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E = mc² Calculator (Mass–Energy)

Convert mass into energy and back with a realistic efficiency — 100 % for antimatter, 0.7 % for fusion, 0.09 % for fission — and read the result in joules, kWh, kcal and tonnes of TNT.

Related tools

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Photon Energy CalculatorEnter a wavelength, a frequency or an energy and get all three from E = hf = hc/λ, in eV, keV, MeV and joules, with the spectral band named and the colour identified for visible light.Weight from mass calculatorCompute the weight (force of gravity) of an object from its mass (W = mg).Kinetic energy calculatorCompute kinetic energy from mass and velocity (½mv²).Potential energy calculatorCompute gravitational potential energy from mass, height and gravity (E = mgh).Spring potential energy calculatorCompute the elastic potential energy stored in a spring (E = ½kx²).Kinetic energy chicken cookerA famous physics joke, made calculable: could you cook a chicken by throwing it? Each throw deposits kinetic energy ½mv²; cooking it needs enough heat to raise it to 74 °C (Q = m·c·ΔT). It reports the energy per throw and how many throws — at your chosen speed — the meal would take.Molar Mass CalculatorCompute the molar mass of any chemical formula, with per-element composition.Appliance Energy Cost CalculatorTurn any appliance's wattage and daily use into a daily, monthly and yearly running cost in your currency.

Open E = mc² Calculator (Mass–Energy) and you get an answer straight away, with no account to create. Its place is under Physics; Photon Energy Calculator and Weight from mass calculator answer the questions closest to this one.

How to use it

  1. Open the tool — no signup or install needed.
  2. Enter your input or adjust the available options.
  3. Get your result instantly, then copy or download it.

Frequently asked questions

What does E = mc² Calculator (Mass–Energy) do?

Convert mass into energy and back with a realistic efficiency — 100 % for antimatter, 0.7 % for fusion, 0.09 % for fission — and read the result in joules, kWh, kcal and tonnes of TNT.

When would I actually use this?

Checking a homework answer, sizing something before building it, and getting an order of magnitude before committing to a design — a torque on a bolt, the force a spring returns, the frequency a circuit resonates at, how long light takes to arrive.

What is the most common mistake?

Feeding in a value in the wrong unit. Physics formulas assume SI throughout, so grams instead of kilograms or centimetres instead of metres shifts the answer by powers of ten without any warning.

How is E = mc² Calculator (Mass–Energy) different from Photon Energy Calculator?

They sit next to each other but answer different questions: Photon Energy Calculator is the one to open when you need it to enter a wavelength, a frequency or an energy and get all three from E = hf = hc/λ, in eV, keV, MeV and joules, with the spectral band named and the colour identified for visible light. Pick whichever matches what you're starting from — both are free.

Is there a tool for the next step?

Weight from mass calculator is the closest one after this: Compute the weight (force of gravity) of an object from its mass (W = mg).

What else is worth having open alongside it?

Kinetic energy calculator and Potential energy calculator — they come up in the same task often enough to be worth a second tab.

Where do the figures come from?

Constants are the CODATA values and the formulas are the textbook ones. Most assume an idealised case — no air resistance, no friction, a point mass — and the tool says so where the simplification matters.

Further reading

All guides →
ExplainerWhat Is Kinetic Energy? The KE = ½mv² Formula ExplainedKinetic energy is the energy of motion, given by KE = ½mv². Learn what the formula means, why speed matters most, and see worked examples in joules.ExplainerHooke's Law Explained: F = kx, Real Spring Constants, and Where It Stops HoldingHooke's law says force is proportional to stretch — but only below the elastic limit. Here is F = kx with worked numbers, what a 200 N/m spring actually feels like, and how springs combine.ExplainerHow Buoyancy Works: Archimedes' Principle, and Why Ice Floats With 10.5 % Above WaterThe upward force equals the weight of the fluid pushed aside. That one sentence decides whether something floats, and if it floats, exactly how much of it stays under.ExplainerProjectile Motion Explained: Range, Height, Flight Time — and Why 45° Is Not Always BestThree formulas cover the whole of projectile motion on level ground. The catch is level ground: the moment launch and landing heights differ, the 45° result stops being true.How-toHow to Calculate Molar Mass, Including Parentheses and HydratesMultiply each atomic weight by its subscript and add. The only three places the arithmetic goes wrong are a subscript after a closing bracket, the hydrate dot, and a capital letter typed as a lowercase one.ExplainerThe Ideal Gas Law Explained: PV = nRT, R in Every Unit, and Where It BreaksPV = nRT holds when the gas is dilute and far from condensing. The value of R depends entirely on the units you feed it, and the temperature is never in degrees Celsius.