About the Volts to Joules Calculator
A voltage tells you how much energy each coulomb of charge carries, so volts become joules once you know the charge. This calculator multiplies the voltage by the charge to give the energy in joules, and also shows it in kilojoules, watt-hours and electronvolts. The charge can be typed in coulombs, milliamp-hours, amp-hours or as a number of electrons, so it works for a phone battery as well as a physics problem about a single particle. Results appear as you type and nothing leaves your browser.
Updated October 2026
How to use the Volts to Joules Calculator
- 1Enter the voltage in volts.
- 2Enter the amount of charge.
- 3Pick the unit of charge: coulombs, mAh, Ah or electrons.
- 4Read the energy in joules.
- 5Check the kJ, Wh and eV rows if you need another unit.
Volts, coulombs and joules
One volt means one joule of energy for every coulomb of charge that passes. That makes the conversion a simple multiplication once the charge is known. One coulomb is the charge moved by a current of one amp in one second, so currents and times can be turned into coulombs too: amps × seconds = coulombs.
Battery energy from the label
Battery labels give the capacity in mAh or Ah and a nominal voltage. Multiply the capacity in amp-hours by 3,600 to get coulombs, then by the voltage to get joules. Divide joules by 3,600 for watt-hours, the unit airlines use for their battery limits.
- 1 mAh = 3.6 C
- 1 Ah = 3,600 C
- 1 Wh = 3,600 J
- 1 eV = 1.602176634 × 10⁻¹⁹ J
Physics problems with particles
For a single electron or ion, choose Electrons as the unit and type the number of elementary charges. An electron accelerated through 1,000 V gains 1,000 eV, which the calculator shows as about 1.6 × 10⁻¹⁶ J. Very large or small results are written in scientific notation so no digits are lost.
Joules or watt-hours?
Joules are the SI unit and suit physics problems; watt-hours are easier for batteries and power banks because they connect to power and time. A 100 Wh power bank holds 360,000 J. If you want to know how long a battery runs a device, divide the watt-hours by the device's watts to get hours of use.
Works on any screen
On a phone the boxes stack one under the other with the answer below; on a computer they sit side by side. The page needs no account and stores nothing. Joules come from charge as well as voltage. Reverse the sum with the Joules to Volts Calculator, use the eV to Volts Calculator for single electrons, and convert battery capacity with the mAh to Wh Calculator.
Features
- Joules, kJ, Wh and eV at once
- Charge in C, mAh, Ah or electrons
- Exact elementary charge (1.602176634 × 10⁻¹⁹ C)
- Handy for batteries and capacitors
- Copy result button
- Runs entirely in your browser
Frequently asked questions
How do I convert volts to joules?
Multiply the voltage by the charge in coulombs: J = V × C. 12 V moving 5 C of charge transfers 60 J.
How many joules is a 3.7 V, 3,000 mAh battery?
3,000 mAh is 10,800 C, so the energy is 3.7 × 10,800 ≈ 39,960 J, or about 11.1 Wh.
Can I convert volts to joules without the charge?
No. A volt is a joule per coulomb, so you need to know how much charge moves. A tiny spark and a car battery can share the same voltage but carry very different energy.
What is the link between joules and electronvolts?
One electronvolt is the energy one electron gains crossing one volt: 1 eV = 1.602176634 × 10⁻¹⁹ J.
Is this the energy stored in a capacitor?
Not quite. A capacitor stores ½ × C × V² because its voltage falls as it discharges. This calculator gives the energy for a charge moved across a steady voltage, as with a battery.
Why does my battery give less energy in real life?
The voltage drops as the battery empties and some energy is lost as heat, so the usable energy is usually 80 to 95% of the figure from the label.