Battery Capacity Calculator: mAh, Ah, Wh and Runtime

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How to use this tool

Set the chemistry first. The voltage that comes with it is the single most important input, because it is the thing that turns a charge figure into an energy figure. Then type the capacity from the label and read the watt hours, or work backwards if you have the energy and need the milliamp hours.

The input people get wrong is voltage on a multi cell pack. A label reading 5000 mAh on a laptop battery is almost never a 3.7 volt figure. If the pack is four cells in series the voltage is 14.8 and the energy is four times what you would get assuming a single cell. Use the series and parallel fields if you know the arrangement, because the difference between assuming one cell and getting it right is a factor of four on the answer.

What the calculation is doing

Watt hours equal milliamp hours multiplied by volts, divided by a thousand to clear the milli. That is the whole formula, and the division by a thousand is bookkeeping rather than physics.

The reason the voltage has to be in there is that milliamp hours are not a measure of energy at all. They measure charge, the number of electrons the pack can push through a circuit. Energy is charge multiplied by the potential those electrons fall through. A coulomb of charge moving through one volt releases one joule. Move the same coulomb through four hundred volts and you get four hundred joules from identical charge.

This is why comparing two batteries on milliamp hours alone is close to meaningless unless they share a voltage. A 5000 mAh phone battery at 3.7 volts holds 18.5 watt hours. A 5000 mAh cordless drill pack at 18 volts holds 90 watt hours, nearly five times the energy, from a label that looks identical. The mAh number tells you almost nothing on its own and the industry keeps printing it because it is the bigger number.

Why the number on a power bank is not the number you get

A power bank rated at 10,000 mAh is rated at the voltage of the cells inside it, which for lithium ion is nominally 3.7 volts. Your phone charges over USB at 5 volts. Getting from one to the other means a boost converter, and the conversion is neither free nor lossless.

Work it through. Ten thousand milliamp hours at 3.7 volts is 37 watt hours. A decent converter runs somewhere between 85 and 90 percent efficient, so call it 33 watt hours reaching the cable. Divide that by the 5 volt output and you get about 6,600 milliamp hours delivered, from a pack sold as 10,000. At 85 percent it drops to about 6,300.

Nothing has gone wrong and nobody has cheated you. The nameplate is honest about the cells and silent about the conversion. But it does mean a 10,000 mAh bank will not give a 5,000 mAh phone two full charges, and the reviews complaining about that are measuring the physics rather than a defect.

A worked check you can repeat on paper

Take the default. Ten thousand milliamp hours at 3.7 volts. Multiply to get 37,000 milliwatt hours, divide by a thousand, and you have 37 watt hours. That sits comfortably under the airline threshold, which is why almost every power bank on sale is 10,000 or 20,000 rather than 30,000.

Find the threshold itself. The IATA carry on limit without approval is 100 watt hours. Rearranged, milliamp hours equal watt hours times a thousand divided by volts, so 100 times 1000 divided by 3.7 gives 27,027 mAh. Anything above that at lithium ion voltage needs airline approval, and the 160 watt hour ceiling lands at 43,243 mAh. Manufacturers know this, which is why you see 26,800 mAh packs marketed hard at travellers. They are sitting deliberately just under the line.

Now runtime. Switch to runtime mode with the same 37 watt hour pack driving a 5 watt load at 90 percent efficiency. Usable energy is 37 times 0.9, which is 33.3 watt hours. Divide by 5 watts and you get 6.66 hours, so 6 hours 40 minutes. Double the load to 10 watts and it halves to 3 hours 20 minutes, because runtime and load are inversely proportional and the curve on the chart is a hyperbola rather than a straight line.

One more, on a bigger pack. A 100 Ah lead acid block at 12 volts is 100,000 mAh, which gives 1,200 watt hours. Run a 100 watt load at 85 percent and you get 10.2 hours. Worth noting that a lead acid battery should not be taken below about half charge if you want it to last, so the practical figure is nearer five hours. The tool gives you the physics and the depth of discharge decision stays yours.

Mistakes that catch people out

Comparing mAh across different voltages. The most common error by a wide margin. A drill pack and a phone battery with the same milliamp hour rating differ by a factor of five in stored energy. Convert both to watt hours before comparing anything.

Assuming cells in parallel raise the voltage. Parallel adds capacity at the same voltage, series adds voltage at the same capacity. A 4S2P pack of 3000 mAh cells is 14.8 volts and 6000 mAh, not 59.2 volts and not 24,000 mAh. The energy comes out the same either way, but the pack voltage decides what it can actually drive.

Using nominal voltage for a fully charged pack. A lithium ion cell is nominally 3.7 volts but sits at 4.2 fully charged and around 3.0 when empty. Nominal is a fair average across the discharge curve and it is the right figure for capacity maths, but if you are sizing a converter you need the 4.2 and the 3.0, not the 3.7.

