What Is an Ampere (Amp)?
The complete guide to the ampere, the SI base unit of electric current.
Last updated: 2026-05-21
Definition
An ampere (symbol: A), often shortened to amp, is the SI base unit of electric current. It measures the rate at which electric charge flows through a conductor. One ampere is defined as one coulomb of charge passing a point each second:
1 A = 1 C/s
Because one coulomb represents about 6.241 × 1018 elementary charges, a current of one ampere means roughly that many electrons drift past a cross-section of wire every second. The amp is one of the seven SI base units, alongside the meter, kilogram, second, kelvin, mole, and candela.
Until 2019, the ampere was defined by the magnetic force between two parallel current-carrying wires. The current SI redefinition instead fixes the elementary charge e at exactly 1.602176634 × 10−19 coulombs. This ties the ampere to a fundamental constant of nature rather than a physical experiment, making it reproducible anywhere with the right equipment.
Current, Voltage & Resistance
Current never acts alone. It is governed by Ohm's Law, which links current (I, in amperes), voltage (V, in volts), and resistance (R, in ohms):
I = V ÷ R
Voltage is the electrical pressure pushing charge through a circuit, and resistance opposes that flow. A useful analogy: voltage is water pressure, resistance is the narrowness of the pipe, and current is the resulting flow rate.
Worked example: A 12-volt car battery is connected across a headlight with a resistance of 4 ohms. The current is I = 12 V ÷ 4 Ω = 3 A. If the same 12 volts were applied across a 6-ohm load, the current would drop to 2 A — higher resistance means less current for the same voltage.
Everyday Examples
Current draw varies enormously between devices. The table below lists typical values to give a sense of scale.
| Device or Situation | Typical Current |
|---|---|
| Single indicator LED | ~0.02 A (20 mA) |
| Smartphone charger | 1–3 A |
| LED light bulb (mains) | ~0.07 A |
| Laptop power adapter | 2–5 A |
| Household lighting circuit | 15–20 A (rated) |
| Electric kettle | ~10–13 A |
| Car starter motor | 100–200 A |
| Lightning strike (peak) | ~30,000 A |
Note that a higher current is not automatically dangerous on its own — what matters is the path it takes. Even currents below 0.1 A across the human body can be lethal, which is why electrical safety focuses on isolation and grounding, not just the size of the supply.
Common Multiples
Because real-world currents span from billionths of an amp in sensors to thousands of amps in industry, SI prefixes are essential.
| Prefix | Symbol | Value in Amperes | Where You See It |
|---|---|---|---|
| Microampere | µA | 0.000001 A (10−6 A) | Sensors, leakage current |
| Milliampere | mA | 0.001 A (10−3 A) | LEDs, microcontrollers |
| Ampere | A | 1 A | Appliances, chargers |
| Kiloampere | kA | 1,000 A (103 A) | Power grids, welding |
Most consumer electronics specs are quoted in milliamps or milliamp-hours (mAh) for battery capacity, while breakers and substations are rated in amps and kiloamps.
History
The ampere is named after André-Marie Ampère (1775–1836), the French physicist and mathematician who pioneered the study of the relationship between electricity and magnetism. Building on Hans Christian Ørsted's 1820 discovery that an electric current deflects a compass needle, Ampère formulated the mathematical laws describing the magnetic force between currents, founding the field he called electrodynamics.
The unit bearing his name was adopted internationally at the 1881 International Congress of Electricians and became one of the SI base units when the modern system was established in 1960. Its 2019 redefinition in terms of the elementary charge marked the final step in anchoring all SI electrical units to fundamental physical constants.
To switch between amperes, milliamperes, microamperes, and other current units, use our current converter for instant, precise results.