With our energy conversion calculator, you can convert energy between a wide range of various units. The magnitude of energy differs significantly in various aspects of life: from the low energies at the microscopic level of atoms to the enormous energies of nuclear bombs. That's why people have introduced many alternative units of energy.

You might use our energy conversion calculator differently, depending on your needs. In the text below, you will find an explanation of how you can convert between some basic energy units, such as:

  • Joules to kcal;
  • Joules to kWh; and
  • Joules to eV.

We have also provided a possibility to compute less frequently used units, like tons of TNT to joules or therms to joules. Did you know that energy can be expressed in units of weight, temperature, or length? Read on if you want to know how our energy converter:

  • Converts mass to energy;
  • Converts temperature to energy; and
  • Converts length to energy.

Joules to kcal and kcal to joules conversion

Kilocalories (kcal) usually measure food energy. One calorie is the amount of thermal energy necessary to raise the water temperature by one degree (in the temperature of 14.5 degrees Celsius). Knowing this, you can compute the equivalent value of kilocalories expressed in joules, for example, with our specific heat calculator. The result is:

1 kcal = 4184 J,

and conversely:

1 J ≈ 0.000239 kcal.

Joules to kWh and kWh to joules conversion

Kilowatt-hours (kWh) are used in everyday electricity. It corresponds to the amount of energy consumed by a device within 1 hour with the power of 1000 watts (one kilowatt). Therefore, we can write:

1 kWh=1 kW×1 h=1000 W×3600 s=3,600,000 J=3.6 MJ\begin{split} 1\ \rm{kWh} &= 1\ \rm{kW} \times 1\ \rm{h}\\ &= 1000\ \rm{W} \times 3600\ \rm{s}\\ &= \text{3,600,000 J}\\ &= 3.6\ \rm{MJ} \end{split}

And conversely:

1 MJ0.278 kWh1\ \rm{MJ} \approx 0.278\ \rm{kWh}

Joules to eV and eV to joules conversion

In physics and chemistry, we often need to use very small energies to describe phenomena on the atomic scale. For that purpose, we can use a convenient unit called electronvolt (eV). One electronvolt is the energy obtained (or lost) by an electron that has traveled in an electric field with a potential difference of 1 volt. This energy equals:

1 eV=1 e×1 V1.60217×1019 C×1 V=1.60217×1019 J\small \begin{split} 1\ \rm{eV} &= 1\ \rm e \times 1\ \rm V\\ &\approx 1.60217 \times 10^{-19}\ \rm C \times 1\ V\\ &= 1.60217 \times 10^{-19}\ \rm J \end{split}

And conversely:

1 J=6.2415×1018 eV\small 1\ \rm J = 6.2415 \times 10^{18}\ \rm{eV}

In the above equation, e1.60217×1019 C\small\rm e \approx 1.60217 \times 10^{-19}\ C is the elementary charge of an electron. Be sure to check our acceleration in the electric field calculator, where you can estimate the acceleration of any charged particle in the electric field.

Tons of TNT and therms to joules

A ton of TNT is the energy unit which is the basic parameter of nuclear weapons, but it is also used to determine the strength of other events, such as meteorite impacts. One gram of TNT releases from 4100 to 4600 joules of energy from the explosion; it is conventionally assumed that it is exactly 4184 J. Therefore:

1 ton ofTNT =1 t×4184 J/g=1,000,000 g×4184 J/g=4.184 GJ=1 Gcal\small \begin{split} \begin{split}1\ \text{ton of}\\ \text{TNT}\ \end{split} &= 1\ \rm t \times 4184\ J/g\\ &= \text{1,000,000 g} \times 4184\ \rm{J/g}\\ &= 4.184\ \rm GJ\\ &= 1\ \rm Gcal \end{split}

A therm is a unit of heat energy that is equivalent to burning 100 cubic feet (or 2.83 cubic meters) of natural gas. Therms are usually used by natural gas companies to calculate the actual energy use. This energy unit is useful for them because it depends only on the composition of the gas (and not on pressure or temperature). The conversion to joules is shown below:

