Battery Life Calculator
Enter battery capacity and current draw to estimate how long it will run, with a real-world efficiency factor.
Battery run time is roughly the battery capacity divided by the current the device draws, times an efficiency factor for real-world losses. A 3,000 mAh battery powering a 200 mA load runs about 3,000 ÷ 200 = 15 hours in theory, or around 12.75 hours at 85% efficiency.
How to Use This Calculator
Enter the battery's capacity in milliamp-hours (mAh) and the average current your device draws in milliamps (mA). The efficiency factor accounts for real-world losses — heat, voltage conversion and the battery not delivering its full rated capacity — and defaults to 0.85.
The calculator returns the estimated run time in hours. Make sure both figures use the same current unit; if your capacity is in amp-hours (Ah), multiply by 1,000 to get mAh first.
How the Calculation Works
In ideal terms, a battery's capacity divided by the current draw gives the run time — a 2,000 mAh battery at 100 mA lasts 20 hours. Both use milliamps, so the units cancel to hours.
Real batteries never deliver their full rated capacity to the load, because of internal resistance, voltage regulation and temperature. Multiplying by an efficiency factor, typically 0.8 to 0.9, gives a more realistic estimate.
run time (h) = (capacity ÷ load) × efficiency
Worked Example
A 3,000 mAh battery powering a device that draws 200 mA lasts, in theory, 3,000 ÷ 200 = 15 hours.
Applying an 85% efficiency factor gives 15 × 0.85 = 12.75 hours, a more realistic expectation for actual use.
Assumptions and Limitations
- Assumes a roughly constant average current draw
- Capacity and load must use the same current unit (mAh with mA)
- Efficiency factor accounts for real losses — 0.8–0.9 is typical
- Actual life varies with temperature, age and load spikes
When to Use a Battery Life Calculator
Makers, electronics hobbyists, and engineers use this to estimate run time from a battery's capacity and the device's current draw. Run it when designing a project, choosing a battery, or predicting how long a device will last on a charge. Enter capacity and draw to get the estimated hours.
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How do you calculate battery life?
Divide the battery capacity by the current draw, then multiply by an efficiency factor. A 3,000 mAh battery at 200 mA is 15 hours ideal, or about 12.75 hours at 85% efficiency.
What is mAh?
Milliamp-hours, a measure of battery capacity. A 3,000 mAh battery can in theory supply 3,000 mA for one hour, or 300 mA for ten hours, before it is depleted.
Why apply an efficiency factor?
Because batteries never deliver 100% of their rated capacity to a device. Internal resistance, voltage conversion and temperature cause losses, so a factor of 0.8–0.9 gives a realistic run time.
How do I convert Ah to mAh?
Multiply amp-hours by 1,000. A 2.5 Ah battery is 2,500 mAh. Keep capacity and current in matching units (mAh with mA, or Ah with A).
Why does my battery last less than calculated?
Cold temperatures, an aging battery, higher-than-average current spikes, and background loads all reduce real run time. The efficiency factor captures some of this, but conditions vary.
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