Electric Charge Converter — coulomb, mAh, faraday

The electric charge converter above moves a value between coulombs, the SI unit, and the practical units people actually meet: milliamp-hours on a battery label, amp-hours on a lead-acid specification, faradays in electrochemistry, and elementary charges in physics. Enter a number, pick the units, and the answer appears live, with the same quantity shown at once in coulombs, mAh, Ah and elementary charges.

Arb Digital publishes this as part of a free engineering converter set. Charge is the quantity where consumer labelling and scientific units diverge most sharply. Nobody sells a battery in coulombs, and no physics paper quotes a reaction in milliamp-hours, yet both describe exactly the same thing. This page connects the two vocabularies.

What This Electric Charge Converter Does

Electric charge is the quantity that flows when current flows. The link is direct: current is charge per unit time, so one ampere is one coulomb per second. Run one ampere for one second and one coulomb has passed. Run one ampere for one hour and 3,600 coulombs have passed, which is why the amp-hour equals exactly 3,600 coulombs — no measurement, just the definition of the hour.

The converter includes the elementary charge, the magnitude of the charge on a single proton or electron. Since the 2019 revision of the SI, this is an exact defined constant: 1.602176634 × 10⁻¹⁹ coulombs. It is no longer a measured quantity with an uncertainty, because the ampere itself is now defined in terms of it. That makes charge one of the few areas where a physics constant and a practical unit meet with zero rounding on either side.

Also included is the faraday, the charge carried by one mole of electrons, equal to the Faraday constant of approximately 96,485.332 coulombs per mole. Electrochemists use it to link current to the amount of substance deposited or dissolved in electrolysis, which is a conversion no battery label will ever need but which turns up constantly in plating and corrosion work.

How to Use It

1. Enter your charge value — a battery capacity, a measured coulomb count, or a physics figure.
2. Pick the source unit from the first dropdown.
3. Pick the target unit from the second.
4. Read the result, which updates on every keystroke; the Convert button stays available for keyboard and screen-reader users.
5. Press Swap units to reverse direction without retyping.

The Formula: How Charge Conversion Is Calculated

Conversions pivot through the coulomb:

result = value × factor(from) ÷ factor(to)

The two factors worth spot-checking are the amp-hour and the elementary charge. The amp-hour is exact by construction: one ampere is one coulomb per second, an hour is 3,600 seconds, so 1 Ah = 3,600 C and 1 mAh = 3.6 C. The elementary charge is exact by definition since the 2019 SI redefinition, fixed at 1.602176634 × 10⁻¹⁹ C in the BIPM SI Brochure. Neither number is rounded here.

The CGS units follow NIST Special Publication 811. The abcoulomb equals exactly 10 coulombs. The statcoulomb, also called the franklin or the electrostatic unit of charge, equals approximately 3.335640951 × 10⁻¹⁰ C — the reciprocal of ten times the speed of light in metres per second, which is why it became exact when the metre was tied to the speed of light.

Reference Table: Every Supported Unit

Each factor is the number of coulombs in one of that unit.

• coulomb (C) — 1 (SI base for this quantity)
• kilocoulomb (kC) — 1 × 10³ C
• millicoulomb (mC) — 1 × 10⁻³ C
• microcoulomb (µC) — 1 × 10⁻⁶ C
• nanocoulomb (nC) — 1 × 10⁻⁹ C
• picocoulomb (pC) — 1 × 10⁻¹² C
• ampere-hour (Ah) — 3,600 C exactly
• milliampere-hour (mAh) — 3.6 C exactly
• ampere-second (A·s) — 1 C exactly; the definition written out
• faraday (F, per mole) — 96,485.33212 C
• elementary charge (e) — 1.602176634 × 10⁻¹⁹ C exactly
• abcoulomb (abC) — 10 C exactly
• statcoulomb (statC, franklin) — 3.335640951 × 10⁻¹⁰ C

Why mAh Is Not a Measure of Energy

This is the single most consequential misunderstanding on this page. Milliamp-hours measure charge. They say nothing about voltage, and therefore nothing about how much energy a battery holds. A 3,000 mAh cell at 3.7 volts and a 3,000 mAh cell at 1.2 volts contain the same charge and very different energy — roughly 11.1 watt-hours versus 3.6 watt-hours.

Energy in watt-hours is charge in amp-hours multiplied by voltage. That is why power banks are honestly labelled in watt-hours as well as mAh, and why a 20,000 mAh power bank does not charge a 4,000 mAh phone five times: the power bank's cells run at a lower voltage than the phone's charging circuit expects, and conversion losses take a further cut. Comparing capacities across devices only works after converting both to watt-hours, which is a job for the energy converter once you have multiplied by the nominal voltage.

