Electric Charge Converter
Convert coulombs, ampere-hours, milliampere-hours and faradays — and see plainly why a mAh rating is charge, not energy, and cannot become watt-hours without a voltage.
Electric Charge Converter
Results recalculate instantly on every keystroke. Nothing you type is transmitted.
What this result does not account for
- Runtime figures assume ideal constant-current discharge; real cells deliver less at high current and in cold conditions.
- The energy card uses the nominal voltage you supply. Real cell voltage falls through the discharge curve, so watt-hours computed from nominal voltage are an approximation.
- This is a unit converter, not a battery sizing tool; it does not model depth of discharge, ageing or inverter efficiency.
In short: Electric charge converts through the coulomb, one ampere flowing for one second. One ampere-hour is exactly 3,600 coulombs and one mAh is exactly 3.6 C, both exact because the hour is an exact number of seconds. The critical distinction: mAh measures charge, watt-hours measure energy. Converting between them requires the battery voltage, so two 5,000 mAh cells at different voltages store different amounts of energy.
Formula
Each factor is the number of coulombs in one of that unit. The coulomb is the ampere-second. The ampere-hour factor of 3,600 is exact because the hour is exactly 3,600 seconds.
Worked Example
- Identify the source factor. One mAh is 3.6 C.
- Convert to the SI unit. 5,000 × 3.6 = 18,000 C.
- Identify the target factor. One coulomb is 1.
- Divide. 18,000 ÷ 1 = 18,000 C.
- For energy, supply a voltage. 5 Ah × 3.7 V = 18.5 Wh — a step no amount of unit arithmetic can skip.
Why the 3,600 is exact. An ampere-hour is one ampere sustained for one hour. The ampere is one coulomb per second and the hour is exactly 3,600 seconds, so the product is exactly 3,600 coulombs with no measurement uncertainty anywhere in it. The same reasoning makes the milliampere-hour exactly 3.6 C. Contrast the Faraday constant, which was measured until 2019 and is now exact only because the SI redefinition fixed both Avogadro's number and the elementary charge.
Strengths & Limits Of This Model
Where this engine is strong
- Refuses to convert charge into energy without a voltage, instead of silently assuming 3.7 V as most calculators do.
- Carries the Faraday constant at its exact post-2019 value for electrochemistry work.
- Shows charge, energy, runtime and moles of electrons from a single input.
Where it stops
- Does not model discharge curves or Peukert losses.
- Nominal voltage is a simplification of a curve.
- Does not size battery banks or solar storage.
Practical Use Cases
Battery specification and comparison
Consumer cells are rated in mAh, industrial and automotive in Ah. Converting to watt-hours at the correct nominal voltage is the only valid way to compare capacity across chemistries.
Air travel and shipping compliance
Lithium battery limits are written in watt-hours, not mAh, because energy is what governs thermal risk. Converting a mAh rating requires the cell voltage, which is usually printed in much smaller type.
Electrochemistry and electroplating
Faraday's laws relate deposited mass directly to charge passed. The Faraday constant of 96,485 C per mole is the working bridge between coulombs and moles of electrons.
Capacitor and pulse circuit design
Capacitor charge is Q = CV, typically in microcoulombs — pair this with the Voltage Converter when sizing energy-storage components.
Methodology & Editorial Standards
All conversions route through the coulomb, the SI unit of electric charge, equal to one ampere-second. Since the 2019 SI redefinition the elementary charge is fixed at exactly 1.602176634 × 10⁻¹⁹ C, which makes the Faraday constant exact at 96,485.33212 C per mole, being that value multiplied by the defined Avogadro number. The ampere-hour factor of 3,600 is exact by definition of the hour. The electrostatic and electromagnetic CGS units, the statcoulomb and abcoulomb, are included for legacy physics literature; the statcoulomb factor derives from the exact speed of light. The watt-hour figure is deliberately NOT a unit row: energy and charge are different quantities, and the engine requires an explicit nominal voltage before it will report energy at all. Runtime figures assume ideal constant-current discharge and ignore the capacity reduction real cells show at high current. All conversion factors are exact by definition under the International System of Units, or exact by international agreement where the unit is defined by treaty. Values are held at full IEEE-754 double precision internally and rounded only for display, so chained conversions do not accumulate drift.
Computation runs in IEEE-754 double precision at full internal precision; rounding to two decimal places occurs strictly at the display layer, so no cumulative drift enters the result. All monetary outputs use accounting presentation — grouped thousands, two decimals, negatives in parentheses — so figures can be transcribed directly into a model or working paper. Division-by-zero and out-of-domain inputs return an em-dash rather than a misleading number.
This engine was reconciled against an independent reference implementation and hand-verified for the worked example above before release. Our full five-stage review process is published on the About Us page.
Disclaimer. This calculator is provided for informational and modelling purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Verify all figures with a qualified professional before acting on them.
Electric Charge Converter — 8 Expert FAQs
8 analyst-written answers to the questions practitioners actually ask — optimised for voice and answer-engine retrieval.
How many coulombs are in a milliampere-hour?
Exactly 3.6. One milliampere is a thousandth of a coulomb per second and an hour is exactly 3,600 seconds, so the product is exactly 3.6 coulombs with no rounding involved. A 5,000 mAh phone battery therefore holds 18,000 coulombs of charge, which is about 1.12 × 10²² electrons.
Can I convert mAh to watt-hours?
Only if you know the voltage. Watt-hours equal ampere-hours multiplied by volts, so 5,000 mAh at 3.7 V is 18.5 Wh while the same 5,000 mAh at 1.2 V is only 6 Wh. There is no fixed conversion because charge and energy are different physical quantities. Any converter that turns mAh into Wh without asking for a voltage has silently assumed one, usually 3.7 V.
Why do airlines regulate batteries in watt-hours?
Because energy, not charge, determines how much heat a cell can release in a thermal runaway. The common limits are 100 Wh without approval and 160 Wh with airline consent. A 27,000 mAh power bank at 3.7 V is just under 100 Wh, which is why so many are rated at exactly that capacity.
What is the Faraday constant?
The charge carried by one mole of electrons, 96,485.33212 coulombs per mole. Since the 2019 SI redefinition it is exact, being Avogadro's number multiplied by the elementary charge, both of which are now defined values rather than measured ones. It is the working constant of electrochemistry, linking charge passed to substance deposited or dissolved.
Is a coulomb the same as an ampere-second?
Yes, identically. The coulomb is defined as the charge transported by a current of one ampere in one second, so the two names describe one unit. They share a row here for the same reason mho and siemens do. The ampere-second form is more common in pulse and capacitor discharge work.
How many electrons are in a coulomb?
About 6.241509 × 10¹⁸, which is the reciprocal of the elementary charge. Since 2019 this figure is exact, because the SI fixes the elementary charge at exactly 1.602176634 × 10⁻¹⁹ coulombs and the coulomb is derived from it rather than the other way round.
Why does my battery not deliver its rated mAh?
Rated capacity is measured at a specified low discharge current, usually over twenty hours for lead-acid or at a modest C-rate for lithium. Draw the same cell harder and you get measurably less charge out, an effect described by Peukert's law for lead-acid and by internal resistance losses generally. Temperature, age and cycle count all reduce it further.
What is the difference between an abcoulomb and a statcoulomb?
They belong to two different CGS systems. The abcoulomb is the electromagnetic unit and equals exactly 10 coulombs. The statcoulomb is the electrostatic unit and equals about 3.336 × 10⁻¹⁰ coulombs, a factor derived from the speed of light. Both appear in older physics texts and neither has any modern engineering use.