Battery Life Calculator: Runtime from mAh

Battery / power bank

Capacity (mAh)

What do you want to work out?

Device current draw (mA)

Efficiency (%)

Estimated runtime

—

Usable capacity—

Estimates only — real results vary with temperature, age and load. Calculations run in your browser.

Why you never get the full mAh

A 10,000 mAh power bank does not put 10,000 mAh into your phone. Voltage conversion (from the cells' 3.7 V to the 5 V USB delivers), heat and circuitry losses typically eat 20–35% of the rated capacity, so real-world output is closer to 6,500–8,000 mAh. That's why this calculator includes an efficiency setting — around 80% is a realistic default for a decent power bank.

Estimating device runtime

To estimate how long a device runs, divide the battery's usable capacity (mAh) by the device's average current draw (mA): a 2,000 mAh battery powering a 200 mA device lasts roughly ten hours. Current draw varies with what the device is doing — screens, radios and motors spike consumption — so use a realistic average for the way you actually use it, not the idle figure.

How many phone charges from a power bank?

Switch this tool to 'phone charges' mode and enter your phone's battery size. Divide the bank's usable capacity by the phone's capacity to estimate full charges. A 10,000 mAh bank at ~80% efficiency delivers about 8,000 mAh, which is roughly two charges of a 4,000 mAh phone. Bigger phones, fast-charging losses and a low battery cut-off mean you may get slightly fewer.

Frequently asked questions

How do I convert mAh to hours of runtime?

Divide the battery's usable capacity in mAh by the device's current draw in mA, then you have hours. For example, 1,000 mAh ÷ 250 mA = 4 hours. Because real batteries lose some capacity to heat and conversion, apply an efficiency factor (this calculator defaults to 80%) for a realistic figure rather than the theoretical maximum.

Why does my power bank give fewer charges than expected?

Rated mAh is the cell capacity at 3.7 V, but charging your phone happens at 5 V (or higher for fast charging), and that voltage conversion plus heat wastes 20–35% of the energy. So a 10,000 mAh bank realistically delivers around 6,500–8,000 mAh to your phone. Fast charging, cold weather and the phone's own losses reduce it further.

What efficiency should I use in the calculator?

Around 80% is a reasonable default for a good-quality power bank or battery setup. Cheaper banks or fast-charging scenarios may be closer to 65–75%; very efficient setups can approach 85%. If you know your specific device's real-world output, use that; otherwise 80% gives a sensible, slightly conservative estimate that won't leave you stranded.