Battery Life Calculator
Battery capacity is quoted in milliamp-hours and drain in milliamps, so runtime is simple division: a 5,000 mAh phone battery feeding 350 mA runs about fourteen and a half hours. The catch is 'average' — devices swing between radio bursts and deep sleep, and cold or an ageing cell shrinks usable capacity below the printed number. This page gives that ceiling honestly: use it to size power banks and compare cells.
Enter values
Results
- Estimated battery life
- 14 h 17 min
- Battery life (hours)
- 14.29
How is the Battery Life calculated?
Find the battery's capacity in mAh on its label or spec sheet, and the average current the device draws while running, in mA — a multimeter across the supply gives the truest figure. The calculator divides capacity by draw, returns the life formatted as hours and minutes, and also shows the decimal hours for quick comparisons. Anything from a coin cell to an e-bike battery works, as long as both numbers speak in milliamps.
Formula
Hours = capacity (mAh) ÷ average draw (mA) where: capacity = charge the battery stores, in milliamp-hours draw = average current the device pulls, in milliamps A 5,000 mAh cell at 350 mA runs 14.29 hours. mAh compares cells at the same voltage only — across voltages, compare energy in watt-hours: Wh ≈ mAh × V ÷ 1,000.
Example
Capacity: 5,000 mAh · Average draw: 350 mA 5,000 ÷ 350 = 14.29 hours ≈ 14 h 17 min A 20,000 mAh power bank is not four full phone charges, though: its capacity sits at the cell's 3.7 V while your phone charges at 5 V, and conversion losses typically deliver only 60–75% of the printed figure into the device.
Things to remember
Treat the result as a ceiling, not a promise. Cold can cut usable capacity by a fifth, cells fade with every cycle, and 'average draw' is where estimates go wrong. For critical sizing — solar banks, e-bike range, remote sensors — measure the real draw, divide with those honest numbers, and add headroom.
Frequently Asked Questions
How do I convert mAh to hours of battery life?
Divide the mAh rating by the device's average mA draw: 2,500 mAh at 250 mA is 10 hours. There is no conversion without the draw figure — mAh alone measures charge stored, not how quickly the device releases it.
Is a higher mAh always a longer battery life?
At the same voltage and the same draw, yes — runtime scales linearly with capacity. Across different voltages, compare watt-hours instead (mAh × volts ÷ 1,000): a 10,000 mAh phone cell stores about 38 Wh, while a 10,000 mAh 12 V pack stores about 120 Wh.
Why does real battery life fall short of the estimate?
Because draw is never constant: radios spike during uploads, screens brighten, and cold weather slows the chemistry itself. The calculation assumes a steady average, so it returns the best case — measured real-world averages usually land 10–30% below it.
Do power banks deliver their full rated mAh?
Rarely. Capacity is stored at the cell's 3.7 V but USB delivers 5 V, and conversion efficiency runs 60–75%, so a 20,000 mAh bank realistically moves the equivalent of about 12,000–15,000 mAh into your phone. Marketing divides by eight; physics divides by something closer to five.
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