Battery Capacity Calculator — mAh and Ah to Watt-Hours, Pack Sizing and the 100 Wh Air-Travel Limit

Background.

Two numbers get printed on batteries and they measure different things. Milliampere-hours and ampere-hours measure charge — how many electrons the cell can push out. Watt-hours and kilowatt-hours measure energy — how much work those electrons can do. Converting between them needs the nominal voltage, and getting that voltage wrong is the single most common mistake people make when working out whether their power bank is allowed on a plane.

The relation is not folklore; it is written into US law. Title 49 of the Code of Federal Regulations, section 171.8, defines the watt-hour rating of a lithium ion cell or battery as the rated capacity in ampere-hours multiplied by its nominal voltage. That is why a 20 000 mAh power bank is a 74 Wh battery: 20 000 mAh is 20 Ah, and 20 × 3.7 = 74. It is not a 100 Wh battery, and it is emphatically not the 100 Wh you would get by multiplying by the 5 V that comes out of the USB socket. The converter inside changes the voltage; the rule is applied to the cell's nominal voltage, which is the lower figure.

That 74 Wh matters because of a second regulation. Section 175.10(a)(18) sets the thresholds for batteries carried by passengers: at or below 100 Wh no special approval is needed, between 100 Wh and 160 Wh you need the operating carrier's approval and may carry at most two spares, and above 160 Wh the passenger exception does not apply at all. Spare batteries must travel in carry-on baggage in every case, never checked, each one individually protected against a short circuit. This calculator puts your pack into one of those bands and tells you which words in the regulation put it there.

Beyond air travel, the same arithmetic is what you need to size anything. A 12 V 100 Ah leisure battery is a 1200 Wh store; a 12.8 V 100 Ah LiFePO4 block is 1280 Wh, which is why the two are not interchangeable on a watt-hour basis despite carrying the same ampere-hour number. Once you know the watt-hours you can divide by a load's wattage to get a runtime, compare a lithium pack against a lead-acid one honestly, or check whether a solar array can refill it in a day.

The calculator also builds packs. Cells in series add voltage while the ampere-hour figure stays put; strings in parallel add ampere-hours while the voltage stays put; the energy scales with both. Six 3.7 V, 3000 mAh cells in series with two such strings in parallel is a 22.2 V, 6 Ah, 133.2 Wh pack — and 22.2 × 6 = 133.2, which is the identity the whole page rests on and which the calculator's own tests assert.

The depth-of-discharge field deserves a word. It defaults to 100 percent, which gives the theoretical figure from the full rated capacity. That is a default, not a recommendation: how deeply a battery should actually be discharged is a property of its chemistry and its manufacturer's specification, and this page will not put someone else's number in front of you as though it were universal. Take the figure from your own datasheet.

One honest limitation goes with every result. The watt-hour rating is the nominal-voltage product, by regulatory definition, and that is what appears on datasheets, customs forms and airline rules. The energy a battery actually delivers is a little different, because the terminal voltage sags as the cell empties and because both discharge rate and temperature move the total. The rating is the right number for the rule; it is an upper estimate for the runtime.

Finally: this page computes a figure. It does not clear a battery for flight. The thresholds here come from US Department of Transportation regulation, other civil aviation authorities mirror them, and the operating carrier's own rules govern and can be stricter. Check with the airline.

What is battery capacity calculator?

Battery capacity is the amount of charge or energy a battery can store, and it is quoted two different ways. Charge capacity, in ampere-hours or milliampere-hours, is the product of a current and a time: a 20 Ah cell can in principle supply 1 A for 20 hours, or 20 A for one hour. Energy capacity, in watt-hours or kilowatt-hours, is that charge multiplied by the voltage it is delivered at, and it is the figure that lets you compare batteries of different voltages against each other and against a load's wattage.

The bridge between the two is the nominal voltage — the voltage the manufacturer designates as the cell's representative working value, roughly the middle of its discharge curve. A lithium-ion cell is nominally 3.7 V while actually spanning about 4.2 V charged to 3.0 V empty; LiFePO4 is nominally 3.2 V; a lead-acid block is nominally 12 V. The nominal figure is the one every rating and every regulation uses.

