Energy Density of Fields Calculator

Created by Dominik Czernia, PhD
Reviewed by Bogna Szyk and Steven Wooding
Last updated: Jan 24, 2023

Did you know that electric and magnetic fields can store a specific amount of energy? Now you can compute that energy with our energy density of fields calculator. It can be especially useful when describing electromagnetic waves – rays of light.

In the context of the physics of conductive fluids, energy density behaves like an additional pressure that adds to the gas pressure. In the text below, we have described what energy density is and how to calculate energy density in a vacuum using the energy density equation.

What is energy density?

Energy density is the amount of energy stored in a given volume (or mass) of a substance. The higher the energy density of a substance, the greater the energy it can store. There are many different types of energy stored in materials, for example: nuclear, chemical, electrochemical, or electrical.

Electric and magnetic fields can also store energy. For example:

  • Lithium-ion batteries stores energy in the electric field for a cell phone (check our battery capacity calculator);

  • Every typical magnet stores energy in the magnetic field; and

  • The heat from an electromagnetic wave (light), where the energy is stored in oscillating electric and magnetic fields.

Energy density equation

The energy density of an electric field and magnetic field in a vacuum can be estimated with the following energy density equation:

u=ε02E2+12μ0B2,u = \frac{\varepsilon_0}{2}\bold{E}^2 + \frac{1}{2\mu_0}\bold{B}^2,


  • uu – Energy density;
  • E\bold{E} – Electric field;
  • B\bold{B} – Magnetic field;
  • ε0\varepsilon_0 – Vacuum permittivity ε08.8541×1012 F/m\varepsilon_0 \approx 8.8541 \times 10^{-12}\ \rm{F/m}; and
  • μ0\mu_0 – Vacuum permeability μ04π×107 H/m\mu_0 \approx 4\pi \times 10^{-7}\ \rm{H/m}.

The total energy density of an electric field and magnetic field in SI units is expressed in joules per cubic meter J/m³. It means that every 1 m³ volume of substance stores 1 J of energy.

Sources of electric and magnetic fields

You can find electric and magnetic fields everywhere! We have already mentioned that sunlight consists of oscillating electric and magnetic fields.

We encourage you to check our other calculators, where we have further presented different sources of both fields in the electric field of a point charge calculator and the solenoid magnetic field calculator.


How can I calculate energy density?

The formula for the energy density of fields is u = (ε0/2)E2+ 1/(2μ0)B2. To calculate it:

  1. Find the energy density for the electric field, e.g., E = 2,000 kN/C:

    uE = (8.8541 x 10-12/2)×(2 × 106)2 = 17.71 J/m3.

  2. Put the value of B = 3 × 10-2 T:

    uB = 1/(2 × 4π × 10-7)× (3 × 10-2)2 = 0.0003581 J/m3.

  3. Sum up: 17.71 J/m3 + 0.0003581 J/m3 = 17.71 J/m3.

Which fuels have the highest energy density?

Uranium has the highest energy density among available fuels. Next are thorium, hydrogen, methane, liquid natural gas, and propane. The higher the energy density, the higher the fuel quality and the simpler the chemical structure. Therefore, gases are generally higher-quality fuels than wood or coal.

What is the energy density of a capacitor having the electric field 50 N/C?

Given E = 50 N/C, energy density is 1.1068 × 10-8 J/m3.

  1. Use formula u = (ε0/2)E2 = (8.8541 x 10-12/2)×(50)2 = 1.1068 × 10-8 J/m3.
  2. Note that magnetic field B = 0.
  3. Total energy density is 1.1068 × 10-8 + 0 = 1.1068 × 10-8 J/m3.

Is there a theoretical limit for energy density?

Yes, energy density determines the total amount of energy you can store in a specific volume, so you cannot add infinite amounts of energy to a finite volume. The energy density limit is proportional to the area enclosing the volume. Of the known objects, some types of stars have the highest energy density.

How can we increase the energy density of a battery?

Mainly by using different battery materials that happen to have better energy density. Also, keep optimum operating temperatures and the battery in good condition by maintaining it properly. These can help to increase the amount of energy stored in a battery.

Dominik Czernia, PhD
Electric field
Magnetic field
Energy density
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