How do I compare two batteries fairly?

Convert both to watt-hours. Milliamp-hour ratings at different voltages are not comparable.

What is the airline limit in mAh?

There is no mAh limit — regulations use watt-hours. The 100 Wh threshold corresponds to about 27,000 mAh at 3.7 V.

Does a higher mAh always mean longer runtime?

Only at the same voltage and with the same device efficiency. Otherwise watt-hours is the meaningful figure.

Wh to mAh Converter | Watt-hours to Milliamp-hours

Conversion Formula

mAh = (Wh × 1,000) ÷ 3.7 V

Divide watt-hours by the nominal voltage of 3.7 V and multiply by 1,000 to express energy as a milliamp-hour charge rating.

This formula uses the nominal cell voltage currently set in the calculator (3.7 V). Change it above and this formula updates with it — the relationship is not a fixed unit pair.

Complete Guide to Watt-hours to Milliamp-hours Conversion

What is a Watt-hours?

A watt-hour (Wh) measures energy. It already includes voltage, which makes it the only fair way to compare batteries of different chemistries and cell configurations — and the basis of aviation safety limits.

What is a Milliamp-hours?

A milliamp-hour (mAh) measures electric charge: one milliampere flowing for one hour. Phone, tablet and power bank marketing uses it because the numbers look impressive, but it says nothing about energy on its own.

How the Conversion Works

Energy equals charge multiplied by voltage. Milliamp-hours become watt-hours by multiplying by the nominal cell voltage and dividing by 1,000 to convert milli-units. This calculator uses 3.7 V, the nominal voltage of a standard lithium-ion cell; substitute 3.2 V for LiFePO4 or 1.2 V for NiMH.

Why This Conversion Matters

Airlines, shipping regulations and honest product comparisons all run on watt-hours. A power bank advertised as 20,000 mAh and a laptop battery advertised as 60 Wh sound unrelated, but converted they are 74 Wh and 60 Wh — nearly the same battery.

Scientific Context

Nominal voltage is an average across the discharge curve. A lithium-ion cell starts near 4.2 V and ends near 3.0 V, and 3.7 V represents the working midpoint. Using the peak voltage instead overstates capacity by roughly 13%.

Step-by-Step Conversion Guide

Follow these steps to convert watt-hours to milliamp-hours manually:

Start with your value in watt-hours

Example: You want to convert 18.5 Wh

Apply the conversion formula

Use the formula: mAh = (Wh × 1,000) ÷ 3.7 V

Calculate the result

18.5 Wh = 5,000 mAh

✓

Verify your answer

Use our calculator above to double-check your manual calculation.

Quick Mental Math Tip

Divide watt-hours by the nominal voltage of 3.7 V and multiply by 1,000 to express energy as a milliamp-hour charge rating.

Common Conversions

Calculated at 3.7 V (nominal cell voltage). Change the value in the calculator above and this table recalculates.

Watt-hours to Milliamp-hours conversion table at 3.7 V
Watt-hours (Wh)	Milliamp-hours (mAh)	Milliamp-hours → Watt-hours
5	1351.3514	5 mAh = 0.0185 Wh
10	2702.7027	10 mAh = 0.037 Wh
18.5	5000	18.5 mAh = 0.0684 Wh
37	10000	37 mAh = 0.1369 Wh
50	13513.5135	50 mAh = 0.185 Wh
74	20000	74 mAh = 0.2738 Wh
100	27027.027	100 mAh = 0.37 Wh
150	40540.5405	150 mAh = 0.555 Wh

When You'll Need This Conversion

Airline battery compliance

Carry-on lithium batteries are limited to 100 Wh without airline approval, and 160 Wh with it.

Comparing power banks honestly

Convert competing mAh claims into watt-hours before deciding.

Device runtime planning

Divide watt-hours by device power draw to estimate hours of use.

Tool battery packs

18 V tool packs quoted in Ah convert to watt-hours the same way.

Pro Tips for Accurate Conversions

Always ask "at what voltage?"

An mAh figure without a voltage is marketing, not a specification.

100 Wh ≈ 27,000 mAh at 3.7 V

The number to remember before flying.

Expect 60–70% real output from a power bank

Voltage boosting and conversion losses eat the rest.

Common Mistakes to Avoid

Comparing mAh across voltages

A 3.7 V and a 12 V pack with the same mAh differ by more than three times in energy.

Convert both to watt-hours first.

Using 5 V instead of cell voltage

Power banks are rated at cell voltage, not USB output voltage.

Use 3.7 V for the rating, then apply a 0.6–0.7 efficiency factor for delivered 5 V charge.

Assuming a bigger battery charges a phone more times

Conversion losses and phone battery size both matter.

Usable Wh ÷ phone battery Wh × 0.85 gives a realistic charge count.

History & Background

Milliamp-hour marketing arrived with mobile phones in the 1990s. Watt-hour labelling became mandatory when aviation authorities needed a consistent way to regulate lithium battery energy in the cabin.