On-line calculator

Calculate magnetic fields from selected UK overhead power lines

Use the following calculator to investigate the factors that affect the magnetic fields produced by some of the standard designs of UK high-voltage overhead power lines.

You can test the effect of changing:

You can choose what range of distances (perpendicular to the line) to calculate over and the calculator will give you a graph and tell you the maximum.

You select or enter these values in the white cells (don’t try to edit the other cells – it will reject any changes to these).

Note: this tool is hosted on the Cloud. If you do not see the calculator, but get a message “proxy server is refusing connections”, this probably means that Cloud services are barred by your internet service. Contact us if you would particularly like to see this tool. The full functionality is supported only for desktops and laptops – mobile devices may not allow all the interactivity.

See our page on the principles of calculating fields for an explanation of the principle this calculator uses.

Please note this calculator is provided to help illustrate the factors that affect the field from a power line.  We do not provide every single line design, and we are unable to tell you the current in specific lines, so it is not really intended as a way of calculating the field in a specific home from a specific line.

Please note:

  • so far, this only calculates magnetic fields not electric fields
  • for the currents: you can include a minus sign to change the direction of the current.  Both currents + or both currents – means the two currents are in the same direction; one + and the other – means opposite directions
  • it calculates by approximating the conductors as infinite straight lines (it ignores the sag) so you need to enter the ground clearance at the point of interest – a lower value if you want to calculate towards the middle of the span (12 m would be a good typical value for a 400 kV line) or a higher value if you want to calculate at a pylon (perhaps 20 m)
  • it calculates for balanced currents within each circuit, though you can choose whatever amount of balance between the circuits you want
  • we have programmed in some of the more common designs of line but we’ll be happy to add any others you’d particularly like – contact us.  We also give the actual dimensions for some of these designs.


We have checked this calculator against other software which in turn we’ve checked against actual measurements, so we believe it gives correct answers, but we cannot be responsible for any errors.

All feedback or requests for improvement welcome!