Forgetting the airline rule is in watt hours, not milliamp hours. Security staff read the watt hour figure printed on the pack. If your battery only shows mAh and volts they will do this calculation themselves, and a pack that shows neither is the one most likely to be refused.

Treating runtime as exact. Real capacity falls with age, with cold, and with high discharge rates. A pack pulling its rated capacity at a gentle current will deliver noticeably less at a hard one, which is the Peukert effect and it hits lead acid far harder than lithium. Treat the runtime figure as a ceiling.

How this was checked

The engine was written twice and the two versions compared. Round trips through eight voltages from 1.2 to 400 volts, at capacities from 50 to 100,000 mAh, all return to the starting value within one part in a billion. The airline thresholds were derived rather than hardcoded, and 27,027 mAh at 3.7 volts comes back as exactly 100.00 watt hours.

The power bank figures were checked against the arithmetic in both directions: 10,000 mAh at 3.7 volts delivering through a 5 volt output gives 6,660 mAh at 90 percent efficiency and 6,290 at 85 percent, which brackets what independent testing of real power banks tends to report. The chart is drawn from the same functions as the readout, so the curve cannot drift from the numbers.

Popular conversions

Each link opens the tool with that figure loaded. The milliamp hour links assume lithium ion at 3.7 volts unless the voltage is named, and you can change it once the page opens.

mAh to Wh at lithium ion 3.7 V. 500 mAh to Wh · 1,000 mAh to Wh · 1,500 mAh to Wh · 2,000 mAh to Wh · 2,500 mAh to Wh · 3,000 mAh to Wh · 3,500 mAh to Wh · 4,000 mAh to Wh · 4,500 mAh to Wh · 5,000 mAh to Wh · 6,000 mAh to Wh · 6,400 mAh to Wh · 7,000 mAh to Wh · 8,000 mAh to Wh · 9,000 mAh to Wh · 10,000 mAh to Wh · 12,000 mAh to Wh · 13,000 mAh to Wh · 15,000 mAh to Wh · 16,000 mAh to Wh · 18,000 mAh to Wh · 20,000 mAh to Wh · 22,000 mAh to Wh · 24,000 mAh to Wh · 25,000 mAh to Wh · 26,800 mAh to Wh · 27,000 mAh to Wh · 30,000 mAh to Wh · 32,000 mAh to Wh · 40,000 mAh to Wh · 50,000 mAh to Wh · 60,000 mAh to Wh · 80,000 mAh to Wh · 100,000 mAh to Wh

Wh to mAh at lithium ion 3.7 V. 1 Wh to mAh · 2 Wh to mAh · 5 Wh to mAh · 10 Wh to mAh · 15 Wh to mAh · 18.5 Wh to mAh · 20 Wh to mAh · 25 Wh to mAh · 30 Wh to mAh · 37 Wh to mAh · 40 Wh to mAh · 45 Wh to mAh · 50 Wh to mAh · 60 Wh to mAh · 70 Wh to mAh · 74 Wh to mAh · 80 Wh to mAh · 90 Wh to mAh · 99 Wh to mAh · 100 Wh to mAh · 110 Wh to mAh · 120 Wh to mAh · 140 Wh to mAh · 160 Wh to mAh · 200 Wh to mAh · 250 Wh to mAh · 300 Wh to mAh · 500 Wh to mAh · 750 Wh to mAh · 1,000 Wh to mAh · 1,200 Wh to mAh · 2,000 Wh to mAh

mAh to Wh at 12 V. 1,000 mAh to Wh · 2,000 mAh to Wh · 3,000 mAh to Wh · 4,000 mAh to Wh · 5,000 mAh to Wh · 6,000 mAh to Wh · 8,000 mAh to Wh · 10,000 mAh to Wh · 12,000 mAh to Wh · 15,000 mAh to Wh · 20,000 mAh to Wh

mAh to Wh at laptop 14.8 V. 2,200 mAh to Wh · 2,600 mAh to Wh · 3,000 mAh to Wh · 4,000 mAh to Wh · 4,400 mAh to Wh · 5,000 mAh to Wh · 5,200 mAh to Wh · 6,000 mAh to Wh · 6,600 mAh to Wh · 8,000 mAh to Wh

mAh to Wh at USB 5 V. 1,000 mAh to Wh · 2,000 mAh to Wh · 3,000 mAh to Wh · 5,000 mAh to Wh · 10,000 mAh to Wh · 15,000 mAh to Wh · 20,000 mAh to Wh · 26,800 mAh to Wh

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• IATA lithium battery guidance is the source of the 100 Wh and 160 Wh thresholds this tool checks against. Individual airlines can be stricter, so confirm with your carrier.