What Is a Coulomb?
The complete guide to the coulomb, the SI unit of electric charge.
Last updated: 2026-05-21
Definition
A coulomb (symbol: C) is the SI unit of electric charge. It is defined in terms of electric current and time: one coulomb is the quantity of charge transported by a steady current of one ampere flowing for one second.
1 C = 1 A·s (one ampere-second)
Charge is a fundamental property of matter, carried by particles such as electrons and protons. The smallest unit of free charge is the elementary charge (symbol: e), equal to approximately 1.602 × 10−19 C. Because each electron carries this tiny amount, one coulomb represents an enormous number of charge carriers:
1 C ≈ 6.242 × 1018 electrons
Since the 2019 redefinition of SI units, the elementary charge is a fixed, exact constant, and the coulomb is derived from it together with the second.
Charge, Current & Time
The relationship between charge, current, and time is one of the most useful formulas in electronics. Electric current is simply the rate at which charge flows past a point, so charge is current multiplied by the time it flows:
Q = I × t
where Q is charge in coulombs, I is current in amperes, and t is time in seconds.
Worked example. Suppose a circuit draws a steady current of 3 amperes for 4 seconds. The charge that passes through it is:
Q = 3 A × 4 s = 12 coulombs
The formula rearranges easily. To find current, use I = Q ÷ t; to find time, use t = Q ÷ I. For example, moving 60 coulombs in 12 seconds means a current of I = 60 ÷ 12 = 5 amperes.
Everyday Examples
| Situation | Approximate Charge |
|---|---|
| 1 ampere flowing for 1 second | 1 C |
| Charge of a single electron | 1.602 × 10−19 C |
| Static shock from a doorknob | ~0.000001 C (1 µC) |
| Smartphone battery (3,000 mAh) | ~10,800 C |
| Car battery (60 Ah) | ~216,000 C |
| Typical lightning strike | ~15 C |
| AA alkaline battery (~2.5 Ah) | ~9,000 C |
A battery's capacity in milliampere-hours (mAh) converts directly to coulombs. A 3,000 mAh phone battery is 3 Ah, and since one ampere-hour is 3,600 coulombs, that battery stores about 3 × 3,600 = 10,800 C of charge.
Related Units
| Unit | Symbol | Value in Coulombs |
|---|---|---|
| Microcoulomb | µC | 0.000001 C (10−6 C) |
| Millicoulomb | mC | 0.001 C (10−3 C) |
| Coulomb | C | 1 C |
| Ampere-hour | Ah | 3,600 C |
| Milliampere-hour | mAh | 3.6 C |
| Faraday constant | F | ~96,485 C per mole |
The ampere-hour and milliampere-hour are the units you will see on battery labels, while the microcoulomb and millicoulomb appear in electrostatics and capacitor work. The Faraday constant links charge to chemistry: it is the charge of one mole of electrons.
History
The unit is named after Charles-Augustin de Coulomb (1736-1806), a French military engineer and physicist. In the 1780s he used a sensitive torsion balance to measure the force between charged objects, establishing that the force is proportional to the product of the charges and inversely proportional to the square of the distance between them.
This relationship, now called Coulomb's law, is the electrostatic counterpart to Newton's law of gravitation and remains foundational to electromagnetism. In recognition of his work, the unit of electric charge was named the coulomb when the practical electrical units were standardized in the late nineteenth century.
Need to move between coulombs, ampere-hours, and milliampere-hours? Use our charge converter for fast, precise results.