Battery Life Calculator
Calculate Battery Life values for science, physics, chemistry, and engineering problems.
This battery life calculator estimates how long a battery will power a device, based on the battery's capacity (in milliamp-hours) and the device's average current draw (in milliamps).
Enter the battery capacity and device current draw, and the calculator returns the estimated runtime.
Worked Calculation Examples
| Scenario | Result | Calculation Step |
|---|---|---|
| 2,000 mAh battery, 200 mA draw | 10 hours | Runtime = 2,000 ÷ 200 = 10 hours (theoretical maximum, before real-world efficiency losses). |
| 3,000 mAh battery, 150 mA draw | 20 hours | Runtime = 3,000 ÷ 150 = 20 hours. |
The Battery Life Formula
Runtime (hours) = battery capacity (mAh) / current draw (mA). A 2000 mAh battery powering a device that draws 200 mA would theoretically last about 10 hours, though real-world runtime is usually somewhat shorter due to efficiency losses and non-constant power draw.
Why Actual Runtime Is Often Shorter Than Calculated
This calculation assumes constant current draw and 100% usable capacity, but real devices often have variable power consumption (spikes during high-demand tasks), and batteries typically can't be fully discharged efficiently, especially as they age - actual runtime is often 10-20% less than the simple theoretical calculation.
How to Use the Battery Life Calculator
- Open the Battery Life Calculator and enter the values requested in the input fields.
- Check the units, percentages, dates, or time periods before reading the answer.
- Review the instant result and adjust any value to compare another scenario.
- Use the formula, example, and FAQs below to understand how the battery life calculator works.
Frequently Asked Questions
How do you calculate battery life?
Divide the battery's capacity in milliamp-hours (mAh) by the device's average current draw in milliamps (mA) to get estimated runtime in hours.
Why is my actual battery life shorter than calculated?
Real devices often have variable current draw (spiking during demanding tasks), and batteries lose some efficiency, especially as they age or in extreme temperatures, so actual runtime is typically somewhat less than the theoretical calculation.
What does mAh mean on a battery?
mAh (milliamp-hours) is a measure of battery capacity - how much current the battery can theoretically supply for one hour before being fully depleted.
How do I use this battery life calculator?
Enter the known values, review the units or settings, and the calculator updates the result instantly. The formula and example on this page show how the answer is produced.
What does the Battery Life Calculator calculate?
Calculate Battery Life values for science, physics, chemistry, and engineering problems. It is designed for fast browser-based calculations without sign-up, downloads, or manual spreadsheet setup.
What formula does this battery life use?
The formula is: Runtime (hours) = Battery Capacity (mAh) ÷ Current Draw (mA). Battery capacity rating in amp-hours dates back to the practical needs of 19th-century lead-acid battery manufacturers, formalized as Italian physicist Alessandro Volta's original 1800 voltaic pile - the first true battery - gave way over the following century to increasingly standardized, ratable battery technologies whose stored energy needed a consistent way to be specified and compared across manufacturers.
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Last updated: September 27, 2026.