Charge, Capacitors and the C = QV Relationship

A capacitor's stored charge is its capacitance multiplied by the voltage across it. Because the farad is defined as a coulomb per volt, that calculation is dimensionally trivial as long as both inputs are in base units. A 470 µF capacitor charged to 12 volts holds 470 × 10⁻⁶ × 12 = 5.64 × 10⁻³ coulombs, or 5.64 millicoulombs — a figure that also equals about 1.57 microamp-hours, which puts capacitor storage in perspective against even the smallest battery.

That comparison is the practical reason capacitors and batteries are not substitutes. Supercapacitors close some of the gap, but the charge a conventional capacitor holds is orders of magnitude below a cell of similar volume. Use the capacitance converter to normalise the farad side before running the multiplication, and remember that the stored energy is half the capacitance times voltage squared, not charge times voltage.

Coulomb Counting in Battery Management

Modern battery management systems estimate state of charge by coulomb counting: integrating current over time to track how much charge has entered and left the cell. In principle this is exact, since charge is conserved. In practice it drifts, because small offset errors in the current measurement accumulate over hours of integration, and because the cell's true full-charge capacity fades with age.

That is why a laptop or phone battery gauge periodically recalibrates against voltage landmarks, typically a full charge and a deep discharge, to reset the accumulated error. It also explains why a battery reporting 4,000 mAh when new may genuinely deliver 3,400 mAh after a few hundred cycles while still reporting a full charge — the counter is accurate about charge moved, but the reference capacity it compares against has changed. Charge conversion is exact arithmetic; battery capacity is an ageing physical property.

Where Charge Sits Among the Other Electrical Quantities

Charge is the flowing quantity, current is its rate, voltage is the energy per unit charge that drives it, and resistance opposes it. For the ohms side use the resistance converter; for stored magnetic energy in coils see the inductance converter; for power in watts, the power converter. This tool deliberately covers charge alone, so no cross-quantity conversion is possible from its unit list.

C-Rate: Charge Capacity Expressed as a Current

Battery datasheets rarely quote charge and discharge currents in amperes. They quote a C-rate, which is a multiple of the cell's rated capacity treated as a current. A 3,000 mAh cell discharged at 1C is delivering 3,000 mA, or 3 amperes, and would in principle be empty after one hour. At 0.5C it delivers 1.5 A for about two hours; at 2C it delivers 6 A for about half an hour.

The reason for expressing it this way is that a current which is gentle for a large cell is abusive for a small one, so an absolute ampere figure would have to be restated for every capacity. C-rate normalises it. The catch is that real capacity falls as C-rate rises: a cell rated 3,000 mAh at 0.2C may deliver noticeably less at 2C, because internal resistance converts more of the stored energy into heat rather than useful output. That effect is described by Peukert's relationship, most pronounced in lead-acid chemistry and much milder in lithium-ion.

So converting 3,000 mAh to 10,800 coulombs is exact, while predicting how many of those coulombs you will actually get out at a given load is not. Keep the two separate: unit conversion is arithmetic, and delivered capacity is a measured performance characteristic that depends on current, temperature and cell age.

Charge in Electroplating and Corrosion Work

Faraday's laws of electrolysis connect charge directly to mass. The amount of a substance deposited or dissolved at an electrode is proportional to the total charge passed, divided by the number of electrons the reaction consumes per atom and multiplied by the molar mass. One faraday — about 96,485 coulombs — deposits exactly one mole of a singly charged ion, half a mole of a doubly charged one, and so on.

This is why plating shops specify a process in ampere-minutes or ampere-hours rather than in time alone. Running 10 amperes for 30 minutes passes 18,000 coulombs regardless of how the current was scheduled, and to a first approximation that determines the deposited mass. It also underpins corrosion rate estimates, where a measured corrosion current is integrated over time and converted into metal loss. Converting a process current and duration into coulombs is the first step in both calculations, and from there the molar mass converter handles the chemistry side.

Common Mistakes to Avoid

• Treating mAh as energy — it is charge, and it means nothing about stored energy without a voltage.
• Comparing power bank and phone capacities directly without converting both to watt-hours.
• Confusing the faraday with the farad — one is a charge per mole, the other is a unit of capacitance.
• Assuming the abcoulomb is small because the CGS name sounds it — it is exactly ten coulombs.
• Trusting a coulomb-counting fuel gauge indefinitely without recalibration, since integration error accumulates.

Related Free Tools From Arb Digital

Pair this with the capacitance converter for charge stored on a capacitor, the energy converter to turn amp-hours plus voltage into watt-hours, and the power converter for charge and discharge rates. The resistance converter and inductance converter cover the rest of the electrical set, and the free online tools hub lists everything.

Frequently Asked Questions