Pick a battery type to fill in a typical capacity, enter the device current draw, then press Calculate to get an estimated runtime.
Choosing a type fills in a typical nominal capacity below. Check the label on your own cell or pack and adjust it if it differs.
Use Ah for larger packs such as e-bike or car batteries, mAh for small cells, power banks, and phones.
Check the device datasheet or a USB power meter for the average current draw. A wireless mouse might draw around 15mA, a Wi-Fi security camera around 200 to 300mA.
A battery rarely delivers its full nominal capacity in real use, because of voltage sag, the discharge rate (C-rate), and converter losses. 85% is a reasonable default, lower it for high-drain devices or older cells.
This is a straight-line estimate based on an average current draw. Real batteries do not discharge in a perfectly straight line, voltage drops faster near the end of the charge, cold temperatures reduce available capacity, and cells lose capacity as they age, so treat the result as a planning figure rather than an exact runtime.
The mistake most people make with a "2500mAh" battery
The number printed on a battery is a nominal capacity measured under a slow, steady lab discharge, not a promise of how long it will run a real gadget. A device pulling current in bursts, at a higher rate, or down to a lower cut-off voltage than the test used, gets noticeably less out of the same cell. This tool applies a usable capacity factor, 85% by default, so the estimate stays closer to what you will actually see, rather than the number on the wrapper.
Typical capacities for common cells and packs
| Type | Typical nominal capacity | Common use in the UK |
|---|---|---|
| AA alkaline | 2000 to 3000mAh | TV remotes, torches, wall clocks |
| AAA alkaline | 1000 to 1200mAh | Small remotes, wireless mice |
| 18650 Li-ion cell | 2500 to 3500mAh | Torches, laptop packs, vapes |
| 9V PP3 | 400 to 600mAh | Smoke alarms, guitar pedals |
| USB power bank | 5000 to 20000mAh | Phone and tablet top-ups |
| E-bike battery pack | 8 to 20Ah at 36V or 48V | Pedal-assist e-bikes, limited to 250W and 15.5mph under UK EAPC rules |
Once a household battery is flat, UK retailers selling more than 32kg of batteries a year are required to take old ones back under WEEE rules, and most supermarkets keep a collection point near the entrance, so a dead 9V or AA rarely needs to go further than the shop it came from.
What this calculator cannot do
The estimate assumes a constant current draw for the whole runtime, which suits a torch or a sensor far better than a phone or an e-bike, where the load changes constantly with screen brightness or hill gradient. It also ignores the non-linear voltage curve of lithium-ion cells, self-discharge while idle, and the gradual capacity loss a rechargeable battery suffers after hundreds of charge cycles. Treat the result as a planning figure, not a countdown timer.
The formula behind the result
Runtime in hours = (nominal capacity in mAh x usable capacity factor) / device current draw in mA, then divided by 24 for a figure in days. A 10000mAh power bank at 85% usable capacity feeding a 200mA device works out to (10000 x 0.85) / 200 = 42.5 hours, close to 1.8 days. For the resistor a low-power LED indicator needs off the same pack, see the LED Resistor Calculator, and the Ohm's Law Calculator covers voltage, current, resistance, and power for any simple DC circuit.