mAh to Wh Calculator

The formula: watt hours are mAh times volts, divided by 1,000

A milliamp hour rating says how much electric charge a battery holds. A watt hour says how much energy it holds, and energy depends on both the charge and the voltage that pushes it. The conversion is one line: Wh = mAh × V ÷ 1,000. A 3,000 mAh phone battery with a 3.7 V cell works out to 3,000 × 3.7 ÷ 1,000 = 11.1 Wh.

That voltage term is the part people drop, and dropping it wrecks the answer. Think of charge as water in a tank and voltage as the pressure behind it. Ten gallons at garden hose pressure and ten gallons at fire hydrant pressure do different amounts of work. The same 3,000 mAh at 3.7 V is 11.1 Wh; at 5 V it would be 15 Wh; at 12 V it would be 36 Wh. The charge count alone tells you none of that.

Watt hours are also the number the outside world asks for. Airlines set their battery limits in Wh, inverter makers size their gear in Wh, and most lithium battery labels print a Wh rating somewhere small. The calculator above runs this conversion in both directions and adds an airline status check and a device runtime estimate, which the rest of this page walks through one step at a time.

Common voltages and where each one applies

The voltage you plug into the formula is the battery's nominal cell or pack voltage, not whatever comes out of the cable. A single lithium ion cell sits at a nominal 3.7 V (some newer phone cells use 3.8 or 3.85 V), rising to about 4.2 V when full and falling near 3.0 V when empty. Packs connect cells in series to add voltage: three cells make the 11.1 V packs inside many laptops, and four cells make the 14.8 V packs that drone pilots fly.

Lead acid works differently. A 12 V car battery is six 2 V cells in series, and its capacity is printed in amp hours instead of milliamp hours. Move the decimal point three places and the same formula applies: 60 Ah becomes 60,000 mAh, and 60 × 12 ÷ 1,000 gives 720 Wh.

USB output adds a twist worth flagging early. A power bank's cells run at 3.7 V inside, but the port delivers 5 V, and some listings print capacity at that output voltage. The energy in the cells never changed; only the accounting did.

The airline 100 Wh line, straight from the FAA and TSA

The FAA's PackSafe rules (current as of 2026) draw the line in watt hours, which is exactly what this page computes. Batteries and power banks up to 100 Wh per battery fly in carry-on bags with no approval. Larger lithium ion batteries from 101 to 160 Wh need airline approval, and each passenger can carry at most two spares at that size. Anything over 160 Wh is forbidden on passenger aircraft, which is where most e-bike packs and portable power stations fail the test.

Placement matters as much as size. Spare lithium ion batteries, including power banks and phone charging cases, must ride in carry-on baggage only, and the TSA enforces the same rule at screening. When a carry-on bag gets checked at the gate or at planeside, all spares and power banks must come out and stay with you in the cabin. Terminals should be taped, bagged or sleeved so they cannot short against coins or keys. Individual airlines are allowed to set stricter limits than the FAA minimum, so confirm with the carrier before travel day.

Run your own pack against the limit before you pack: a 20,000 mAh bank at 3.7 V is 74 Wh and passes without a word to anyone. A 30,000 mAh bank at 3.7 V is 111 Wh, so it needs airline approval and counts toward the two-spare cap. The FAA publishes the passenger rules on its PackSafe lithium battery page and its airline passenger battery guide, and the TSA screening rules cover what happens at the checkpoint.

Why the same 20,000 mAh is not the same energy

Capacity marketing lives at the cell level, and that is where comparisons fall apart. A 20,000 mAh power bank holds its charge in 3.7 V cells, so the stored energy is 20,000 × 3.7 ÷ 1,000 = 74 Wh. If a listing quotes that same 20,000 mAh at 5 V, it is describing the USB output after the boost converter runs, and only on paper: 100 Wh at the port cannot come out of 74 Wh of cells, because the conversion loses 10 to 20 percent along the way.

