Topsoil Weight
Topsoil weight is calculated by multiplying the number of cubic yards of topsoil by the weight per cubic yard.
The weight of a cubic yard of topsoil is about 1 ton, but this will depend on the moisture content of the soil. This is enough topsoil to spread over about 300 square feet to a depth of 1 inch.
A landscaping firm may estimate the amount of topsoil they need for a grass lawn based on this estimate of coverage.
Here is a handy tool that you can use to give a rough idea of how much material to order: