Joules to Volts Conversion Calculator

What this calculates

Voltage is defined as energy per unit of charge — one volt is one joule per coulomb. So if you know how much energy was transferred and how much charge carried it, dividing the first by the second gives the potential difference they moved through. This calculator does exactly that division.

It shows up whenever the energy and charge are the known quantities and voltage is the unknown: checking a capacitor’s stored energy against its charge rating, working out the effective voltage a battery delivered over a discharge, or the potential-difference part of a physics problem framed in joules and coulombs.

The joules to volts formula

is voltage in volts, is energy in joules, and is charge in coulombs. If charge isn’t given directly, it comes from current and time:

amps times seconds. A circuit that carried 4 A for 2.5 seconds moved 10 coulombs.

Worked example

The calculator’s defaults are 120 J of energy and 10 C of charge.

The charge moved through an average potential difference of 12 volts. Check it against power: if those 10 C moved in 5 seconds, the current was 2 A, the power was 24 W, and 24 W for 5 s is 120 J — consistent with the input.

Joules to Volts Conversion Calculator worked example diagram — A visual read of the worked example above — the same numbers the calculator returns for its default inputs.

Energy and charge to voltage

Energy	1 C	10 C	100 C	3,600 C (1 Ah)
12 J	12 V	1.2 V	0.12 V	0.0033 V
120 J	120 V	12 V	1.2 V	0.033 V
1,200 J	1,200 V	120 V	12 V	0.33 V
43,200 J	43,200 V	4,320 V	432 V	12 V

The bottom-right cell is a battery-scale figure: 43,200 J delivered by one amp-hour of charge works out to 12 V, the same result the Wh = Ah × V shortcut gives.

What the result assumes

The formula returns the average voltage over the transfer. It assumes the voltage held steady while the charge moved. That’s fine for a battery under a light, slow load, but a capacitor’s voltage falls as it discharges, so dividing its total energy by its total charge gives a figure partway between its start and end voltage — specifically half the peak for a capacitor taken fully to zero. For a live circuit, a voltmeter is the better tool.

Common mistakes

Using amp-hours where coulombs are expected. Multiply amp-hours by 3,600.
Expecting a capacitor’s rated voltage. Its energy is ½ C V², so energy ÷ charge gives half the rated voltage, not the full value.
Confusing this with joules to watts. That one needs time, not charge, and gives power rather than voltage.

For the reverse, use the volts to joules calculator. To find power from the same energy figure, see joules to watts.

About the formula: Use the stated electrical relationship and enter values in the displayed units.

Frequently Asked Questions

How does this joules to volts conversion calculation work?+

Voltage is energy per unit of charge, so dividing energy in joules by charge in coulombs gives voltage directly. 120 J moved by 10 C means 12 V of potential difference.

Does this assume the voltage stayed constant?+

Yes. The result is the average voltage over the process. If the voltage changed while the charge moved — as it does in a discharging capacitor — this gives the average, not the starting or ending value.

How do I get the charge in coulombs?+

From current and time: Q = I × t, amps times seconds. A circuit passing 2 A for 5 seconds moved 10 coulombs. From a battery spec, multiply amp-hours by 3,600 to get coulombs.

Why doesn't this match my capacitor's rated voltage?+

A capacitor stores energy as ½ × C × V², not V × Q. If you take its rated energy and rated charge and divide, you get half its rated voltage, because the voltage climbed from zero to rated as it charged. This calculator assumes a fixed voltage.

Is this the same as joules to watts?+

No. Joules to watts divides energy by time to get power. This divides energy by charge to get voltage. Different second quantity, different result — pick the one matching what you know.

Where is this actually useful?+

Mostly in physics work and rough capacitor or battery estimates: you have an energy figure and a charge figure and want the potential difference between them. For a live circuit, a voltmeter is faster and more reliable.