Wh to mAh Calculator

Watt-hours measure energy, milliamp-hours measure charge

Watt-hours (Wh) say how much energy a battery holds. One watt-hour is one watt of power delivered for one hour. Milliamp-hours (mAh) say how much electric charge moved. One milliamp-hour is one thousandth of an amp flowing for one hour. Voltage is the only thing that links the two numbers.

Inside one product family the charge rating works as a size label, because every battery in that family sits at the same voltage. It stops working the moment you compare a 3.7 V phone pack with an 18 V tool battery. The 18 V pack holds more energy for the same charge number, so a bigger label does not always mean a bigger battery.

That is why airlines, laptop makers and power banks quote watt-hours. The FAA limits batteries by watt-hour rating, and a laptop battery sticker shows both units. Converting between them takes one division and one multiplication, and the result changes with every voltage you pick.

The conversion formula and the voltage you should use

The formula is mAh = Wh / V x 1,000, where V is the voltage the label refers to. Divide the energy by that voltage, then move the decimal three places to the right.

Picking V is the whole decision. Use the voltage printed on the battery pack, not the voltage coming out of a USB port. A power bank sold as 20,000 mAh usually carries four 3.7 V lithium cells wired in parallel. The pack label reads 3.7 V and 74 Wh. Its USB port hands out 5 V, and converting with 5 V answers a different question: how much charge the port can push out, not how much charge the cells hold.

Each row is 74 / V x 1,000. Nothing about the pack changed between rows: same energy, different number under the fraction. The USB Implementers Forum describes how USB Power Delivery negotiates fixed steps such as 5 V, 9 V, 15 V and 20 V, which is why the last row exists at all.

Worked example: a 74 Wh power bank at 3.7 V

The best-selling power bank size is 20,000 mAh, and its label usually shows 74 Wh. Check the label against the cells: 20,000 x 3.7 / 1,000 = 74. The arithmetic agrees, so the pack holds 74 watt-hours of energy.

Run the same pack through the formula in the direction this page is built for: 74 / 3.7 x 1,000 = 20,000 mAh. Round numbers come out because the manufacturer picked the label from the cells, not the other way round.

Now take the larger pack, 26,800 mAh. 26,800 x 3.7 / 1,000 = 99.16 Wh. That sits 0.84 Wh below the 100 Wh carry-on ceiling, and the margin is deliberate: makers size the biggest consumer pack to land just under the line where airline approval starts. The same figures show how much more energy the big pack holds: 99.16 / 74 = 1.34, about a third more than the 74 Wh model, which the charge labels alone do not spell out.

Why the 5 V output gives a smaller number

The cells store energy at 3.7 V and the port delivers it at 5 V, so the same watt-hours buy fewer milliamp-hours at the higher voltage: 74 / 5 x 1,000 = 14,800 mAh.

Conversion also costs energy. A boost converter inside a power bank typically keeps 85 to 90 percent of what it takes in, and the rest leaves as heat. At 85 percent the usable energy is 74 x 0.85 = 62.9 Wh, which at the port works out to 62.9 / 5 x 1,000 = 12,580 mAh. That is 15 percent below the 14,800 mAh the port could deliver with no losses, and 37 percent below the 20,000 mAh printed on the box, which is why people think their power bank is broken.

A phone battery rated 5,000 mAh at 3.85 V holds 5,000 x 3.85 / 1,000 = 19.25 Wh. The naive sum looks like 20,000 / 5,000 = 4 full charges. The honest sum is 62.9 / 19.25 = 3.27, so three charges and a partial fourth is the realistic outcome. If you want runtime instead of charge, the mAh to hours calculator takes the same capacity and divides by current draw.

Airline limits are written in watt-hours

The FAA PackSafe guidance (checked 2026) grades lithium-ion batteries by watt-hour rating, not by milliamp-hours. Up to 100 Wh is allowed on passenger aircraft without approval. From 101 to 160 Wh the carrier has to approve, and no more than two spare batteries per person. Above 160 Wh the battery is forbidden in both carry-on and checked baggage. Spare batteries and power banks travel in the cabin only, with terminals taped or bagged so they cannot short.

The tier is energy, so it treats packs of different sizes alike. A 26,800 mAh power bank at 3.7 V (99.16 Wh) and an 8,700 mAh laptop battery at 11.4 V (99.18 Wh) both sit under the ceiling even though their charge labels differ by a factor of three. The full chart is on the FAA PackSafe lithium battery page, and the rule behind it is 49 CFR 175.10. Individual airlines often set stricter counts, so check with the carrier before you pack a large bank.

Real batteries converted to watt-hours

The table converts common batteries both ways, so you can see which ones clear an airline tier and which ones never will. Each row is mAh x V / 1,000.

The e-bike row is the useful warning. 504 Wh is more than three times the 160 Wh ceiling, so that battery cannot fly at all, whatever the airline says. The drill pack at 90 Wh would pass as a spare if it travels in the cabin and is protected, though tool batteries inside checked devices follow their own rules.

Going the other way: mAh to Wh

The reverse is Wh = mAh x V / 1,000. Multiply charge by voltage and move the decimal three places to the left. A 5,000 mAh phone cell at 3.85 V gives 5,000 x 3.85 / 1,000 = 19.25 Wh, and the same 19.25 Wh read at 3.7 V would come out as 5,203 mAh (19.25 / 3.7 x 1,000 = 5,203), which is why a spec sheet and a battery teardown never match exactly.

A useful inverse: a 100 Wh allowance equals 100 / 3.7 x 1,000 = 27,027 mAh at cell voltage, but only 100 / 11.4 x 1,000 = 8,772 mAh at 11.4 V. The same allowance covers three times the charge in a low-voltage pack. If you have the watt-hours and want current instead of charge, the amps to watts calculator covers the power side of that conversion.

Mistakes that throw the number off

Using 5 V for a pack labelled at 3.7 V. The answer drops by 26 percent before any losses are counted: 20,000 mAh turns into 14,800 mAh. Both numbers are true in their own context, but only one belongs on a spec sheet.

Mixing Ah and mAh. A drill pack marked 5 Ah is 5,000 mAh. Drop the kilo and the watt-hour figure falls by a factor of 1,000.

Using the full-charge voltage. Lithium cells charge to 4.2 V but are rated at 3.7 V nominal. At 4.2 V the same 74 Wh reads 74 / 4.2 x 1,000 = 17,619 mAh, a number no manufacturer prints because it describes a state the pack holds for minutes.

Treating label mAh as delivered mAh. Every conversion, every cable and every charge controller takes a cut. Expect 80 to 90 percent of the arithmetic in real use, and less in cold weather.

Rounding at the 100 Wh line. A pack at 100.4 Wh needs airline approval even though the label rounds to 100. Compare the printed watt-hours to the tier, not a rounded version of it.