Example 1
A charger supplies a steady 2 A. In the first 10 s, 20 C flows into the phone. That is a short slice of charging, not the whole battery.
Q = 20 C
Electric Charge Calculator
Enter current I in amperes and time t in seconds. The calculator multiplies those numbers with Q = I·t and reports charge in coulombs, ampere-hours, and as a count of elementary charges. For 2 A and 10 s the result is 20 C, and for 1 A and 3600 s it is 3600 C, which is 1 Ah. The model assumes steady current: it does not integrate a changing waveform and it does not apply Coulomb’s law.
Result
Enter current and time. Charge, ampere-hours, and the elementary-charge count appear here.
For the inverse problem, current from charge and time, use the I = Q/t calculator. For electric power, use P = U·I, and for capacitance, C = Q/U.
How it works
I2 A
t10 s
Q20 C
The charge that passes at a steady current is the product Q = I·t. I is current in amperes, t is time in seconds, and Q is charge in coulombs. At 2 A and 10 s the result is 2 × 10 = 20 C. At 1 A and 3600 s the result is 3600 C, exactly 1 Ah, because 1 Ah = 3600 C.
The calculator uses the two numbers you enter: current I and time t. A comma and a period are the same decimal separator. A minute is 60 s, and an hour is 3600 s. Milliamperes are entered as a fraction of an ampere, for example 0.02 instead of 20 mA.
What the result panel shows. Besides charge in coulombs, the calculator reports the same result in ampere-hours (Q/3600) and milliampere-hours (Q/3.6), the number of elementary charges n = |Q|/e with e = 1.602176634×10⁻¹⁹ C, and the current and time you entered. For 20 C that is exactly 1/180 Ah and 20/3.6 mAh, and the elementary-charge count is about 1.248×10²⁰. The chart under the result draws Q = I·t at fixed current as time grows.
Limits of the model. The calculator does not evaluate the integral for a changing current, does not compute capacitance C = Q/U, and does not compute electrical work W = U·I·t. Negative time is rejected. Zero time gives Q = 0 C. A negative current means the opposite direction and gives charge with the same sign. Coulomb’s law, the force between two charges, is a separate calculator.
Examples on this page: 2 A and 10 s give 20 C; 0.5 A and 120 s give 60 C; 1 A and 3600 s give 3600 C; 0.02 A and 500 s give 10 C; 5 A and 720 s give 3600 C. The button “Fill this example” enters those same numbers and refreshes the panel and the chart.
Formula
Q = I · t
[Q] = A·s = C; 1 Ah = 3600 C, 1 mAh = 3.6 C, e = 1.602176634×10⁻¹⁹ C
Model: charge at steady current, the rearranged definition of current I = dQ/dt. This is not Coulomb’s law. The calculator multiplies the entered I and t in SI units, then converts Q to ampere-hours, milliampere-hours, and the elementary-charge count n = |Q|/e.
Theory, derivation, and scope
Electric charge and the coulomb
Electric charge Q is how much charge has passed a cross-section of a conductor, or collected on a plate. For a steady current, the definition I = Q/t rearranges to the product Q = I·t. The SI unit of charge is the coulomb: 1 C = 1 A·s. The coulomb is not a unit of capacitance. Capacitor capacitance C = Q/U is measured in farads.
A coulomb is large on an everyday scale. A current of 1 A for one second already carries 1 C, while a coin cell is often rated in tens of milliampere-hours, which is tens of coulombs. That is why the panel shows ampere-hours next to coulombs.
History: Coulomb, Ampère, and the ampere-hour
In 1785 Charles-Augustin de Coulomb measured the force between charges with a torsion balance and stated F = k·q₁·q₂/r². This calculator does not compute that law: there is no distance and no second charge. André-Marie Ampère studied the forces between currents, and the unit of current carries his name. The name coulomb for the unit of charge entered the SI later.
The ampere-hour is not an SI unit. It grew up as a practical battery measure: one ampere for one hour. An hour is 3600 seconds, so 1 Ah = 3600 C exactly, and 1 mAh = 3.6 C exactly.
