Joules to Volts Calculator (V = energy ÷ charge, coulombs, mAh or electrons)

Understanding the joules to volts calculator — concept and principle

This calculator turns an energy in joules and an amount of charge into a voltage, using the definition of the volt. The charge can be in coulombs, milliampere-hours, ampere-hours or a number of electrons. It also converts the charge to mAh and the energy to watt-hours and electronvolts, which helps with battery, capacitor and physics problems.

A volt is one joule per coulomb, so 9 J carried by 2 C is 4.5 V.

How it is calculated

V = W ÷ Q

1 V = 1 J/C.
Charge units: 1 mAh = 3.6 C; 1 Ah = 3,600 C; one electron = e = 1.602176634 × 10⁻¹⁹ C.
Energy units: 1 Wh = 3,600 J; 1 eV = 1.602176634 × 10⁻¹⁹ J.

The principle

Voltage is energy per unit charge. Moving one coulomb of charge through a potential difference of one volt takes or releases one joule. That is why joules and volts cannot be converted directly: they measure different things, and the charge connects them. The electronvolt follows from the same definition: an electron crossing 1 V gains 1 eV, so an energy in eV divided by a number of electrons gives the voltage directly.

Worked example

Default inputs: 9 J, charge 2 C.

V = 9 ÷ 2 = 4.5 V.
Charge in mAh = 2 ÷ 3.6 = 0.555556 mAh.
Energy in Wh = 9 ÷ 3,600 = 0.0025 Wh.
Energy in eV = 9 ÷ 1.602176634 × 10⁻¹⁹ = 5.617 × 10¹⁹ eV.

Each point is the calculator re-run with only “Charge” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Charge	Voltage (V)
0.5	18
1	9
1.5	6
2	4.5
2.5	3.6
3	3
4	2.25

How to read the result

The voltage is the average energy each coulomb carries. For a battery, energy ÷ charge gives its average voltage over the discharge: a 10 Wh battery rated 2,700 mAh averages about 3.7 V. For a capacitor, the energy stored is ½CV², so energy ÷ charge gives half the final voltage, not the full value.

Common mistakes

Trying to convert joules to volts without a charge.
Using mAh without converting to coulombs (× 3.6).
Applying V = W/Q to a capacitor's stored energy without the factor of ½.
Mixing Wh and J.

Keep in mind: The result is an average voltage. A real battery's voltage changes during discharge and under load.

Frequently asked questions

How many joules are in 1 mAh at 3.7 V?

3.6 C × 3.7 V = 13.32 J.

Is a volt the same as a joule?

No. A volt is a joule per coulomb.

What is an electronvolt?

The energy one electron gains crossing 1 V, 1.602176634 × 10⁻¹⁹ J.

How do I get a battery's energy from mAh and voltage?

Multiply the charge in coulombs by the average voltage, or mAh × V ÷ 1,000 for Wh.

Sources: BIPM, The International System of Units (SI Brochure), 9th edition (2019)

Checked 9 Nov 2026. Definition of the volt (1 V = 1 J/C) and the exact elementary charge of the 2019 SI — standard values.

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