Voltage changes what a milliamp hour means, so Wh is the only fair way to compare packs built differently. A 10,000 mAh pack built from two cells in series at 7.4 V carries the same 74 Wh as the 20,000 mAh single cell pack. Two banks that look like half and double on the store shelf can hold identical energy.

When a datasheet gives you a Wh figure and an mAh figure, trust the Wh figure. It already contains the voltage, so it cannot be quoted two ways.

How long the battery runs a device

Energy turns into hours with one division: hours = usable Wh ÷ device watts. The word usable carries the weight. Converting a 3.7 V bank up to 5 V or 12 V costs energy in the converter, and cables and charging circuits eat more after that. A realistic plan uses 80 to 90 percent conversion efficiency, and the calculator above defaults to 85 percent.

Take the 74 Wh bank from the last section. At 85 percent, 74 × 0.85 = 62.9 Wh reach the device. A phone charging at about 10 W draws that down in 62.9 ÷ 10 = 6.3 hours of continuous output, which works out to roughly three or four full phone charges once the phone's own charging losses take their cut. A 45 W laptop pulls the same bank dry in about 1.4 hours.

Devices that cycle on and off, like a 12 V fridge on a thermostat, draw their rated watts only part of the time, so use the average draw rather than the rated draw before trusting the hours. And when a datasheet lists current in milliamps instead of watts, the mah to hours calculator on this site takes that route, since mAh divided by mA needs no voltage at all.

Working backward: watt hours to mAh

Flip the formula to size a battery against a Wh limit: mAh = Wh × 1,000 ÷ V. The FAA line makes this concrete. At 3.7 V, 100 Wh works out to 100 × 1,000 ÷ 3.7 = 27,027 mAh, which is why 27,000 mAh sits near the practical ceiling for a power bank that flies without approval. At a 3.6 V lithium iron phosphate cell the same 100 Wh is 27,778 mAh, and at a 14.8 V drone pack it is only 6,757 mAh, because each volt carries more energy per milliamp hour.

Large batteries print amp hours, so shift the decimal: 720 Wh at 12 V is 60 Ah. The same arithmetic sizes a house battery for a fridge or a bank for an inverter, and it doubles as a sanity check on listings that quote capacity without a voltage. The wh to mah calculator on this site runs this direction in more detail if a Wh target is where your problem starts.

How this page differs from the other two battery tools

This site has three battery calculators that overlap on purpose. This one answers a single question: how much energy does this pack hold. Milliamp hours and a voltage go in, watt hours come out, with the airline status and a device runtime attached. The wh to mah calculator runs the reverse direction, turning a Wh figure or a Wh limit back into capacity at a given voltage. The mah to hours calculator skips voltage entirely, because mAh divided by a draw in mA gives hours directly; it is the right tool when a datasheet lists current instead of power.

A quick routing guide helps. Checking a power bank for a flight starts here. Sizing a replacement pack from an energy budget starts at the reverse tool. Estimating runtime for a device rated in milliamps, like many small electronics and radios, starts at the hours tool. All three agree on the underlying physics; they start from different fields on the datasheet.

Checks before you trust the number

• Nominal, not peak: use 3.7 V for a lithium cell, not the 4.2 V full charge voltage. Calculating with 4.2 V inflates the Wh figure by about 13 percent.
• Series and parallel: cells in parallel add mAh at the same voltage; cells in series add voltage at the same mAh. Wh comes out identical either way, which is one more reason it is the honest comparison across pack layouts.
• Label first: when a battery prints both Wh and mAh, the Wh figure is the safer number, because whoever printed it already picked the voltage.
• Marketing mAh: treat a 5 V capacity claim on a 3.7 V bank as an output claim, not stored energy, and expect less after conversion losses.
• Flying: the FAA lets airlines set stricter battery rules than the federal minimum, and gate checked bags must give up every spare, so a quick check with the carrier beats a surprise at the gate.

Do the math once, write the Wh figure on the case with a marker, and both the airline question and the runtime question get easier every time after that.