Assumptions of Q = I·t
Q = I·t is valid when the current is constant over the whole time interval. The calculator treats the number in the current field as that constant value. Time must be zero or positive, because the interval does not run backward. A negative current is the direction opposite the positive convention, and Q then has the same sign.
The product does not include how charge is spread in space, the electric field, or the Coulomb force. The result is the charge that would pass at a steady current, not a battery capacity read from a label.
Derivation from the definition of current
Step 1. Current is defined as the derivative of charge with time: I = dQ/dt. Step 2. When I is constant, dQ = I·dt, and the integral from zero to t is Q = I·t. Step 3. An ampere times a second is a coulomb. Step 4. At 2 A and 10 s the product is 20 C. At 1 A and 3600 s the product is 3600 C, which is 1 Ah.
When current is a function of time, step 2 stays an integral, Q = ∫ I(t) dt. One number in the current field does not describe that function, so the result remains the product alone.
Ampere-hours, milliampere-hours, and the elementary charge
The conversions follow from the definitions: Ah = Q/3600 and mAh = Q/3.6. For 20 C that is 20/3600 = 1/180 Ah and 20/3.6 mAh. Since 20 May 2019 the ampere has been defined by fixing the elementary charge at e = 1.602176634×10⁻¹⁹ C exactly, as stated in the 9th edition of the SI Brochure (BIPM). The number of elementary charges is n = |Q|/e. For 20 C that is about 1.248×10²⁰. The sign of Q is the direction; n is a count.
What this calculator does not compute
The calculator does not turn a battery’s ampere-hours into runtime, because that also needs voltage and efficiency. It does not compute electrical work W = U·I·t or power P = U·I, because the formula has no voltage. It does not compute Joule heat. The RMS value of an alternating current is not the same thing as the steady current in the field, even when both numbers are written in amperes.
Capacitance C = Q/U is a separate page. Current from a known charge and time, I = Q/t, is also a separate page: the same definition, a different unknown.
Sources
The definition, the unit, and the derivation come from textbooks and metrology documents: D. Halliday, R. Resnick, and J. Walker, Fundamentals of Physics; H. D. Young and R. A. Freedman, University Physics; D. J. Griffiths, Introduction to Electrodynamics (the definition I = dQ/dt); C.-A. de Coulomb, memoirs on electricity and magnetism, 1785; BIPM, The International System of Units (SI), 9th edition, including the defining constants and the fixed value of e; NIST Special Publication 330. This page is for educational purposes.
How to use
- Enter current I in amperes, for example 2. A negative value is the opposite direction.
- Enter time t in seconds, for example 10. One hour is 3600, not 1.
- Click Calculate or wait for the live result. At 2 A and 10 s the calculator shows 20 C.
- Read the panel: coulombs, ampere-hours, milliampere-hours, the elementary-charge count, and the chart of Q against time.
- Clear removes the inputs and hides the panel. Current from charge and time is calculated on the page for I = Q/t.
Symbol glossary
Q is charge, I is steady current, and t is time. At 2 A and 10 s the result is 20 C.
- Q
- Charge [C]. The product I·t. At 2 A and 10 s it is 20 C. This is not capacitance in farads.
- I
- Current [A], constant over the whole time t. A minus sign is the opposite direction.
- t
- Time [s]. A negative value is rejected. One hour is 3600 s.
- Ah
- Ampere-hour. 1 Ah = 3600 C. The panel computes Ah = Q/3600.
- mAh
- Milliampere-hour. 1 mAh = 3.6 C. The panel computes mAh = Q/3.6.
- e
- Elementary charge, 1.602176634×10⁻¹⁹ C exactly (SI, 2019). The count is n = |Q|/e.
Worked examples
Example 2
A small motor draws 0.5 A for 120 s, which is two minutes. 60 C passes through the winding.
Q = 60 C
Example 3
The ampere-hour definition: 1 A flows for a full hour, 3600 s. The charge is 3600 C, exactly 1 Ah.
Q = 3600 C (1 Ah)
Example 4
A brief circuit test: 3 A for 5 s. The charge that passes is 15 C.
Q = 15 C
Example 5
An LED runs at 20 mA, which is 0.02 A, for 500 s. In that time, 10 C passes.
Q = 10 C
Example 6
A model pulse: 10 A for half a second. Despite the high current, the pulse carries only 5 C.
