% charge is reached at one time constant, ~95% at 3τ, and ~99% near 5τ. The calculator reports each checkpoint automatically.
How do I use the RC Time Constant?
Select the units for R (Ω, kΩ, MΩ) and C (F → nF). Enter the numeric values for resistor and capacitor. Read τ in seconds plus the time to reach 63%, 95%, and 99% of the final voltage.