E = mc² calculator
Enter a mass to see how much energy it is equivalent to by Einstein's formula E = mc², or enter an energy to find its mass. Masses can be in kilograms, grams, milligrams, pounds or atomic mass units; energies in joules, megajoules, kilowatt-hours or electronvolts.
Runs in your browser: nothing you enter is sent anywhere.
How to use it
- Choose whether to convert a mass to energy or an energy to mass.
- Enter the value and pick its unit.
- Read the result in your unit and in joules, kilowatt-hours and tonnes of TNT.
- Energy: E = m × c², with c = 299,792,458 m/s exactly.
- Mass: m = E ÷ c².
- Units: 1 kWh = 3.6 × 10⁶ J, 1 eV = 1.602176634 × 10⁻¹⁹ J, 1 tonne of TNT = 4.184 × 10⁹ J.
Worked example
1 g of mass: E = 0.001 × 299,792,458² = 8.98755 × 10¹³ J. That is 24,965,421.6316 kWh, and the TNT equivalent is 21,480.7643 t.
Going the other way, 1 kWh corresponds to 3.6 × 10⁶ ÷ 299,792,458² = 4.0055 × 10⁻¹¹ kg.
Tips
- This is rest energy. It is the total energy locked in the mass, not what a reaction releases. Fission converts under 0.1 % of the fuel’s mass into energy.
- Moving objects have more energy, γ × mc²; see the time dilation calculator for γ.
- Very large or small numbers are shown in scientific notation, for example 8.98755E16 J for 1 kg.
Frequently asked questions
What does E = mc² mean?
Mass and energy are equivalent. A mass m at rest has an energy E = m × c², where c = 299,792,458 m/s is the speed of light. Because c² is about 9 × 10¹⁶ m²/s², a tiny mass stands for an enormous energy.
How much energy is in 1 gram of matter?
0.001 kg × 299,792,458² = 8.98755 × 10¹³ J, which is about 25 million kWh or the energy of 21,481 tonnes of TNT.
How many MeV is one atomic mass unit?
1 u = 1.66053906660 × 10⁻²⁷ kg corresponds to 931.494 MeV. Nuclear physicists use this to turn the mass defect of a nucleus into its binding energy.
Does a charged battery weigh more?
In principle, yes. Storing 1 kWh adds 3.6 × 10⁶ J ÷ c² = 4.0 × 10⁻¹¹ kg, about 40 nanograms, far too little to weigh.