E = mc² Calculator (Mass–Energy Equivalence)
By Santaji GadepatilinUpdated November 2, 2026
Einstein's equation E = mc² says that mass and energy are two forms of the same thing, linked by the square of the speed of light. This E = mc² calculator converts a mass into its rest energy in joules, kilowatt-hours and TNT equivalent, and works backwards to find the tiny mass change behind chemical and nuclear energy.
How to use the E = mc² calculator
Enter a mass in grams, kilograms or another unit. The calculator returns the equivalent rest energy in kWh (switch to J, MJ or BTU), in terajoules and in kilotonnes of TNT. To find the mass equivalent of an energy, enter the energy and clear the mass field.
Mass–energy equivalence formula
E = m c² · c = 299,792,458 m/s
Example
Converting 1 gram of mass entirely into energy:
- E = 0.001 × 299,792,458² = 8.9876 × 10¹³ J
- = 24,965,422 kWh (24.97 GWh)
- = 21.48 kilotonnes of TNT
Rest energy of particles
| Particle | Mass | Rest energy |
|---|---|---|
| Electron | 9.1094 × 10⁻³¹ kg | 0.511 MeV |
| Proton | 1.6726 × 10⁻²⁷ kg | 938.272 MeV |
| Neutron | 1.6749 × 10⁻²⁷ kg | 939.565 MeV |
| Atomic mass unit (1 u) | 1.6605 × 10⁻²⁷ kg | 931.494 MeV |
Mass equivalent of everyday energies
| Energy source | Energy | Equivalent mass |
|---|---|---|
| 1 kWh of electricity | 3.600 × 10⁶ J | 4.006 × 10⁻¹¹ kg |
| 1 kg of petrol burned (≈ 46 MJ) | 4.600 × 10⁷ J | 5.118 × 10⁻¹⁰ kg |
| 1 tonne of TNT | 4.184 × 10⁹ J | 4.655 × 10⁻⁸ kg |
| 1 kg of U-235 fully fissioned (≈ 8.2 × 10¹³ J) | 8.200 × 10¹³ J | 9.124 × 10⁻⁴ kg |
Notes
- 1 kilotonne of TNT is defined as 4.184 × 10¹² J.
- In nuclear reactions the energy released equals the mass defect × c²; in radioactive decay this energy is carried by the emitted particles — see the radioactive activity calculator.
Frequently asked questions
How much energy is in 1 gram of mass?
E = 0.001 kg × (299,792,458 m/s)² ≈ 8.99 × 10¹³ J — about 25 million kWh, or 21.5 kilotonnes of TNT.
Does burning fuel convert mass into energy?
Yes, but the mass change is tiny. Burning 1 kg of petrol releases about 46 MJ, which corresponds to only about 5 × 10⁻¹⁰ kg of mass.
How much mass is converted in a nuclear reactor?
Fission of uranium-235 converts roughly 0.09% of the fuel's mass into energy, millions of times more per kilogram than chemical reactions.
What units does E = mc² use?
With mass in kilograms and c in metres per second, energy comes out in joules. The calculator converts grams, ounces and other units automatically.