A battery pack is built from cells in two directions at once. Series connections put cells end to end so their voltages add: four 3.7 V cells in series make a 14.8 V pack of the same ampere-hour capacity as one cell. Parallel connections put cells side by side so their capacities add: four 3000 mAh strings in parallel make 12 000 mAh at unchanged voltage. Packs are described as "6S2P" and similar, meaning six in series and two such strings in parallel, and the energy is the cell's energy multiplied by both numbers.

Depth of discharge is the fraction of the rated capacity actually taken out before recharging. It is a separate concept from capacity itself: a 100 Ah battery used to 50 percent depth of discharge delivers 50 Ah per cycle. How deep is acceptable varies enormously by chemistry and by manufacturer, and the trade-off is against cycle life.

How to use this calculator.

1. 01Enter the capacity printed on a single cell, module or pack, and choose whether that figure is in mAh, Ah, Wh or kWh.
2. 02Enter the nominal voltage of that cell — 3.7 V for typical lithium-ion, 3.2 V for LiFePO4, 1.2 V for NiMH, 12 V for a lead-acid block. Do not use the fully charged voltage or a USB converter's output voltage.
3. 03If you are describing a multi-cell pack, enter how many cells are in series and how many strings in parallel. Leave both at 1 for a single cell or a pack whose figures you already have.
4. 04Set the depth of discharge to whatever your battery's datasheet supports, or leave it at 100 % for the theoretical maximum.
5. 05Choose the chemistry. The watt-hour air-travel bands apply to lithium ion only; anything else suppresses the verdict and explains why.
6. 06Read the watt-hour figure as the headline — that is the number datasheets, customs forms and airline rules mean by capacity.
7. 07Check the air-travel classification beside it, then confirm with your operating carrier, whose own rules govern and may be stricter.

The formula.

The defining relation is statutory rather than empirical, which is unusual for a calculator and worth being explicit about. Title 49 of the US Code of Federal Regulations, section 171.8, states: "Watt-hour (Wh) means a unit of energy equivalent to one watt (1 W) of work acting for one hour (1 h) of time. The Watt-hour rating of a lithium ion cell or battery is determined by multiplying the rated capacity of a cell or battery in ampere-hours, by its nominal voltage." So Wh = Ah × V, with the nominal voltage, by definition. Milliampere-hours divide by 1000 to give ampere-hours; watt-hours divide by 1000 to give kilowatt-hours. Running the definition backwards gives Ah = Wh ÷ V, which is how the watt-hour and kilowatt-hour input modes work.

Pack arithmetic follows from how cells are wired. Series connections stack voltages, so the pack voltage is the cell voltage times the number in series while the ampere-hour figure is unchanged — the same charge passes through every cell in the chain. Parallel strings share the current, so the ampere-hours add while the voltage is unchanged. Energy, being the product of the two, scales with both: Wh_pack = Wh_cell × S × P. That is algebraically the same as V_pack × Ah_pack, and the calculator's test suite asserts the two expressions agree to within a part in a billion for a 6S2P pack, which is a real check on the code rather than a restatement of the algebra.

Usable capacity is the rated figure times the depth of discharge as a fraction. The page defaults to 100 percent deliberately: no recommended depth of discharge is baked in, because the right value depends on chemistry, on the manufacturer, and on how many cycles you want. Shipping one number as though it were universal would be worse than leaving the field to the reader.

The air-travel classification is applied to the pack's total watt-hours, unrounded, before any display rounding. Section 175.10(a)(18) of the same title governs batteries carried by passengers, and its wording sets the edges. For lithium ion, "the Watt-hour rating must not exceed 100 Wh" defines the band that needs no approval, so exactly 100.0 Wh is inside it. Batteries "exceeding 100 Wh, but not exceeding 160 Wh" are permitted with the operating carrier's approval, and "no more than two individually protected lithium ion batteries" in that range may be carried per person as spares in carry-on baggage — so exactly 160.0 Wh is inside that band too. Above 160 Wh the passenger exception does not apply. Both edges are inclusive-below, and the calculator's tests check 99.99, 100, 100.01, 159.99, 160 and 160.01 Wh, plus 100.000000001 Wh to prove the classification uses the unrounded value.

