How much topsoil do I need?
Soil is the order people underestimate most, because a couple of inches across a whole yard is a great deal more than it sounds. Here is the arithmetic, the depth each job actually wants, and what the names on the price list mean.
How much topsoil do you need?
Working out a whole yard rather than one bed? Sketch it to scale in the planner and every area is measured, costed and totalled together.
The formula
Same one that governs everything sold by depth:
cubic yards = (square feet × depth in inches) ÷ 324
A cubic yard is 27 cubic feet and an inch is a twelfth of a foot, so 12 × 27 = 324 converts square feet and inches straight into the unit the yard sells in. A 1,000 square foot lawn at 4 inches is (1000 × 4) ÷ 324 = 12.3 cubic yards, which is two full truckloads.
That is the number worth sitting with. Four inches sounds like nothing. Across a small suburban back lawn it is twelve tons of soil to be moved by wheelbarrow.
How deep
| Job | Depth | Why |
|---|---|---|
| Topdressing an existing lawn | ¼ – ½ in | Any more smothers the grass you are trying to help |
| New lawn from seed or sod | 4 – 6 in | Root depth for turf grass |
| Flower beds and borders | 6 – 12 in | Perennials root deeper than grass |
| Raised vegetable beds | 10 – 18 in | Root crops need the depth; this is the expensive one |
| Levelling a dip | whatever the dip is | Measure it; do not guess it |
Topdressing is the one people get wrong in the other direction. A half inch over a lawn is a light dusting you can still see grass blades through, and that is correct. Burying an existing lawn under two inches of soil kills it as effectively as a tarp.
Topsoil, fill dirt and garden mix
These are three different products at three different prices, and the names are used loosely enough that it is worth asking what is actually in the pile.
- Fill dirt is subsoil. No organic matter, cheap or free, and it compacts hard. Right for building up a level or backfilling a hole, wrong for anything that has to grow.
- Screened topsoil is the usual buy: native soil run through a screen to take the rocks and roots out. Quality varies enormously by supplier because it is whatever was dug locally.
- Garden mix or triple mix is topsoil blended with compost and sand. Costs more, and worth it for beds and vegetables. Overkill under a lawn.
If you are filling a deep hole, the sensible order is fill dirt for the bulk and good soil for the top six inches. Paying garden-mix prices to fill the bottom of a two-foot dip is money burned where no root will ever reach.
Ask to see it before it is tipped, if you can. Screened topsoil that arrives grey, sticky and full of clay is a season of trouble, and it is a great deal easier to refuse a load than to remove one.
Weight and delivery
A cubic yard of dry topsoil is roughly 2,000 to 2,400 pounds, and wet soil goes past 3,000. A half-ton pickup carries about half a yard safely. This is not a material you collect. Anything past a yard is a delivery, and most yards have a two or three yard minimum anyway, which is why small jobs end up on bags.
Bagged soil is roughly 0.75 cubic feet a bag, so it takes 36 bags to make a cubic yard at three to five dollars each. Against forty dollars for a delivered yard, the crossover is under half a yard. Bags are for a single container or a small patch; anything bigger is bulk.
Settling
Fresh soil settles ten to twenty per cent as it is watered in, and more if it was fluffy and dry when it arrived. Order the depth you want to end up with plus about fifteen per cent, and rake it high rather than level.
For a raised bed, fill it proud and expect to top it up next spring. For a lawn, water it and let it sit a week before you seed or sod, so the dips show up while there is still something you can do about them.
Ordering
- Work out the yardage, add fifteen per cent for settling, then round up to the next half yard.
- Say where the pile goes before the truck arrives, and remember every yard of it moves by barrow from there.
- Do not order soil you have nowhere to put. A yard is a heap about five feet across and three high.
- If it is going under sod, order the soil first and the sod for a few days later. Sod is perishable; soil is not.
Work out your yardage in the planner
Paint topsoil under the lawn with Add layer and the planner counts both: the soil going in and the sod going over it, each at its own depth and its own price.