What is the strength of the earth’s magnetic field?

The surface field strength of the earth is about 0.5 gauss, but it varies by as much as 10% depending on the strength of the crustal field. A range from 0.85 to 0.45 can be found across the globe. Geomagnetic storms can cause changes of between 1% to 5% that last from a few hours to a full day.

How does a magnet’s strength drop off over distance?

The strength of a magnetic field drops off more or less 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 governing equation for field strength relative to distance?

For a circular magnet with a radius of R and length L, the field Bx at the centerline of the magnet at 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.

There are additional formulae that can be used to calculate the field from a rectangular magnet and magnets in other configurations.

What can I use to block a magnetic field?

Only materials that are attracted to a magnet can "block" a magnetic field. Depending on how thick the blocking piece is, it will partially or completely block the magnetic field.