Q = 5 C
Example 7
A bicycle lamp draws 0.1 A for 60 s. In one minute, 6 C passes.
Q = 6 C
Example 8
A charger supplies 5 A for 12 minutes, which is 720 s. The product is 3600 C, or 1 Ah.
Q = 3600 C (1 Ah)
Example 9
A portable battery delivers 2 A for half an hour, 1800 s. The charge is 3600 C, or 1 Ah.
Q = 3600 C (1 Ah)
Example 10
A slow charge: 0.5 A for two hours, 7200 s. The charge is again 3600 C, or 1 Ah.
Q = 3600 C (1 Ah)
Ways to use this calculator
- You check how many coulombs pass at a steady current before you convert the result to ampere-hours.
- You compare a pulse of 10 A for 0.5 s, which is 5 C, with a charge of 0.5 A for 7200 s, which is 3600 C.
Common questions
What does this calculator compute?
The calculator multiplies steady current in amperes by time in seconds and reports charge Q = I·t in coulombs. The panel also shows ampere-hours, milliampere-hours, and the number of elementary charges. At 2 A and 10 s the result is 20 C.
Which units do I enter for current and time?
Enter current in amperes and time in seconds. The main result is in coulombs. A minute is 60 seconds, and an hour is 3600 seconds. Enter milliamperes as a fraction of an ampere: 20 mA is 0.02 A.
Why is an hour 3600 in the time field, not 1?
The time field takes seconds, because a coulomb is an ampere times a second. A 1 means one second, not one hour. At 1 A for 3600 s the charge is 3600 C, which is exactly 1 Ah.
How is an ampere-hour different from a coulomb?
Both units measure charge. The coulomb is the SI unit: 1 C = 1 A·s. The ampere-hour is the practical battery unit: 1 Ah = 3600 C, and 1 mAh = 3.6 C. The calculator converts the same product into both scales.
How is this different from the current page I = Q/t?
Here the unknown is charge, and the inputs are current and time. On the current page the unknown is current, and the inputs are charge and time. It is the same definition written two ways.
How is charge Q different from capacitance?
Charge on this page is in coulombs and is the product of current and time. Capacitance C = Q/U is in farads and describes the charge stored for each volt of potential difference. The letter C names two different quantities.
Can time be zero or negative?
Negative time is rejected, because the formula describes an interval that does not run backward. Zero time gives a charge of 0 C, because no charge has been carried yet. Both fields need a number before a result appears.
What does a negative current mean?
A negative current is the direction opposite the one taken as positive. Charge Q then has the same sign. The elementary-charge count on the panel is the absolute value |Q| divided by the elementary charge e.
Does the calculator handle a current that changes?
No. Q = I·t needs a current that stays constant over the whole interval. When current changes, charge is the integral of current over time, not the product of one number and the time.
Do a comma and a period mean the same current?
Yes. 0.5 and 0,5 are the same current. At 0.5 A for 120 s the charge is 60 C.
How do I calculate electric charge with Q = I·t?
Multiply current in amperes by time in seconds. The result is in coulombs. For 2 A and 10 s: Q = 2·10 = 20 C.
What is Q at 2 A and 10 s?
20 C. In ampere-hours that is 20/3600 ≈ 0.00556 Ah, and the elementary-charge count is about 1.25×10²⁰.
How many coulombs is 1 Ah?
Exactly 3600 C, because 1 A for 3600 s gives Q = 1·3600 = 3600 C. 1 mAh is 3.6 C.
How do I convert minutes to seconds before calculating?
Multiply minutes by 60. Five minutes is 300 s; at 2 A that gives Q = 2·300 = 600 C. One hour is 3600 s, not “1” in the time field.
Is Q = I·t Coulomb’s law?
No. Coulomb’s law describes the force between charges in space. Here the calculator finds charge flow at steady current: the product form of I = dQ/dt.
Sources
The definition, the unit, the history, and the derivation come from physics textbooks and metrology documents.
- Wikipedia — Electric charge
- BIPM — SI Brochure, 9th edition
- BIPM — SI measurement units, including the coulomb and the ampere
- NIST SP 330 — The International System of Units
Page updated November 3, 2026.