Charge and Current Calculator: Q = It

Charge, current and time, with amp-hours for batteries.

Calculated from standard textbook formulas, with every step shown. The formulas are fixed, so the working does not need checking: what matters is the numbers you type in. Use these calculators only if you are competent to choose the inputs (loads, material strength, dimensions, supports) and check your units and assumptions. For learning and checking your own working: design and sign-off are for a qualified engineer. Drafting Central does not store the numbers you type.

How this calculator works

Choose which quantity you want to find.

Enter the other values in the units shown.

The calculator gives the answer with the formula, the substituted numbers, and the answer in other units.

Formulas used

  • Q = I\,t
  • I = \frac{Q}{t}
  • t = \frac{Q}{I}

Worked example

With the default numbers in the calculator above:

  1. The formula, arranged to find the charge: charge = current x time
  2. Put the values in (every value in its SI unit): charge = 2 x 3600
  3. The answer: charge: 7200 C
  4. The same answer in other units: 2 Ah; 2000 mAh; 44938.9 electrons (x 10^18)

Change the numbers in the calculator to see the full working, every step, for your own case.

Common questions

What is the formula for charge?

Charge equals current times time. 2 A for 3600 s is 7200 C, which is 2 Ah.

How do I convert mAh to coulombs?

Multiply by 3.6. A 3000 mAh battery holds 10 800 C.

How long will a battery last?

Divide the capacity in amp-hours by the current in amps to get the time in hours.

Related calculators

Got a question about this? Ask an engineering question and get a plain-language answer with references.

Further reading: Any school physics text (electric charge and current).

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