mAh to Wh Calculator for Battery Capacity

Milliamp-Hours (mAh) to Watt-Hours (Wh) Conversion Calculator formula — The Milliamp-Hours (mAh) to Watt-Hours (Wh) Conversion formula

Making the mAh number mean something

A milliamp-hour rating on a phone or power bank tells you about charge, not energy. On its own it can’t be compared across devices that run at different voltages. Watt-hours can — it’s charge times voltage, the actual amount of stored energy. Converting mAh to Wh is what lets you compare a power bank to a laptop battery, check a pack against the 100 Wh airline carry-on limit, or work out how many phone charges a bank really holds.

The milliamp-hours to watt-hours formula

Scale the milliamp-hours down to amp-hours, then multiply by the nominal voltage. For a single lithium-ion cell that voltage is about 3.7 V; for a multi-cell pack, use the pack’s rating (7.4 V for 2S, 11.1 V for 3S).

Worked example

The calculator’s defaults are 5,000 mAh at 3.7 V.

Amp-hours first:

Then watt-hours:

So a 5,000 mAh cell at 3.7 V holds 18.5 Wh.

Milliamp-Hours (mAh) to Watt-Hours (Wh) Conversion Calculator worked example diagram — A visual read of the worked example above — the same numbers the calculator returns for its default inputs.

What “20,000 mAh” really buys you

A 20,000 mAh power bank at 3.7 V is Wh of stored energy. Its USB output runs at 5 V, and the boost conversion is only about 85–90% efficient, so usable output is closer to 60–65 Wh. A phone with a 3,000 mAh, 3.7 V battery needs about 11 Wh per full charge. That’s why a “20,000 mAh” bank realistically delivers four to five phone charges, not the six or seven the raw ratio suggests.

mAh to Wh at common voltages

mAh	3.7 V	7.4 V	11.1 V
2,000	7.4 Wh	14.8 Wh	22.2 Wh
5,000	18.5 Wh	37.0 Wh	55.5 Wh
10,000	37.0 Wh	74.0 Wh	111 Wh
26,800	99.2 Wh	198 Wh	297 Wh

The 26,800 mAh row at 3.7 V lands at 99.2 Wh — just under the 100 Wh airline threshold, which is exactly why that odd-looking capacity is so common on travel power banks.

What the result assumes

Nominal voltage is an average across the discharge; a full cell reads about 4.2 V, a flat one about 3.0 V. Delivered energy also drops slightly with fast discharge, cold, and age. The figure is the nameplate rating.

Common mistakes

Using 3.7 V for a multi-cell pack, which undercounts the energy.
Ignoring conversion losses when estimating device charges from a power bank.
Comparing mAh across voltages instead of converting to Wh.

For the reverse, use the watt-hours to mAh calculator. For full-size batteries, see amp-hours to watt-hours.

About the formula: Use the stated electrical relationship and enter values in the displayed units.

Frequently Asked Questions

How do I convert mAh to Wh?+

Divide mAh by 1,000 to get amp-hours, then multiply by voltage: Wh = (mAh ÷ 1,000) × V. A 5,000 mAh cell at 3.7 V holds 18.5 Wh.

How many Wh is a 20000 mAh power bank?+

At the 3.7 V cell voltage, 20,000 mAh is 74 Wh. The USB output at 5 V delivers less than that after conversion losses — usually around 60 to 68 Wh of actual usable energy, which is why a '20,000 mAh' bank charges a 3,000 mAh phone four or five times, not six or seven.

Nominal or fully-charged voltage?+

Nominal — roughly 3.7 V for a lithium cell, the average across a discharge. Off the charger a cell reads about 4.2 V; using that would overstate the energy.

Why does the airline carry-on limit use Wh, not mAh?+

Because Wh measures actual energy and fire risk scales with energy, not charge. The common limit is 100 Wh without airline approval. A 26,800 mAh bank at 3.7 V is 99 Wh — just under — which is why that capacity is so common.

Will I get the full calculated Wh out of the battery?+

Usually slightly less. Internal resistance, fast discharge, and cold reduce delivered energy below the nameplate figure.

What voltage for a multi-cell pack?+

The pack's nominal voltage. A 2S lithium pack is 7.4 V, a 3S is 11.1 V. Using 3.7 V for a multi-cell pack undercounts the energy.

Milliamp-Hours (mAh) to Watt-Hours (Wh) Conversion Calculator