Fill Dirt vs Topsoil, and How to Layer Them

The short answer

Fill dirt does the depth. Topsoil does the surface. Not one or the other, both, stacked in that order.

How many inches of topsoil depends only on what is going on it. Four for a lawn from seed, two to three under sod, six to twelve for a planting bed. Everything below that number is fill, whatever the total depth happens to be.

And if nothing is growing there, the answer collapses to one material. Filling a void, backfilling a wall, raising ground under a patio: all fill dirt, bottom to top. Topsoil in any of those is money spent on a layer that has no job.

So the decision is not really about the materials. It is about which parts of the depth have a job beyond holding the level up.

What fill dirt is

Everything topsoil is not, and priced accordingly. It is heavy, it is stable once compacted, and nothing grows in it. Set against topsoil all three sound like faults, and here they are the whole point.

Judge it by what it does under load, not by what it looks like. Good fill packs down tight and then stops moving. That is exactly what you want under a slab, behind a retaining wall, or beneath a lawn that has to stay flat for twenty years.

Grade matters less than people expect. Common fill at $8 to $15 a yard covers most jobs. Screened or clean fill costs more, and it earns that in two cases only: a structure is going over it, or a permit wants paperwork on where the material came from.

One thing is worth checking before it arrives, though. Ask what is in it. A load with buried timber or root balls will leave voids as they rot, and you will be filling the same dip again in three years.

What topsoil is

The opposite trade. You are buying biology rather than bulk, and paying 3 times as much a yard for it.

Screened topsoil runs $20 to $50 a yard and is the default for anything planted. Compost blends go $35 to $75. That is money well spent in a vegetable bed and largely wasted under sod, since turf roots into the top few inches and never sees the rest.

Depth is where the money leaks. Topsoil is priced by volume, so every extra inch across a big area is real money. Past root depth it buys nothing at all. A lawn does not care whether there are four inches or twelve underneath it.

It also stays loose on purpose. Fill gets compacted in layers, while topsoil goes down light and is then left alone. Compact the growing layer and you have made a surface roots cannot get into.

Cost comparison

Common fill is $8 to $15 a cubic yard. Screened topsoil is $20 to $50. Call it a 3x markup on the inches that have roots in them.

Now put that gap against a real job. A 600 sq ft area raised 12 inches takes 22.2 yards. All in screened topsoil that is $878 delivered.

Layered instead, it is 17.8 yards of fill and 7.4 of topsoil, so $614. The finished surface is identical and you keep $264, which is 30% of the bill.

The two layers add up to more than 22.2 yards, and that is not an error. Fill gets compacted, so it has to be ordered loose and larger. Topsoil does not, so it is ordered as measured.

Two things push the gap wider. Bigger areas, because the saving scales with volume while the delivery charges barely move. And deeper raises, since every extra inch below root depth is another inch bought at the cheap rate instead of the expensive one.

What narrows it is a small job. Under five or six yards most suppliers add a short-load fee, and paying that twice on two deliveries can eat the saving on a tiny area. So on anything under a pickup load or two, price it both ways before ordering.

When to use each

Ask one question. Will anything be rooting into this depth? If yes it is topsoil, if no it is fill, and most jobs answer both ways at different depths.

The bottom two rows are the ones people get wrong in the expensive direction. Good soil behind a wall or under a slab is not just wasted, it is actively bad. The organic matter in it breaks down, and the level above drops with it.

But the lawn row goes wrong in the other direction. Skipping the topsoil saves a few hundred and leaves a lawn that never thickens. Fixing that later means stripping the surface and starting again.

The garden bed row has a catch worth knowing. Roots go deeper than turf roots do, so the growing layer is deeper too, and on a raised bed of a foot or less there is no fill layer at all. Fill only appears once you are lifting the ground higher than the plants will reach.

And one job is not on the table because it does not need either. If your ground is already at the right level and just tired, you are not raising anything. Work compost into what you have instead.

Layering fill dirt and topsoil correctly

Fill first, in six inch lifts, compacting each one before the next goes on. This is the step that gets skipped and it is the one that decides whether the finished level holds.

Stop the fill at the depth of your growing layer, not at the finished level. If you are raising twelve inches and seeding after, the fill stops at 8 and the last 4 are topsoil. Filling to the top and then adding soil leaves the whole thing proud.

Now break the surface between the layers before the topsoil goes on. Compacted fill ends up smooth enough that roots treat it as a floor, and water sits on top of it. Scratch it up an inch or two and the two layers knit.

Then leave the topsoil alone. No plate compactor, no rolling beyond what settles it lightly. Water it in and let it find its own level over a week or two instead.

And allow for one more thing. The fill will settle a little further whatever you do, so finishing a touch high on a large area is normal practice. The alternative is a dip you are topping up next spring.

Black dirt, and where it fits

Black dirt is the third name people meet, and the least standardized of the three. It describes a color. So what turns up varies far more than either fill or screened topsoil does.

Usually it means soil with high organic content, often from low ground or old wetland. At its best that is genuinely rich material. At its worst it is largely peat, which looks superb wet, then shrinks hard as it dries and takes your finished level down with it.

So treat it as a maybe rather than an upgrade. Ask what it is and where it came from. If the answer is vague, screened topsoil at a known grade is the safer buy for the same money.

Where it does shine is mixed into beds rather than used alone. As a soil improver it is hard to beat. As the entire growing layer under a lawn it is a gamble on shrinkage.

How much of each to order

Work the top layer first, then let the fill take whatever is left. That order matters because the topsoil depth is fixed by the planting and the fill depth is simply the remainder.

The splitter above does it, but the arithmetic is short enough to check. Area × topsoil depth ÷ 12 ÷ 27 gives the topsoil yards. Area × the remaining depth, same divisors, plus a settlement allowance, gives the fill.

Only one of the two numbers gets that allowance, and the splitter applies it to the lower layer alone. Ten to twenty percent covers most ground. Heavy clay wants the top of that range, sandy ground the bottom.

Order them as two deliveries and expect two delivery fees. Having both dropped at once is tempting, but they cannot share a truck bed. And mixing the piles on a driveway turns your growing layer into fill by accident.

Related pages

• Fill dirt calculator for the lower layer on its own, with settlement in the order.
• Topsoil calculator for the growing layer, with depth presets by what you are planting.
• Fill dirt cost, where the haul turns out to matter more than the dirt.
• Topsoil cost per yard for grades, delivery and the bagged crossover.
• Compost calculator if the plan is to improve the soil you already have rather than import more.