In a pile by the curb these three can look almost identical, but they do very different jobs. Using the wrong one means either plants that will not grow or money spent on premium soil where cheap fill would have done just as well.
Topsoil
The upper layer of soil, screened of rocks and debris, with some organic content. It is the all rounder for building beds, leveling a lawn, and topping up planting areas. Screened topsoil weighs roughly 1.1 tons per cubic yard when fairly dry, more when wet. Quality varies a lot, so ask whether it is screened and where it came from.
Fill dirt
Subsoil with little or no organic matter, used to fill holes, raise grades and build up low spots. It is cheaper than topsoil and compacts well, which makes it good structural fill, but plants will struggle in it because it lacks nutrients and structure. Use fill to get to rough grade, then cap it with topsoil for anything you intend to plant.
Compost
Fully decomposed organic matter, rich in nutrients and biology. It is a soil amendment, not a standalone growing medium. Mixed into topsoil at perhaps a quarter to a third by volume, it transforms a tired bed. Used pure, it is too rich and drains too fast for most plants, and it settles a lot as it continues to break down.
Side by side
| Topsoil | Fill dirt | Compost | |
|---|---|---|---|
| Organic content | Some | Little | High |
| Grows plants | Yes | No | As an amendment |
| Compacts well | Moderate | Yes | No |
| Relative cost | Medium | Low | High |
A typical layered approach
For a raised bed or a regraded area: fill dirt to build the bulk and set the grade, topsoil for the main root zone, and compost worked into the top layer for fertility. This costs far less than filling the whole depth with premium soil, and it puts the good soil where the roots actually are.
Estimating quantities
All three are calculated the same way, by volume. Work out area times depth, convert to cubic yards, and order with a small allowance for settling. Remember that compost and loose topsoil settle over the first few weeks, so a freshly filled bed will drop a little, which is normal.