Topsoil Calculator

How much topsoil do I need?

Every topsoil order comes down to one line of arithmetic: area multiplied by depth, divided by 27 to turn cubic feet into cubic yards. The area is the easy part. The depth is where people underestimate, because a number that sounds small in inches becomes a truck-sized number once it is spread over a garden. Two inches over 500 square feet is more than three cubic yards — around a ton and a half of material to move by wheelbarrow.

Worked example: a 25 ft x 10 ft planting bed

Say you are building a new bed along a fence, 25 feet long and 10 feet wide, and you want the standard 4 inches of good soil for annuals and perennials. Area is 25 x 10 = 250 square feet. Four inches is 0.333 feet, so the volume is 250 x 0.333 = 83.3 cubic feet. Divide by 27 and you get 3.1 cubic yards. At the usual bulk density for screened dry loam, roughly 1.35 tons per cubic yard, that is about 4.2 tons of soil.

Now the same bed at 6 inches for a raised frame: 250 x 0.5 = 125 cubic feet, or 4.6 cubic yards. Adding two inches of depth added half again as much soil. That is the single most useful thing to internalise before you call a supplier — depth, not area, is what moves the price.

The bag maths nobody enjoys

Bagged topsoil in the US is typically sold as a 40 lb bag holding about 0.75 cubic feet. The 250 square foot bed above needs 83.3 cubic feet, which is 112 bags. At three to five dollars a bag that is four to five hundred dollars, plus a car boot filled six times over. The same 3.1 yards delivered in bulk usually costs 20 to 50 dollars per yard plus a flat delivery fee, so somewhere between 150 and 250 dollars in one drop.

The practical crossover is about one cubic yard, roughly 36 bags. Below that, bags win on convenience and you avoid a pile on the driveway. Above it, bulk wins on both cost and your back. One caveat on the numbers: bagged product is drier, screened finer and often blended with organic matter, so 112 bags do not weigh the 4.2 tons the bulk figure quotes. The volume matches; the weight does not.

Depth by job

Do not bury a living lawn

The most common expensive mistake is treating an existing lawn like a new bed. Grass crowns sit at the soil surface and suffocate under anything thicker than about half an inch. Topdress at a quarter to half an inch, drag it into the canopy with the back of a rake so the blades still show, and repeat next season if the surface is still uneven. If you genuinely need four inches on a lawn area, you are not topdressing, you are rebuilding — strip or till the old turf first and reseed.

Buying checklist

Ask whether the soil is screened, and to what size; half-inch screening is normal for beds. Ask what it was before it was topsoil, because stripped construction site soil can carry compaction, weed seed and the occasional lump of concrete. Ask for a recent test or at least the pH range if you are planting acid-lovers. Look at a sample wet: good topsoil crumbles, poor topsoil ribbons like clay. And ask about organic matter, since a soil with 3 to 5 percent will behave far better than a mineral fill that happens to be dark.

Limitations of any calculator

These numbers describe a level surface and a uniform depth. Real ground is neither. Add roughly 10 percent for settling, uneven subgrade and the soil that stays stuck in the barrow, and 15 percent for compost-rich blends, which shrink noticeably in the first season as the organic fraction breaks down. Weight is the softest figure here: moisture content swings bulk topsoil between about 1.0 and 1.7 tons per cubic yard, which is why suppliers who sell by the ton should always be asked what conversion they are using that week. If you have a choice, buy by volume and let the moisture be their problem.