Battery C-Rate Calculator

Enter capacity + current to get C-rate and ideal runtime (or solve in reverse). Includes unit conversions and Show Work.

How to Use

  1. Enter battery capacity (Ah or mAh).
  2. Enter current (A or mA), or enter a C-rate.
  3. Use “Solve For” to choose what you want to compute.
  4. Open “Show Work” for formulas and step-by-step math (base units).
Battery Lab View
Visual feedback: higher C-rate = higher stress + shorter ideal runtime.
Capacity
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Current
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C-Rate
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Ideal Time
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Load: —
C-rate gauge Higher C = faster discharge/charge (ideal math). Manufacturer limits vary. 0C 0.5C 1C 2C 4C Indicator scales up to 4C for visualization; calculations are not capped.
Inputs & Settings
Capacity + current gives C-rate and ideal runtime. You can also solve for current from C-rate.
Examples: 3000mAh (18650), 5Ah (tool pack), 60Ah (car battery)
If solving for current, leave this blank and enter C-rate instead.
Definition: C = I / Capacity(Ah). 1C means current equals capacity in amps.
Applies to ideal runtime only (losses, heat, inverter, etc.). Leave blank for 100%.
Quick Adjust
1.0 C

Tip: Slider is for quick “what-if” testing. Use Solve For if you want C-rate to drive current.

Show Work (step-by-step)
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Work is shown in base units (Ah, A, hours). Efficiency (if set) adjusts ideal runtime only.

C-Rate Formulas

Definition: C = I / Capacity(Ah)

  • C-rate: C = I / Ah
  • Current: I = C × Ah
  • Ideal runtime: t(h) = Ah / I (and if efficiency is used: t × (eff/100))
Notes: This is ideal math. Real runtime depends on chemistry, temperature, voltage cutoff, internal resistance, and manufacturer specs.

FAQ

What does “1C” mean?

1C means the current equals the capacity in amps. Example: 2Ah at 1C = 2A (ideal ~1 hour).

How do I convert mAh to Ah?

Ah = mAh / 1000. Example: 3000mAh = 3Ah.

Why is my real runtime shorter than the ideal?

Losses and limits: voltage cutoff, internal resistance heating, high-load sag, temperature, and battery health.

Can I use this for charging C-rate too?

Yes—the math is the same. Always follow the battery’s specified charge current and max C-rate.

Tool Info

Last updated:

Updates may include UI improvements, unit support, and calculation edge-case handling.