Energy Unit Converter

What is an energy unit converter?#

Energy is one quantity — the capacity to do work — but it gets counted in wildly different units depending on who is asking. A physicist measures a particle in electronvolts (eV), an engineer sizes a motor in joules (J), a dietitian counts food Calories (kcal), an electricity bill is in kilowatt-hours (kWh), and an HVAC spec sheet speaks in BTU. They are all measuring the same thing; the only thing that differs is the scale factor.

This page converts between nine common energy units through one exact bridge: every unit is defined by its value in joules (the SI base unit). To go from one unit to another, the tool passes through joules — value × (unit_in_joules) ÷ (target_in_joules). Because each factor is an exact defined constant (the thermochemical calorie, the International Table BTU, the 2019-SI exact electronvolt), conversions round-trip cleanly instead of accumulating error.

How to use it#

1. Type the Value you want to convert.
2. Pick the From unit from the first dropdown — J, kJ, cal, kcal, Wh, kWh, BTU, eV, or ft·lbf.
3. Pick the To unit from the second dropdown. It defaults to kcal because the most common question is “how many food Calories is this?”
4. The Result line updates with the converted value.
5. Keep the All units section open to see the same value expressed in all nine units at once — useful when you are not sure which unit the answer is cleanest in.

Key features#

• Nine units, one exact bridge. Every conversion passes through joules using defined constants, so there is no compounding rounding error.
• Food Calorie aware. kcal is the food Calorie (1 Cal on a nutrition label = 1 kcal = 1000 cal), correctly distinguished from the small cal.
• Scientific-notation friendly. Electronvolt conversions span ~19 orders of magnitude; results are rounded to 6 significant digits so they stay readable without losing precision.
• Live “all units” table so you can scan across J / kJ / cal / kcal / Wh / kWh / BTU / eV / ft·lbf in one glance.
• 100% local. Nothing is uploaded.

Worked example#

A nutrition label says a snack has 250 food Calories. You want that in plain joules and in watt-hours. Enter 250 in Value, set From to kcal, and read the All units table:

So 250 food Calories is about 1.05 megajoules of chemical energy — roughly the same energy as a smartphone battery (~11 Wh) discharged about 26 times over, or about a third of a kilowatt-hour of electricity. The eV value (6.5 × 10²⁴) looks absurd only because the electronvolt is inconceivably tiny; it is the right number, just written in scientific notation.

For an electricity-bill question, set Value = 1, From = kWh: 1 kWh = 3,600,000 J = 3.6 MJ = 0.00029 kWh worth in BTU is 3412 BTU, which is why portable air conditioners are often rated “10,000 BTU” — that is about 2.9 kWh of cooling capacity.

FAQ#

What is the difference between a Calorie and a calorie?#

A food Calorie (capital C, the one on nutrition labels) is a kilocalorie — 1 Cal = 1 kcal = 1000 small calories. A small calorie is the energy to warm 1 gram of water by 1°C. The tool’s kcal field is the food Calorie; the cal field is the small one, a thousandth of it. Mixing them up is the single most common energy-unit mistake.

Are these thermochemical or International Table units?#

The calorie here is the thermochemical calorie (1 cal = 4.184 J exactly). The BTU is the International Table BTU (1 BTU = 1055.05585 J exactly). These are the standard choices in physics and food science respectively; they differ from the steam-table or 15°C calorie by under 0.1%, which only matters at laboratory precision.

Why is the electronvolt value so enormous (or tiny)?#

Because one electronvolt is 1.602176634 × 10⁻¹⁹ J — the kinetic energy a single electron gains crossing 1 volt. Everyday energies contain a staggering number of particles, so the eV count is astronomically large. The tool switches to scientific notation automatically so the number stays legible.

How do I convert my electricity bill to joules?#

1 kWh = 3.6 MJ exactly (1 W·s = 1 J, times 3600 seconds). So a monthly bill of 200 kWh is 720 megajoules — set From = kWh, Value = 200, and read the joule row. This is also the cleanest way to compare the energy in your electricity use against, say, the food energy you eat in a month.