Energy Converter
J, kJ, Wh, kWh, BTU, cal, ft·lbf — plus the eV for the physicists.
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The engineering
Energy wears a different suit in every room: utilities bill kWh, US HVAC and combustion quote BTU (and MMBtu on the gas bill), chemists log thermochemical calories, mechanical legacy work keeps ft·lbf, and semiconductor or nuclear people count electron-volts — 1.602×10⁻¹⁹ J each, included here as a row because band gaps and food labels occasionally share a meeting.
The classic traps are the near-twins: the IT calorie (4.1868 J) vs the thermochemical calorie used here (4.184 J), the IT BTU (1055.056 J) vs the thermochemical BTU (1054.35 J), and the food 'Calorie' that is secretly a kilocalorie — three names, factors of 1.0007 to 1000 apart. State which table you used; this card uses the IT BTU and thermochemical calorie, both NIST-listed.
Where this math comes from
That heat and work are the same currency took a century to price. Count Rumford's 1798 cannon-boring experiments showed friction makes heat without limit; Julius Robert Mayer argued the equivalence in 1842; and James Prescott Joule nailed the exchange rate from 1843 to 1850 with paddle-wheel experiments precise enough to feel — about 4.2 units of mechanical work per unit of heat, measured on his honeymoon-adjacent waterfall thermometry and in his Manchester brewery lab.
The unit was named the joule in 1889 by the British Association, and the 9th CGPM (1948) made it the SI unit of heat as well as work, formally demoting the calorie — which promptly refused to die in chemistry and on food labels. The kilowatt-hour, meanwhile, won the billing meter in the 1880s because selling electricity by the joule produces absurdly large numbers, and 3.6 MJ per kWh has been a conversion-table staple ever since.
- 1798Count RumfordCannon-boring: friction makes heat without limit.
- 1842Julius Robert MayerStates the mechanical equivalent of heat.
- 1843James Prescott JoulePaddle-wheel experiments price heat in work, refined through 1850.
- 1889British AssociationThe joule named in Joule's honor.
- 19489th CGPMJoule adopted for heat — the calorie officially deprecated.
See the full timeline of the math behind every calculator →
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