Energy Density of Fields Calculator

By Dominik Czernia, PhD candidate
Last updated: Jul 06, 2018

Did you know that fields, both electric and magnetic, 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 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 vacuum using energy density equation.

What is energy density?

Energy density is the amount of energy that can be stored in a given volume (or mass) of a substance. The higher the energy density of a substance is, the greater amount of energy is stored. 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 battery stores energy in the electric field for a cell phone (check our battery capacity calculator),
  • every typical magnet stores energy in the magnetic field,
  • the heat which warms us up is from an electromagnetic wave (light), where the energy is stored in oscillating electric and magnetic fields.

Energy density equation

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

U = E² * (ϵ₀ / 2) + B² / (μ₀ * 2)


  • U is the energy density,
  • E is the electric field,
  • B is the magnetic field,
  • ϵ₀ is the vacuum permittivity ϵ₀ ≈ 8.8541 * 10^(-12) F/m,
  • μ₀ is the vacuum permeability μ₀ ≈ 4π * 10^(-7) H/m.

The total energy density of 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

Electric and magnetic fields can be found 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:

Dominik Czernia, PhD candidate