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Electric Charge Converter
Convert between coulombs, ampere-hours and milliampere-hours.
Your phone battery says 4,500 mAh. Your physics textbook measures charge in coulombs. The two units describe the same underlying quantity — the amount of electric charge that has moved — but one was built for engineers reading a datasheet and the other for the international system of measurement, and the conversion factor between them is 3,600.
The units, plainly
- Coulomb (C) — the SI unit of charge. Since 2019 it has been defined exactly, as the charge of 6.241509074×10¹&sup8; elementary particles, rather than derived from other measurements.
- Ampere-hour (Ah) — the charge delivered by a current of one ampere flowing for one hour. Equal to exactly 3,600 coulombs.
- Milliampere-hour (mAh) — one thousandth of an ampere-hour, and the unit almost every consumer battery is rated in because the full ampere-hour number would look tiny and unimpressive.
Why mAh is not the same as battery life
A 4,500 mAh battery does not tell you how many hours your phone will last, because that depends on voltage and how much power the screen, radio and processor are actually drawing. A battery’s real energy content is charge multiplied by voltage, giving watt-hours — the figure airlines actually care about for carry-on limits.
This is also why you cannot directly compare the mAh rating of a phone battery to a laptop battery and conclude the phone lasts longer. Phone batteries run around 3.7 V; laptop batteries often run at 11 V or higher, so the same mAh number represents roughly three times the stored energy in the laptop.
Charge unit conversions
| From | To | Multiply by |
|---|---|---|
| 1 Ah | coulombs | 3,600 |
| 1 mAh | coulombs | 3.6 |
| 1 coulomb | Ah | 0.0002778 |
| 1,000 mAh | Ah | 1 |
| 1 Ah | mAh | 1,000 |
Where you actually run into charge units
- Phone, laptop and power-bank batteries, always rated in mAh on the label.
- Car batteries, usually rated in Ah — a typical starter battery holds 45–70 Ah.
- Electroplating and electrolysis calculations in chemistry, where coulombs relate directly to the amount of metal deposited via Faraday’s laws.
- Airline carry-on battery rules, which are actually written in watt-hours, not mAh, catching out anyone who only checks the mAh figure before a flight.
Electric charge questions
How do I convert mAh to coulombs?
Multiply the mAh figure by 3.6. A 2,000 mAh battery holds 2,000 × 3.6 = 7,200 coulombs of charge. The factor comes from there being 3,600 seconds in an hour and the milli- prefix contributing another 0.001.
Is a higher mAh rating always a better battery?
Only if the voltage is the same. mAh measures charge, not energy, so comparing two batteries at different voltages by mAh alone is misleading. For a fair comparison, convert both to watt-hours by multiplying mAh by voltage and dividing by 1,000.
What is the difference between a coulomb and an amp?
An amp (ampere) is a rate — charge per second. A coulomb is an amount — one amp flowing for one second delivers exactly one coulomb. Confusing the two is like confusing speed with distance.
Why did the definition of the coulomb change in 2019?
The SI system redefined its base units, including the ampere, in terms of fixed fundamental constants rather than physical prototype objects. As a result the coulomb is now defined exactly through the elementary charge, making it more precisely reproducible in any lab worldwide.
Can I convert mAh directly to watt-hours?
Not without the voltage. Multiply mAh by the battery’s voltage and divide by 1,000 to get watt-hours — a 3,000 mAh battery at 3.7 V holds 11.1 Wh. Skipping the voltage step is the most common mistake when comparing battery capacities.
