Energy Conversion Calculator

Convert energy between joules, calories, kilowatt-hours, and microscopic units.

Supports 9 energy forms with nano to mega prefixes and SI-exact constants.

Updated August 20, 2026

CreatorFrank Zhao

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Introduction / overview

The Energy Conversion Calculator is a single-stop converter for every way energy is quoted — from everyday kitchen and electricity bills to spectroscopy and particle physics. Instead of juggling separate factors by hand, you type a value in any one field and the other eight update automatically.

SI & everyday

Joule with nJ to MJ, Calorie (cal / kcal / Mcal) and Watt-hour (Wh / kWh / MWh), plus Therm and ton of TNT for gas and explosive equivalents.

Microscopic

Electronvolt from neV to MeV, wavenumber as reciprocal length (mm⁻¹ to yd⁻¹), and mass-energy via $E=mc^{2}$ .

Thermal

Temperature as $k_{B}T$ in K, °C and °F — useful when a paper quotes an energy as "200 K" or "17 meV".

Typical users are students checking a textbook conversion, cooks and engineers flipping between kcal and kWh, and researchers moving between eV, wavenumbers and kelvins without re-typing constants. All nine fields are bidirectional — edit any one and the rest become derived results.

How to use / quick start

Pick the field that matches your starting number

For example, choose Joule if you have joules, Calorie if the label says kcal, or Temperature if a paper gives an energy as kelvins.

Choose the display unit inside that field

Each field has its own unit menu — for instance Joule offers nJ through MJ, Calorie offers cal / kcal / Mcal, and Temperature offers K / °C / °F. The conversion is applied instantly.

Type the value — the other eight fields compute at once

Edit any field at any time. Clearing a field leaves it blank until you type again; you do not need a separate reset between conversions.

Read the result in the unit you need

Change the target field's unit without re-typing — the displayed number re-renders while the underlying energy stays the same.

Worked example — food label to electricity

A snack is labelled 250 kcal. How many kilowatt-hours is that, and what is the photon wavenumber of the same energy?

E = 250\ \mathrm{kcal}

=

250 \times 4184\ \mathrm{J}

=

1{,}046{,}000\ \mathrm{J}

E_{\mathrm{kWh}}

=

E/3{,}600{,}000

=

0.2906\ \mathrm{kWh}

\tilde{\nu}

=

E/(h c)

=

5.265\times 10^{28}\ \mathrm{m^{-1}}

=

5.265\times 10^{26}\ \mathrm{cm^{-1}}

Type 250 into Calorie → kcal. The Watt-hour field shows about 0.291 kWh and Reciprocal length shows on the order of 10²⁶ cm⁻¹ — the same joule amount seen in three different unit systems without re-entering the number.

Real-world examples

Spectroscopy check

A paper lists a vibrational mode at 500 cm⁻¹. What is that in meV and in kelvins?

Enter 500 in Reciprocal length → cm⁻¹. You get about 62.0 meV and 719 K.

E = h c \times 50{,}000\ \mathrm{m^{-1}}

\approx

9.93\times10^{-21}\ \mathrm{J}

Useful when comparing infrared data (cm⁻¹) with electronic energies (eV) or thermal scales.

Particle mass to energy

One electron rest mass — how much energy is that in MeV and in joules?

Enter 1 in Mass → me. You get about 0.511 MeV (≈ 8.19×10⁻¹⁴ J). Switching Mass to kg shows the underlying $9.11\times10^{-31}\ \mathrm{kg}$ .

E = m_{e}c^{2}

=

0.511\ \mathrm{MeV}

The same field also handles proton, neutron and atomic-mass-unit equivalents.

Tips & best practices

Enter once, read everywhere. You do not need to press convert or cycle fields. Editing any field automatically makes the other eight derived — pick whichever starting unit your source actually uses.
Use the unit menu, not manual factors. Selecting kcal vs. Mcal or Wh vs. MWh applies the exact factor (e.g. 1 kcal = 4184 J, 1 kWh = 3 600 000 J). Retyping the same number in a different field with the wrong prefix is the most common slip.
Temperature is an energy scale here. This calculator maps temperature via $E = k_{B} T$ . A "negative energy" is blocked, but a negative °C or °F is valid because it still corresponds to a positive kelvin value.
Compare at the same prefix. When checking eV vs. wavenumber, switch both fields to meV and cm⁻¹ first — the ratio is easier to judge than mixing MeV with mm⁻¹.

Next step: Converting a rate rather than an amount? Try the Power Converter for watts and horsepower. For the underlying distance or mass constant itself, use the Length Converter or Weight Converter.

Calculation method

Every field stores the same physical quantity in joules. The visible unit only changes the display factor. When you edit any field, that field becomes the source and the other eight are recomputed through that common joule value.

