Battery Watt-Hour Calculator

How to use

1. 1Pick a direction: mAh to Wh if you have a capacity rating from a cell or power bank, or Wh to mAh if you have a watt-hour figure (laptop, e-bike) and want the equivalent capacity.
2. 2Enter the capacity or energy value, then set the cell voltage. Use a voltage preset if you are not sure: 3.7 V is right for almost every Li-ion phone, power bank, or laptop cell.
3. 3Set Cells in series and Cells in parallel to model a pack. Leave both at 1 for a single cell. Series multiplies pack voltage; parallel multiplies pack capacity. Both multiply total energy.
4. 4Enter the load you want to power and pick the unit (W, A, or mA). For amps, the tool uses the pack voltage so the runtime matches what a multimeter would read at the load.
5. 5Adjust the Efficiency percentage if your system is more or less efficient than the 85% default; lower it for inverter-driven loads and raise it for a clean DC-DC setup near the cell.
6. 6Read the result panel for the converted value, pack voltage, pack capacity, pack energy, and estimated runtime, and check the Airline tier panel before flying with a lithium-ion battery.

About this tool

Battery Watt-Hour Calculator converts between milliamp-hours and watt-hours at any cell or pack voltage, builds a pack from cells arranged in series and parallel, estimates how long the pack runs under a load, and classifies the pack against the standard IATA / FAA / TSA airline tiers for lithium-ion batteries. mAh on its own is not energy; it is capacity measured at a specific voltage, which is why a 10000 mAh power bank at 3.7 V (37 Wh) does not store the same energy as a 10000 mAh laptop pack at 11.4 V (114 Wh). The conversion uses the textbook relationship Wh equals mAh times volts divided by 1000, and the inverse mAh equals Wh times 1000 divided by volts; the tool always shows both numbers side by side so the relationship between the two is obvious. Pack math follows the standard rules cells in series multiply the pack voltage, cells in parallel multiply the pack capacity, and both contribute to total energy, so a 4S2P pack of 3.7 V 3000 mAh 18650 cells is a 14.8 V 6000 mAh 88.8 Wh pack. Voltage presets cover the chemistries people actually look up nominal 3.7 V Li-ion / Li-Po, 3.85 V high-voltage Li-ion, 3.2 V LiFePO4, 1.2 V NiMH, 1.5 V alkaline, 2.0 V lead-acid cell, and common bus voltages 5 V USB, 12 V automotive, 24 V scooter, 36 V e-bike, and 48 V e-bike or solar. Runtime is estimated as pack energy times efficiency divided by load watts; the efficiency slider defaults to 85% to capture inverter, voltage-conversion, BMS, and low-voltage-cutoff loss that any real portable system experiences, and the load can be entered in watts, amps, or milliamps. A list of realistic loads (phone idle, USB-C laptop charge, drone, 12 V cooler, CPAP, Starlink, e-bike) one-click fills the field at sensible mid-range numbers. The airline panel maps total pack watt-hours to the three tiers in the IATA Dangerous Goods Regulations (Packing Instruction 965 and 967) and US 49 CFR 175.10 (a)(18): under 100 Wh allowed in carry-on without approval, 100 to 160 Wh allowed with prior airline approval and up to two spares, and over 160 Wh forbidden on passenger aircraft. The page surfaces the underlying formulas and the assumptions behind every number so the math is verifiable. Useful for sizing a power bank, comparing two laptop batteries with different chemistries, designing a small solar or e-bike pack from 18650 or 21700 cells, estimating drone flight time, planning a CPAP or camping fridge runtime, or just checking whether a battery is allowed in carry-on luggage before a flight. Everything runs in the browser; nothing is uploaded.

Free to use. Works in your browser. No signup, no login.