Lithium metal batteries are deliberately not classified. Section 171.8 measures them by lithium content in grams, and section 175.10(a)(18) sets those limits at 2 grams, or up to 8 grams for portable medical devices with the operator's approval. Grams of lithium cannot be derived from a watt-hour figure, so selecting a non-lithium-ion chemistry switches the output text to say exactly that instead of producing a verdict that would be confidently wrong.

One nuance is recorded rather than smoothed over. The regulation defines the watt-hour rating as exactly the nominal-voltage product; there is nothing approximate about it for regulatory purposes. The PVCDROM teaching reference from Arizona State University, consulted independently, notes that the physical energy a battery actually delivers is only approximated by that product, because terminal voltage falls through the discharge and the total depends on rate and temperature. Both statements are true about different things. This page computes the regulatory definition, because that is what the 100 Wh threshold, the datasheet and the customs form all mean — and says plainly that real delivered energy will be lower at high discharge rates or low temperatures.

A worked example.

Frequently asked questions.

References& sources.

1. [1]US Government Publishing Office / US Department of Transportation, Code of Federal Regulations, 2024 edition, Title 49 § 171.8 (definitions). Primary and statutory source for the defining relation. Retrieved text: "Watt-hour (Wh) means a unit of energy equivalent to one watt (1 W) of work acting for one hour (1 h) of time. The Watt-hour rating of a lithium ion cell or battery is determined by multiplying the rated capacity of a cell or battery in ampere-hours, by its nominal voltage." The same section defines a "Lithium metal cell or battery" and notes that its lithium content "is measured when the cell or battery is in an undischarged state" — the reason lithium metal is not classified by watt-hours on this page. Access: open. Retrieved 2026-07-29.
2. [2]US Government Publishing Office / US Department of Transportation, Code of Federal Regulations, 2024 edition, Title 49 § 175.10(a)(18) — the passenger exception for portable electronic devices and spare batteries. Primary and statutory source for the air-travel bands: for lithium ion "the Watt-hour rating must not exceed 100 Wh"; with operator approval, batteries "exceeding 100 Wh, but not exceeding 160 Wh" are permitted and "no more than two individually protected lithium ion batteries each exceeding 100 Wh, but not exceeding 160 Wh, may be carried per person as spare batteries in carry-on baggage"; lithium metal limits are 2 grams, or "exceeding 2 grams, but not exceeding 8 grams" for portable medical devices with operator approval; spare batteries must be carried in carry-on baggage. Access: open. Retrieved 2026-07-29.
3. [3]OpenStax (Rice University), University Physics Volume 2, §9.5 "Electrical Energy and Power". Secondary check on the watt-hour as an energy unit: "E = Pt" for power delivered at a constant rate, "The energy unit on electric bills is the kilowatt-hour (kW·h), consistent with the relationship E = Pt", and "1 kW·h = 3.6 × 10⁶ J". The section does not discuss batteries; it is cited only for the unit. Access: open. Retrieved 2026-07-29.
4. [4]US Federal Aviation Administration, PackSafe programme, "Lithium Batteries" passenger guidance, which restates the 100 Wh and 160 Wh thresholds for travellers. ACCESS: GATED — faa.gov returned HTTP 403 to the retrieval used for this page, as did the equivalent TSA page. Nothing is quoted from it and no threshold on this page rests on it: every figure comes from the CFR text cited above, which was retrieved in full. Listed so readers can find the plain-language version, and declared as unopened rather than presented as verified. Retrieval attempted 2026-07-29.
5. [5]NIST, Special Publication 811, "Guide for the Use of the International System of Units (SI)", 2008 edition (DOI 10.6028/NIST.SP.811e2008). Used only for unit symbols and typography — W, V, A, Wh, kWh — not for any battery relation. NIST's own landing page notes that SP 811 "has not yet been updated to reflect the changes in the SI that came into effect on May 20, 2019"; none of those changes affect the symbols used here. Access: open. Retrieved 2026-07-29.