1 them105.5 MJ\small 1\ \rm them \approx 105.5\ MJ

Mass to energy conversion

You probably heard about the famous E = mc2 equation. Albert Einstein proposed that every mass in the world is just energy waiting to be set free. Although it is not achievable yet, we can use this formula to express the energy in the units of weight. The 1 J\small 1\ \rm J of energy "weights":

m=E/c2(1 J)/(2.988×108 m/s)21.113×1017 kg\begin{split} m &= E/c^2\\ &\approx (1\ \rm J)/(2.988 \times 10^8\ m/s)^2\\ &\approx 1.113 \times 10^{-17}\ \rm kg \end{split}

where c2.998×108 m/s\small c \approx 2.998 \times 10^8\ \rm m/s is the light speed.

This unit of energy is, for example, used to describe the annihilation process where an electron (with the mass me=9.109×1031 kg\small m_{\rm e} = 9.109 \times 10^{-31}\ \rm kg) collides with a positron (particle with the same mass but opposite charge) and creates two photons with the energy 1.022 MeV\small 1.022\ \rm MeV in total.

You can check it with our energy conversion calculator! Just remember that electron and positron will have the mass of 2 me2\ m_{\rm e} in total.

Temperature to energy conversion

In the classical kinetic theory of gases, the average kinetic energy per one particle is proportional to the product kB×T\small k_{\rm B} \times T, where T\small T is the temperature and kB1.3806×1023 J/K\small k_{\rm B} \approx 1.3806 \times 10^{-23}\ \rm J/K is the Boltzmann constant. Therefore, we can say that 1 J\small 1\ \rm J of energy has the temperature of:

T=E/kB(1 J)/(1.3806×1023 J/K)7.2432×1022 K\begin{split} T &= E/k_{\rm B}\\ &\approx (1\ \rm J)/(1.3806 \times 10^{-23}\ J/K)\\ &\approx 7.2432 \times 10^{22}\ \rm K \end{split}

We sometimes use this unit of energy in physics or chemistry. For example, when we want to describe the magnitude of thermal vibrations in the crystal lattice of the material. If we say that the energy of vibrating atoms is 200 K\small 200\ \rm K, it means that their vibrational energy is 17.235 meV\small 17.235\ \rm meV (check it with our energy converter).

Length to energy conversion

One of the central parts of the theory of quantum mechanics is matter waves. Every moving particle with specific kinetic energy behaves like a wave and can be described as having a specific wavelength. Check our De Broglie wavelength calculator to learn more about it.

In spectroscopy (the study of the interaction between matter and waves), measuring the energy of particles or light in units of reciprocal centimeters (cm⁻¹) is common. The formula for appropriate conversion is provided by Planck's equation:

E=hcλE = \frac{hc}{\lambda}

where h=6.6261×1034 m2kg/s\small h = 6.6261 \times 10^{-34}\ \rm m^2\cdot kg /s is the Planck constant, c2.998×108 m/s\small c \approx 2.998 \times 10^8\ \rm m/s is the light speed and λ\small \lambda is the wavelength. If we write down the above equation in a different form, we can say that 1 J\small 1\ \rm J of energy will have:

1/λ=E/hc=(1 J)/(6.6261×1034 m2kg/s×2.998×108 m/s)=5.034×1024 m1=5.034×1022 cm1\footnotesize \begin{split} 1/\lambda &= E/hc\\ &= (1\ \rm J)/(6.6261 \times 10^{-34}\ m^2\cdot kg/s\\ &\quad \times 2.998 \times 10^8\ \rm m/s)\\[.3em] &= 5.034 \times 10^{24}\ \rm m^{-1}\\ &= 5.034 \times 10^{22}\ \rm cm^{-1} \end{split}

You can check with our energy conversion calculator that a reciprocal centimeter describes rather low energies. For example, 50 meV=403.28 cm1\small 50\ \rm meV = 403.28\ cm^{-1}.

Dominik Czernia, PhD
SI unit for energy
Joule
J
Big energies
Calorie
kcal
Watt-hour
kWh
Therm
therms
Ton of TNT
tons of TNT
Microscopic world
Electronovolt
eV
Temperature
K
Reciprocal length
cm
⁻¹
Mass
me
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