A resource explaining how the strength of a magnetic field drops off over distance.
||Field at Distance x
How Does a Magnet’s Strength Drop Off Over Distance?
The strength of a magnetic field drops off roughly exponentially over distance. Here is an example of how the field (measured in Gauss) drops off with distance for a Samarium Cobalt Grade 18 disc magnet which is 1″ in diameter and 1/2″ long.
What is the equation for field strength relative to distance?
For a circular magnet with a radius of R and Length L, the field at the centerline of the magnet a distance X from the surface can be calculated by the following formula (where Br is the Residual Induction of the material):
There are additional formulae that can be used to calculate the field from a rectangular magnet and magnets in other configurations, but the formulae get too long and complex looking to include here!