Math & Science
Electric Charge Calculator
Charge, current or time — give it any two and it solves for the third.
Electric Charge Calculator
About the Electric Charge Calculator
Charge, current and time are locked together by one short equation: charge equals current multiplied by time. Give this calculator any two of them and it solves for the third.
Current can be entered in amps, milliamps, microamps or kiloamps, and time in seconds, minutes or hours — everything is converted internally before the arithmetic. Results come in coulombs, in amp-hours (the unit batteries are actually rated in), and as a count of individual electrons.
This is the physics calculation: coulombs of charge moved by a current over time. It does not estimate the cost of charging an electric car, which is a different question about energy and tariffs rather than charge.
How to use the Electric Charge Calculator
- Choose which variable to solve for.
- Enter the other two with their units.
- Select Calculate.
- Amp-hours is the unit batteries are rated in — useful for comparing capacity.
- The electron count shows the scale of a coulomb.
The formula
Q = I × t
I = Q ÷ t
t = Q ÷ I
Q = charge in coulombs, I = current in amperes, t = time in seconds.
Worked example
A 2.5 A current flows for 4 minutes. How much charge passes?
Convert the time: 4 minutes = 240 seconds.
Q = 2.5 × 240 = 600 coulombs
As a count of electrons: 600 ÷ 1.602176634 × 10−19
600 C — about 3.74 × 1021 electrons — 0.167 amp-hours
What a coulomb is
One coulomb is the charge carried past a point by a current of one ampere flowing for one second. It is a big unit in everyday terms: a single AA battery delivers only a few thousand coulombs over its whole life, while one coulomb contains roughly 6.24 × 1018 electrons.
Since the SI redefinition took effect in 2019, the ampere is fixed by defining the elementary charge as exactly 1.602176634 × 10−19 coulombs. The coulomb is then derived from the ampere and the second. This calculator uses that exact value for its electron count.
Current is charge in motion
Current measures the rate at which charge flows, so one ampere is one coulomb per second. The relationship is exactly like speed and distance: charge is how much has passed, current is how fast it is passing.
| Device | Typical current | Charge in 1 hour |
|---|---|---|
| LED indicator | 20 mA | 72 C |
| Smartphone charging | 2 A | 7,200 C |
| Laptop | 3.5 A | 12,600 C |
| Household kettle | 13 A | 46,800 C |
| Car starter motor | 200 A | 720,000 C |
| Lightning strike | 30,000 A | 15 C in ~0.5 ms |
The lightning row shows why time matters as much as current. An enormous current for half a millisecond moves less total charge than an LED does over an hour.
Amp-hours, and why batteries use them
An amp-hour is the charge delivered by one amp for one hour, so 1 Ah = 3,600 coulombs. Batteries are rated this way because it maps directly onto how long they last at a given draw.
Runtime (hours) = capacity (Ah) ÷ current draw (A)
A 3,000 mAh phone battery is 3 Ah, or 10,800 coulombs. Drawing 0.5 A, it lasts about six hours. Real batteries do a little worse, since capacity falls at higher discharge rates and with age.
Amp-hours measure charge, not energy. To compare batteries at different voltages you need watt-hours: Wh = Ah × volts. A 3 Ah battery at 3.7 V holds 11.1 Wh; the same 3 Ah at 12 V holds 36 Wh. Charge alone does not tell you how much work a battery can do.
Common mistakes
- Leaving time in minutes or hours. The formula needs seconds unless you convert. This calculator handles it, but hand calculations often do not.
- Confusing mA with A. A factor of 1,000, and an easy one to miss.
- Treating amp-hours as energy. They measure charge; multiply by voltage for energy.
- Expecting full rated capacity. Manufacturers quote capacity at a gentle discharge rate. Draw harder and you get less.
Charge tells you how much has flowed, not how much work it can do — for that you need energy. The kinetic energy calculator covers the mechanical side, and the velocity calculator handles the same rate-over-time relationship for distance.
Battery and EV charging time and cost
Charging a battery is charge flowing over time, so the same idea applies. For small batteries in amp-hours (Ah):
Charge time (h) ≈ battery capacity (Ah) ÷ charging current (A)
Electric cars are rated in kilowatt-hours (kWh), so use power instead:
Charge time (h) ≈ energy to add (kWh) ÷ charger power (kW)
Charge cost = energy to add (kWh) × price per kWh
Worked example
An EV needs 45 kWh to go from 20% to 80%. A home charger gives 7.4 kW. Electricity costs $0.16 per kWh.
Time: 45 ÷ 7.4 = 6.1 hours. Cost: 45 × $0.16 = $7.20.
About 6 hours and $7.20
Real charging loses roughly 10% as heat, and fast chargers slow down above 80%, so allow a little extra.
Electric field of a point charge
A charge creates an electric field around it. Its strength falls with the square of the distance.
E = k × q ÷ r² (k ≈ 8.99 × 109 N·m²/C²)
A 1 microcoulomb charge at 0.1 m gives E = 8.99 × 109 × 10−6 ÷ 0.01 = 899,000 N/C. Double the distance and the field drops to a quarter.
Other ways to use this calculator
- Coulombs calculator — the electric charge formula is Q equals It: current × time.
- Amp hours calculator — 1 amp-hour is 3,600 coulombs; convert battery ratings either way.
- EV charge time calculator and EV charge cost calculator — battery kWh ÷ charger kW gives a rough charging time, and kWh × price per kWh gives the cost.
- Electric field of a point charge formula — E = kq ÷ r²; see the section above.
Frequently asked questions
A coulomb is the SI unit of electric charge — the amount carried past a point by a current of one ampere in one second. It contains roughly 6.24 × 1018 electrons. Since 2019 the SI has fixed the elementary charge at exactly 1.602176634 × 10−19 C, and the coulomb follows from the ampere and the second.
Current is the rate at which charge flows, so one ampere is one coulomb per second. The equation is Q = I × t: a 2.5 A current flowing for 240 seconds moves 600 coulombs. The relationship works like distance and speed — charge is how much has passed, current is how fast it is passing, and time connects the two.
An amp-hour is the charge delivered by one amp flowing for one hour, equal to 3,600 coulombs. Batteries are rated in amp-hours because it maps onto runtime: capacity divided by current draw gives roughly how many hours you get. A 3,000 mAh phone battery is 3 Ah, or 10,800 coulombs, lasting about six hours at a 0.5 A draw.
About 6.24 × 1018 — roughly six quintillion. The figure comes from dividing one coulomb by the elementary charge, which the SI fixes at exactly 1.602176634 × 10−19 C. This calculator reports the electron count for any charge you enter, which is a useful reminder of just how small an individual charge is.
Divide the energy you need in kWh by the charger's power in kW. Adding 45 kWh on a 7.4 kW home charger takes about 6 hours. Allow around 10% extra for charging losses.
Sources and further reading
- NIST — Elementary charge — the exact value 1.602176634 × 10⁻¹⁹ C
- NIST — SI unit of electric current: ampere — the 2019 redefinition of the ampere and coulomb
- BIPM — The International System of Units (SI Brochure) — definitions of the coulomb, ampere and second