Mass-energy equivalence, made practical

Convert mass, energy, light, and photons.

Enter the quantities you know. EISMC² fills the rest using compatible SI units, keeping Einstein's most famous relationship visible while you work.

Interactive calculator

Known values

Type numbers into any fields. Calculated fields are labelled automatically; edit a calculated value to make it one of your known values.

Core: energy equals mass times the speed of light squared

Energy E J joules

Mass m kg kilograms

Speed of light c m/s exact value in vacuum

Photon energy and wavelength

Frequency f Hz cycles per second

Wavelength λ m lambda, metres

Planck constant h J s exact SI constant

Photon momentum p kg m/s p = E/c

Energy scales

Electronvolts eV eV particle energy unit

Kilowatt-hours kWh kWh 1 kWh = 3.6 MJ

TNT equivalent kg TNT 1 kg TNT = 4.184 MJ

Enter at least two related values to begin.

Formula sheet

E = mc²Energy equals mass times the speed of light squared.
m = E/c²Mass can be read as the mass equivalent of an energy.
E = hfA photon's energy equals Planck's constant times frequency.
c = fλLight speed equals frequency times wavelength in vacuum.
E = hc/λPhoton energy can be found from wavelength.
p = E/cPhoton momentum equals energy divided by the speed of light.
1 eV = 1.602176634 x 10^-19 JElectronvolts convert particle-scale energy to joules.
1 kWh = 3.6 x 10⁶ JKilowatt-hours convert everyday electrical energy to joules.
1 kg TNT = 4.184 x 10⁶ JTNT equivalent gives a familiar energy scale.

Reference

All calculations use SI units internally. The exact constants are editable so you can explore the relationships, then recalculate them when needed.

E: Energy, measured in joules J.
m: Mass, measured in kilograms kg. A lowercase m can also mean metres when used as a unit.
c: Speed of light in vacuum, exactly 299,792,458 metres per second.
f: Frequency, measured in hertz Hz, or cycles per second.
λ: Wavelength, measured in metres m.
h: Planck constant, exactly 6.62607015 x 10^-34 joule seconds.
p: Photon momentum, measured in kilogram metres per second, kg m/s.
eV: Electronvolt, a particle-physics energy unit equal to 1.602176634 x 10^-19 joules.
kWh: Kilowatt-hour, an energy unit equal to 3,600,000 joules.
TNT: One kilogram of TNT equivalent is defined here as 4,184,000 joules.