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Free Battery Capacity (mAh) Calculator

This battery capacity calculator computes runtime, energy content, or required capacity for any battery-powered device. Enter your battery's mAh rating, nominal voltage, and average current draw to instantly get operating time and watt-hours.

Choose a calculation mode, enter your values, and get instant results.

Efficiency accounts for self-discharge and circuit losses. Typical values: 80–90% for Li-ion, 70–85% for NiMH.

Results
Theoretical runtime23 h 20 min
Adjusted runtime (with efficiency)19 h 50 min
Battery energy content12.95 Wh

Runtime = Capacity (mAh) / Current (mA). Energy = mAh × V / 1,000. Li-ion nominal voltage: 3.7 V. LiFePO4: 3.2 V. NiMH: 1.2 V. Alkaline: 1.5 V.

Quick answer

Battery runtime is calculated by dividing battery capacity in milliamp-hours (mAh) by the device's average current draw in milliamps (mA): Runtime (h) = Capacity (mAh) / Current (mA). Energy content in watt-hours equals capacity times voltage divided by 1,000: Energy (Wh) = mAh × V / 1,000. To find the capacity needed for a target runtime, multiply current draw by desired hours: mAh = Current (mA) × Time (h). Typical consumer batteries range from 50 mAh for earbuds to 100,000 mAh for large power banks.

Formula & method

Runtime (h) = Capacity (mAh) / Current Draw (mA)
  • Capacity — Battery capacity in milliamp-hours (mAh)
  • Current Draw — Average current consumed by the device in milliamps (mA)
  • Runtime — Estimated operating time in hours

Runtime in hours. Both capacity and current must use the same prefix (mA and mAh). For real devices, apply an efficiency factor of 0.7–0.9 to account for self-discharge and circuit overhead.

Energy (Wh) = Capacity (mAh) × Voltage (V) / 1000
  • Energy — Energy stored in the battery in watt-hours (Wh)
  • Capacity — Battery capacity in milliamp-hours (mAh)
  • Voltage — Nominal battery voltage in volts (V)

Converts mAh to watt-hours using the nominal battery voltage. Use the nominal (not maximum) voltage — typically 3.7 V for Li-ion, 3.2 V for LiFePO4, 1.2 V for NiMH, 1.5 V for alkaline.

Required Capacity (mAh) = Current Draw (mA) × Target Runtime (h)
  • Required Capacity — Minimum battery capacity needed in mAh
  • Current Draw — Average current consumed by the device in mA
  • Target Runtime — Desired operating time in hours

Use this to size a battery for a new design. Divide the result by your expected efficiency factor (e.g. 0.8) to add a safety margin.

Examples

Example 1: Smartphone with 3,500 mAh battery at 150 mA draw
Input
Capacity = 3,500 mAh, Voltage = 3.7 V, Current Draw = 150 mA
Result
Runtime = 23.33 h (23 h 20 min), Energy = 12.95 Wh
Why
Runtime = 3500 / 150 = 23.333 hours, which is 23 hours and 20 minutes. Energy = 3500 × 3.7 / 1000 = 12.95 Wh. In real-world use the screen, cellular radio and other loads increase average draw well above 150 mA, cutting actual life to 8–15 hours.
Example 2: Wireless earbuds with 55 mAh cell at 10 mA draw
Input
Capacity = 55 mAh, Voltage = 3.7 V, Current Draw = 10 mA
Result
Runtime = 5.5 h (5 h 30 min), Energy = 0.2035 Wh
Why
Runtime = 55 / 10 = 5.5 hours. Energy = 55 × 3.7 / 1000 = 0.2035 Wh. This matches the ~5 h playback spec common on compact true-wireless earbuds. The charging case typically carries 3–4× this capacity to top up the earbuds multiple times.
Example 3: IoT sensor needing 8 hours on a 3.3 V supply at 25 mA
Input
Target Runtime = 8 h, Voltage = 3.3 V, Current Draw = 25 mA
Result
Required Capacity = 200 mAh, Energy = 0.66 Wh
Why
Required capacity = 25 mA × 8 h = 200 mAh. Energy = 200 × 3.3 / 1000 = 0.66 Wh. Adding a 25% safety margin (÷ 0.8 efficiency) raises the practical target to 250 mAh, so a standard 300 mAh LiFePO4 coin cell or small pouch cell would be a suitable choice.
Example 4: Power bank rated 20,000 mAh at 5 V output
Input
Capacity = 20,000 mAh, Voltage = 5 V, Current Draw = 1,000 mA
Result
Runtime = 20 h at 1 A draw, Energy = 100 Wh
Why
Runtime = 20000 / 1000 = 20 hours of continuous 1 A output. Energy = 20000 × 5 / 1000 = 100 Wh. Note that airline carry-on rules permit lithium batteries up to 100 Wh without approval, so this 20,000 mAh / 5 V power bank sits right at that boundary.

Frequently asked questions

What does mAh mean on a battery?

mAh stands for milliamp-hours. It measures how much charge a battery can store and deliver over time. A 3,000 mAh battery can theoretically supply 3,000 mA (3 A) for one hour, or 300 mA for ten hours. The higher the mAh rating, the more total energy the battery holds, assuming the same voltage.

How do I convert mAh to watt-hours (Wh)?

Multiply the capacity in mAh by the nominal battery voltage in volts, then divide by 1,000: Wh = mAh × V / 1,000. For example, a 5,000 mAh battery at 3.7 V holds 5,000 × 3.7 / 1,000 = 18.5 Wh. Watt-hours are a better comparison metric when batteries have different voltages.

Why does my device not last as long as the calculator predicts?

Real runtime is shorter than the theoretical value for several reasons: battery capacity degrades with age and charge cycles, peak loads (screen, radio, motor) spike well above the average current, temperature reduces usable capacity, and the circuit itself has conversion losses of 10–30%. Apply an efficiency factor of 0.7 to 0.85 for a more realistic estimate.

What voltage should I use — nominal, maximum, or cut-off?

Use the nominal voltage for runtime and energy calculations. Nominal voltage is the stable mid-discharge voltage: 3.7 V for lithium-ion, 3.2 V for LiFePO4, 1.2 V for NiMH, and 1.5 V for alkaline. The maximum (fully charged) voltage is higher (e.g. 4.2 V for Li-ion) and the cut-off voltage is lower; the nominal value best represents average energy delivery.

How do I choose the right battery capacity for my project?

Calculate the required capacity using: mAh = Current Draw (mA) × Target Runtime (h). Then divide by an efficiency factor (typically 0.8) to add a safety margin. For example, a device drawing 50 mA for 24 hours needs 50 × 24 = 1,200 mAh of theoretical capacity, and 1,200 / 0.8 = 1,500 mAh of rated capacity to meet the target reliably.

Can I add battery cells in series or parallel to increase capacity?

Connecting cells in parallel doubles capacity (mAh) while keeping voltage the same — two 2,000 mAh cells in parallel give 4,000 mAh at the same voltage. Connecting cells in series doubles voltage while keeping capacity the same — two 2,000 mAh cells in series give 2,000 mAh at twice the voltage. Most multi-cell packs combine both configurations for higher voltage and capacity.

Sources & references

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