Everyday energies

E_{J}=E_{\text{cal}}\times 4.184

;

E_{J}=E_{\text{Wh}}\times 3600

E_{J}=E_{\text{therm}}\times 105\,506\,000

;

E_{J}=E_{\text{TNT}}\times 4\,184\,000\,000

1 thermochemical calorie is defined as 4.184 J. Wh, therm and ton of TNT follow by exact multiplication; SI prefixes (nJ, mJ, kJ, MJ, etc.) scale by powers of ten.

Microscopic — electronvolt

E_{J}=E_{\text{eV}}\times 1.602176634\times10^{-19}

The 2019 SI defines the elementary charge exactly, so 1 eV is an exact number of joules. Nano to mega prefixes scale it by $10^{-9}$ to $10^{6}$ .

Thermal — temperature as energy

E_{J}=k_{B}T_{K}

,\quad

k_{B}=1.380649\times10^{-23}\ \mathrm{J\,K^{-1}}

T_{K}

=

T_{C}+273.15

=

(T_{F}-32)\tfrac{5}{9}+273.15

The display can be K, °C or °F; the calculator first converts to kelvins, then multiplies by $k_{B}$ .

Spectroscopy — wavenumber

E_{J}=h\,c\,\tilde{\nu}

,\quad

h=6.62607015\times10^{-34}\ \mathrm{J\,s}

,\quad

c=299\,792\,458\ \mathrm{m\,s^{-1}}

\tilde{\nu}_{[\mathrm{m^{-1}}]}

=

N_{[\mathrm{cm^{-1}}]}/0.01

=

N_{[\mathrm{mm^{-1}}]}/0.001

The field shows $N$ as "per mm/cm/m/inch/ft/yd". The underlying wavenumber in m⁻¹ is $N / \ell$ where $\ell$ is that length in meters.

Mass-energy

E_{J}=m\,c^{2}

,\quad

c^{2}=8.987551787\times10^{16}\ \mathrm{m^{2}s^{-2}}

Display units range from µg/mg/g/dag/kg through grains, drachms, ounces and pounds to particle masses $m_{e}$ , $m_{p}$ , $m_{n}$ and atomic mass units $u$ . Each is first converted to kilograms, then multiplied by $c^{2}$ .

Variables

$E_{\text{J}}$ — energy in the common base, joules.
$k_{B}$ — Boltzmann constant, $1.380649\times10^{-23}\ \mathrm{J/K}$ .
$h,\ c$ — Planck constant and speed of light.
$\tilde{\nu},\ m,\ T$ — wavenumber, mass and temperature displays.

Related concepts

Why therm and TNT appear alongside eV

All nine fields hold the same joule amount under different conventions: the therm is a gas-billing convention (≈ 100 ft³ of natural gas), the ton of TNT is a convention of 4.184 GJ per metric ton, and the electronvolt is the SI exact charge times one volt. The calculator does not estimate any fuel property — it applies those fixed definitions.

Temperature vs. heat, wavenumber vs. wavelength

Here "temperature" is just $E/k_{B}$ , not the heat content of an object, and "reciprocal length" is $\tilde{\nu}=1/\lambda$ , not the wavelength itself. If you need the wavelength in nanometers, use $\lambda = 1/\tilde{\nu}$ after reading the cm⁻¹ value.

Need the rate form? Energy per time is power — see the Power Converter for W, kW and horsepower.

Frequently asked questions

1Is this the same calorie as on a food label?

Yes, food "Calories" with a capital C are kilocalories. Select Calorie → kcal for the label number. If a science source truly means the small calorie, switch that same field to cal (1 kcal = 1,000 cal = 4,184 J).

2Why does a negative value show an error?

Energy itself cannot be negative, so the Joule, Calorie, Watt-hour, Therm, TNT, Electronovolt, Reciprocal length and Mass fields are constrained to ≥ 0 and keep a negative entry visible with an error. Temperature is the exception — −40 °C or −40 °F corresponds to a positive kelvin energy and converts normally.

3Can I edit the result and have the input update?

Yes. All nine fields are peers. Editing an automatically computed value makes it the new source and the previous source becomes derived — the underlying math is symmetric (slope 1 through joules).

4Which mass units are available?

Micrograms through kilograms plus decagrams, grains, drachms, ounces, pounds, and the particle scales electron mass $m_{e}$ , proton $m_{p}$ , neutron $m_{n}$ and atomic mass units $u$ — all via $E=m c^{2}$ .

Limitations & disclaimers

•This is a unit-definition converter. It applies exact SI definitions (2019) and fixed conventions for therm and TNT; it does not model fuel composition, combustion efficiency, or measurement uncertainty of a real system.
•Very large or very small energies display in scientific notation and respect the calculator's display precision. Copy the full value from the input if you need every digit for a downstream calculation.
•Temperature here means thermal energy $k_{B}T$ . For thermodynamic design, calorimetry or safety cases, validate against domain references and, where relevant, a qualified professional.

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