Wh to mAh Calculator

To convert Wh to mAh, multiply the watt-hours by 1,000 and then divide by the battery’s nominal voltage. A 74Wh lithium pack at the standard 3.7V cell voltage is 20,000mAh, because 74 × 1,000 ÷ 3.7 = 20,000. Divide by the cell voltage, never the 5V a USB port puts out.

The Wh to mAh formula, explained

Three quantities are involved, and only one of them causes any trouble.

• Watt-hours (Wh) — energy. What the battery holds, independent of voltage: one watt-hour runs a one-watt load for one hour. It is the unit power stations, laptops and airlines are specified in.
• Volts (V) — the pack’s nominal voltage. The average the cells sit at across a discharge, not what a full cell reads on a meter and not what the pack outputs. 3.7V for lithium-ion, 3.2V for a LiFePO4 cell, 12.8V for a 12V LiFePO4 battery.
• Milliamp-hours (mAh) — charge. How much current, for how long. On its own it describes nothing useful, because the same charge at twice the voltage carries twice the energy.

The 1,000 is only the milli- prefix. Divide watt-hours by volts for amp-hours, then multiply by 1,000: a 50Wh pack at 3.7V is 13.5Ah, or 13,514mAh.

If the battery is already rated in amp-hours, do not force it into milliamp-hours — three extra zeros and no extra information. Use watt-hours to amp-hours instead. The reverse direction is the mAh to Wh calculator.

Conversion table at 3.7V

Every row assumes 3.7V nominal, the single-cell lithium-ion voltage behind phone batteries and every power-bank rating printed on a box. Two rows are not round figures on purpose: 37Wh and 74Wh are what the two commonest bank sizes actually contain.

The last row is where the unit stops working: 54,054mAh is arithmetically correct and useless as a shopping number, because nobody selling a 200Wh unit publishes the pack voltage it came from.

Why this conversion matters for power stations and solar

There is a hard line in this market at about 100Wh. Below it, everything is sold in milliamp-hours: power banks, phone batteries, camera cells. Above it, everything is sold in watt-hours: portable power stations, home batteries, solar generators. Two halves of one market, two units for the same physical thing.

A 25,000mAh power bank holds 92.5Wh. A 12V battery described as 25,000mAh holds 300Wh. Same headline number, more than three times the energy. Any mAh figure quoted without a voltage is an incomplete specification.

Three places the conversion earns its keep:

• Sizing a power station against what you already carry. If a 20,000mAh bank covers a day of phones and a tablet, that day costs 74Wh — so a 500Wh station is not “a bit bigger”, it is nearly seven days of the same load. Our guide to what size power station you need works in watt-hours, and so does the comparison tool.
• Sizing a 12V bank for a van or an RV. Say you need 1,000Wh of AC load a day. Allow 15% for the inverter and about 1,180Wh has to leave the battery, which at 12V is 98,000mAh, or 98Ah. Hold a lithium bank to 80% depth of discharge and you are shopping for a nominal 120Ah; lead-acid, usable to 50%, needs roughly double.
• Matching solar to storage. Panels are rated in watts, and watts × usable sun hours gives watt-hours. Milliamp-hours cannot be divided into watts, so a day’s harvest must be in Wh before it meets a bank. The solar battery calculator does that step; how to size a solar system covers the array.

Same watt-hours, four voltages, four answers

One column of energy, four valid milliamp-hour ratings for it, spread over three and a half to one. This is the argument for watt-hours in a single screenshot.

The multiplier for any voltage

Each figure below is 1,000 ÷ V. Multiply your watt-hours by it and you are finished.

A 60Wh ultrabook battery on an 11.1V pack is 5,405mAh; a 500Wh e-bike battery at 36V is 13,889mAh, or 13.9Ah; a 16Wh camera pack at 7.4V is 2,162mAh. Pick a neighbouring row by mistake and you are out by a third.

One trap: a 12V LiFePO4 battery is nominally 12.8V, so dividing by 12V overstates the milliamp-hours by about 7%.

The 100Wh flight limit, in milliamp-hours

The airline sets its limit in watt-hours; the battery in your bag is labelled in milliamp-hours. That is the most common real reason to run this conversion. Under the rules the FAA and most carriers apply, spare lithium batteries fly in carry-on only:

• Up to 100Wh — carried without approval. At 3.7V, 27,027mAh.
• 100 to 160Wh — permitted with airline approval, normally two spares per passenger. The ceiling is 43,243mAh at 3.7V.
• Above 160Wh — not permitted in passenger baggage at all.

The practical rule is a bank rated 27,000mAh or below. A 20,000mAh bank is 74Wh and has never been a problem; a 30,000mAh bank is 111Wh and needs approval. That is why so many large banks stop at 26,800mAh rather than a round 30,000 — 99.2Wh, parked deliberately under the line. Check the casing for a printed Wh figure first: at a security desk a printed number beats your arithmetic.

What the pack actually delivers

Everything above is what a battery holds. What reaches your phone is less, because stepping 3.7V up to 5V costs energy. At 85–90% conversion efficiency, generous for a cheap bank, a 10,000mAh bank holds 37Wh and delivers about 31–33Wh, which is 6,300–6,700mAh measured at 5V. A 20,000mAh bank holds 74Wh and delivers 63–67Wh, or 12,600–13,300mAh at 5V.

That settles the usual complaint. A 20,000mAh bank does not fill a 5,000mAh phone four times; it manages about three, because the phone battery is also a 3.7V pack and also loses something on the way in. Power stations shed energy the same way, more through an AC inverter than a DC port. Convert first, then discount.

Frequently asked questions