Last updated: 2026-06-30

Battery Capacity (mAh ⇄ Wh)

Battery capacity is commonly expressed in milliampere-hours (mAh), but watt-hours (Wh) measure the actual stored energy. Converting between the two requires knowing the nominal cell voltage. This calculator handles both directions: enter a capacity and voltage to get energy in watt-hours, or enter watt-hours and voltage to recover the milliampere-hour rating.

The conversion formula

The relationship between charge capacity and energy follows directly from the definition of electrical power:

where $Q$ is the capacity in milliampere-hours and $V$ is the nominal cell voltage in volts. Rearranging gives the inverse:

The factor of 1000 converts milliampere-hours to ampere-hours (1 Ah = 1000 mAh), and 1 Ah × 1 V = 1 Wh.

Why mAh and Wh describe different things

Milliampere-hours measure charge — how many milliamperes of current a battery can deliver for one hour. Watt-hours measure energy — how much work the battery can do. A higher-voltage battery stores more energy per unit of charge, which is why mAh alone does not uniquely determine energy content.

Consider two power banks both rated at 10,000 mAh:

The second pack stores twice the energy despite the same mAh rating. This is why airlines and safety bodies regulate power banks in watt-hours: fire risk scales with energy, not charge.

Choosing the right nominal voltage

The nominal voltage to use depends on the cell chemistry and configuration:

• Lithium-ion (Li-ion): 3.6–3.7 V per cell. The most common chemistry in smartphones and portable electronics.
• Lithium polymer (LiPo): 3.8 V per cell. Common in thin consumer devices.
• Alkaline AA / AAA: 1.5 V nominal (1.2 V for NiMH rechargeable).
• USB output: Do not use 5 V for this calculation. Power banks boost the internal 3.7 V cell to 5 V at the USB port, and conversion losses mean that the 5 V figure overstates the usable cell energy.

The value to use is the cell nominal voltage, which is printed on the internal battery label or listed in the product specification sheet.

Worked example

A power bank lists its capacity as 20,000 mAh. The internal cell voltage is 3.7 V. Is it within the IATA 100 Wh carry-on limit?

At 74 Wh, this power bank is well within the 100 Wh IATA limit and can be carried on without advance airline approval.

Airline carry-on limits

The International Air Transport Association (IATA) Dangerous Goods Regulations set the following limits for lithium battery power banks carried in cabin baggage:

• Up to 100 Wh: permitted without approval, limit of two spare batteries per person.
• 100–160 Wh: permitted with airline approval, limit of two spare batteries.
• Above 160 Wh: not permitted in cabin or checked baggage for personal use.

Individual airlines may apply stricter limits, so confirm with your carrier before travel.

Limitations of this calculator

This calculator assumes 100% energy conversion efficiency and uses a single nominal voltage for the entire discharge. In practice, actual usable energy is lower due to:

• Internal resistance losses during discharge.
• Voltage sag at the end of the discharge cycle.
• Temperature effects on cell chemistry.
• Boost converter losses when stepping up from cell voltage (3.7 V) to USB output voltage (5 V).

Manufacturers typically rate power bank output capacity at the USB port, which accounts for some of these losses. Internal cell capacity (in mAh at cell voltage) and rated output capacity (in mAh at 5 V output) can therefore differ by 15–25%.