The equation for pressure is:
The force responsible for providing the pressure in the tyres is the weight of the bicycle plus rider.
But the weight is distributed across two tyres equally (we assume this). So the actual force involved is:
Now rearrange the pressure equation to solve for the contact area:
The area is small so it might be more convenient to use a prefix, i.e.
That is a reasonable value for the area of bicycle tyres in contact with the ground.
When converting areas into a prefix the conversion factor must be squared that's why I multiplied by
Find out more about prefix conversions here: