Calculate battery runtime from mAh capacity and current draw, the mAh capacity you need, or convert mAh to Wh and Ah at a nominal voltage.

mAh is charge (milliamps × hours), not milliamps per hour.

Use the current drawn from the battery, not a converter’s output current.

Optional settings
Blank means 100% (ideal). Enter a lower value if only part of the rated charge is usable.

Milliamp-hour (mAh) / Runtime Formula

CmAh = ImA × tₕ\ tₕ = (CmAh) / (ImA)\ ImA = (CmAh) / (tₕ)

Variables:

  • CmAh is the battery capacity in milliamp-hours (mAh)
  • ImA is the average current in milliamps (mA)
  • th is the time in hours (h)

These are ideal charge relationships: mAh means milliamps multiplied by hours, not milliamps per hour. With usable fraction u, runtime = capacity × u / battery-side current, and required rated capacity = current × time / u. Blank usable capacity means 100% (u = 1).

How to Calculate Battery Capacity (mAh) or Runtime

The following steps outline how to use mAh, current draw, and time together.

  1. First, determine the average current draw in milliamps (mA).
  2. Next, determine the time in hours (h).
  3. Next, gather the formula from above (for capacity: CmAh = ImA × th; rearrange as needed for runtime or current).
  4. Finally, calculate the value you need (capacity, runtime, or average current).
  5. After inserting the variables and calculating the result, check your answer with the calculator above.

Example Problem : 

Use the following variables as an example problem to test your knowledge.

average current draw (mA) = 74

time (hours) = 20

battery capacity = 74 × 20 = 1480 mAh

Frequently Asked Questions

What is the significance of calculating milliamp-hours (mAh)?

mAh is a common way to express a battery’s capacity (electric charge). If you know a device’s average current draw in mA, you can estimate runtime with runtime (h) ≈ capacity (mAh) ÷ current (mA) using average battery-side current and a known usable-charge fraction.

Can the mAh formula be used for any type of battery?

The basic relationships between capacity, current, and time apply broadly, but real-world runtime can vary with temperature, discharge rate, cutoff voltage, aging, and efficiency of the power circuitry.

How can I improve the accuracy of my mAh/runtime estimate?

Measure the device’s average current during typical use (not just peak current), and account for system efficiency and the fact that some batteries deliver less usable capacity at higher discharge rates or low temperatures.

Is it possible to convert mAh to watts?

Not directly. You can convert mAh to watt-hours (Wh) if you know voltage: Wh ≈ (mAh × nominal V) / 1000. To get watts (W) (average power), you also need time: W = Wh / hours.