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Battery Life Calculator

Estimate how long a battery will last.

How it is calculated

Runtime is the capacity divided by the load: ideal hours = capacity (mAh) / load (mA). The tool then shows a second, more cautious figure: realistic hours = ideal hours × 0.85. The fixed 0.85 factor is a rule-of-thumb allowance for conversion losses and for a battery that cannot be drained completely; it is not measured for your cell, temperature or discharge rate. Both inputs must be greater than zero, and both results are given in hours with two decimals. Because the formula is only a ratio, the same arithmetic works for watt-hours divided by watts, as long as both inputs use matching units. It assumes a constant load, which real devices rarely have.

Worked example

A 2000 mAh battery powers a sensor that draws 150 mA. Ideal runtime: 2000 / 150 = 13.33 hours, shown as 13.33 h. Applying the 0.85 allowance: 13.33 × 0.85 = 11.33 hours, shown as 11.33 h. In practice you would plan around the second figure, so the sensor should run for just over eleven hours before the battery needs charging, assuming the load really stays at 150 mA.

Try this example →

Common mistakes

  • Mixing amps and milliamps, for example a 2000 mAh battery with a load entered as 0.15 for 0.15 A. The units must match, so enter 150 mA, otherwise the answer is off by a factor of 1000.
  • Using the average current for a device that spends most of its time asleep and wakes in short bursts. Use the duty-cycle average, otherwise a battery lasts far longer than the peak current suggests.
  • Treating the printed capacity as usable. Cheap or old cells often deliver less than the label, and cold weather reduces capacity further, so the realistic figure may still be optimistic.
  • Comparing batteries of different voltage by mAh alone. Two packs with the same mAh but different voltages store different energy; convert to watt-hours (volts × amp-hours) first.
  • Entering the charger or adapter rating as the load. Use the current the device actually draws from the battery, not the current its charger can supply.

About this calculator

This calculator estimates runtime as hours = capacity ÷ load — battery capacity in milliamp-hours (mAh) divided by the device's current draw in milliamps (mA), or watt-hours divided by power draw in watts for larger systems. It's a simplified model that assumes constant current and ignores the efficiency losses and voltage sag that real batteries show as they discharge.

Electronics hobbyists use it to size a battery for an Arduino or RC project before buying one, off-grid and solar system designers use the watt-hour version to match battery banks to expected loads, and it gives anyone a quick sanity check on a device's advertised battery life against its actual capacity and power draw.

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Comments (1)

  • Editorial team

    When several parts run at the same time, such as a sensor, a radio and an LED, add their currents in milliamps first and enter the sum as the load. The realistic line applies one fixed 0.85 factor, so a measured runtime much shorter than it points to a real load higher than you assumed or a tired cell. If your measured runtime differs a lot from the realistic figure, post your capacity and load.

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