Topsoil is sold by volume, so estimating it is the same as any bulk material: work out the volume of the layer you want, convert to cubic yards, and decide between bags and a bulk load. Here is the whole process.
Topsoil is sold by volume
Multiply length × width × depth (with depth in feet) for cubic feet, then divide by 27 for cubic yards — the unit bulk suppliers use. The topsoil calculator does this and also shows tons and bags.
What one cubic yard of soil covers
A single cubic yard of topsoil covers about 100 square feet at 3 inches deep, weighs roughly 1.1–1.3 tons (screened, and heavier when wet), and equals about 13–14 of the standard 2-cubic-foot bags. That last figure is why bags get expensive fast for anything but a small job.
How deep should it go?
- New lawn: 3–4 inches of quality topsoil (more if the subsoil is poor).
- Garden and planting beds: 6–12 inches so roots have room.
- Leveling or top-dressing a lawn: a thin 1/4–1/2 inch pass.
The math, step by step
To spread 3 inches over a 20 × 10 ft area: convert depth to feet (3 ÷ 12 = 0.25), then 20 × 10 × 0.25 = 50 cubic feet. Divide by 27 and you need about 1.85 cubic yards. Enter your numbers in the topsoil calculator for the exact yards, tons, and bag count.
Bags or bulk?
Bags suit small beds and easy cleanup, but past about 10–12 bags a bulk load is far cheaper by volume — see bulk vs. bags for the break-even. Either way, order about 10% extra: soil settles and grades are never perfectly even.