Garden Soil Calculator: Estimate Topsoil, Compost, and Mulch
Use our Garden Soil Calculator to easily determine the precise amount of soil and organic matter required to cover your open garden beds, lawns, and backyard landscapes.
Table of Contents
How Much Topsoil, Compost, or Mulch Do You Need?
The formula is the same for all three: length (ft) × width (ft) × depth (ft) ÷ 27 gives you cubic yards. Depth is where they differ most — a new garden bed usually needs 6 inches of topsoil, a lawn top-dressing needs just 1 inch of compost, and a mulch border typically wants 2-3 inches of mulch. For a 100 sq ft area at 3 inches deep, you’re looking at roughly 0.93 cubic yards of material. Use the calculator below for your exact area, material, and project type.
Garden Soil Calculator
Estimates topsoil, compost, or mulch by area and depth, suggests the right depth for your project, and converts to cubic yards and bags.
Standing in the landscaping aisle trying to guess how many bags of topsoil, compost, or mulch to load into your cart is such a familiar feeling — and it’s exactly how most people either overspend or end up making a second trip. Maybe you’ve wondered… Aren’t these three basically the same thing? How deep am I actually supposed to go? Do I even need all three? They look similar enough that it’s tempting to treat them the same way, but they’re used at completely different depths and for completely different jobs. Get the math right once, and you’ll never have to guess again.
Topsoil vs Compost vs Mulch: What’s the Difference?
These three materials get lumped together all the time, but each one really does a different job in your yard.
| Material | What It’s For | Where It Goes |
|---|---|---|
| Topsoil | Base growing medium — provides structure and body for roots | Mixed into or filling garden beds, lawns, and raised beds |
| Compost | Decomposed organic matter — feeds soil microbes and improves fertility | Worked into existing soil, or as a thin lawn top-dressing |
| Mulch | Surface layer — retains moisture, blocks weeds, moderates temperature | Spread on top of soil, never dug in |
Since they serve such different purposes, they also get applied at very different depths — which is exactly why a single “6 inches for everything” rule of thumb leads people astray.
The Volume Formula, Step by Step
Want to see the actual math? Here’s the same formula our calculator runs behind the scenes for any of the three materials.
Example: a 10 ft × 10 ft flower border, adding 3 inches of mulch.
1. Convert depth to feet: 3 ÷ 12 = 0.25 ft.
2. Find the area: 10 ft × 10 ft = 100 sq ft.
3. Multiply by depth: 100 sq ft × 0.25 ft = 25 cubic feet.
4. Convert to cubic yards (optional): 25 ÷ 27 = 0.93 cubic yards.
What this tells you: at under 1 cubic yard, bagged mulch is the easier and more practical route for a border this size.
Recommended Depth by Project
This is where topsoil, compost, and mulch really part ways. Here’s a practical reference for the most common projects you’ll run into.
| Project | Topsoil | Compost | Mulch |
|---|---|---|---|
| New garden bed | 6 in | 3 in (worked in) | 2 in (on top after planting) |
| Lawn top-dressing | 1-2 in | 1/4-1/2 in | Not applicable |
| New sod / seed prep | 3-5 in | 1-2 in worked in | Not applicable |
| Raised bed fill | 8-12 in | 3-6 in of the mix | 1-2 in as top layer |
| Mulch bed / border | Not applicable | 1-2 in | 2-3 in |
Notice how thin the compost layers are compared to topsoil? That’s not a typo — compost is a concentrated amendment, not a bulk filler. Piling it on too thick, especially on an established lawn, can actually smother what’s growing underneath rather than help it.
Volume by Area for Common Depths
Once you know your square footage and target depth, this reference table should get you a ballpark number fast, in both cubic feet and cubic yards.
| Area | 1″ Deep | 3″ Deep | 6″ Deep | 12″ Deep |
|---|---|---|---|---|
| 50 sq ft | 4.2 cu ft | 12.5 cu ft | 25 cu ft | 50 cu ft |
| 100 sq ft | 8.3 cu ft | 25 cu ft | 50 cu ft | 100 cu ft |
| 200 sq ft | 16.7 cu ft | 50 cu ft | 100 cu ft | 200 cu ft |
| 500 sq ft | 41.7 cu ft | 125 cu ft | 250 cu ft | 500 cu ft |
How Many Bags Will You Need?
Bag sizes vary quite a bit between materials and brands, which can change your bag count more than you’d expect. Here’s how many bags it takes to cover 100 cubic feet across the common sizes.
| Bag Size | Bags for 25 cu ft | Bags for 50 cu ft | Bags for 100 cu ft |
|---|---|---|---|
| 0.75 cu ft | 34 bags | 67 bags | 134 bags |
| 1 cu ft | 25 bags | 50 bags | 100 bags |
| 1.5 cu ft | 17 bags | 34 bags | 67 bags |
| 2 cu ft | 13 bags | 25 bags | 50 bags |
| 3 cu ft | 9 bags | 17 bags | 34 bags |
Bagged Material vs Bulk Delivery
Once you know your total cubic feet, the next question is how to actually buy it. Bagged material is convenient, but it stops making financial sense once your order gets large.
Bagged material is usually easier — no delivery minimum to hit, and you can buy exactly what you need without leftovers.
Bulk delivery almost always wins on price per cubic foot, and saves you from hauling dozens of individual bags.
Accounting for Settling
Here’s something worth keeping in mind no matter which material you’re using: don’t buy exactly to the decimal. Topsoil and compost both settle and compact as organic matter breaks down and air pockets compress, sometimes losing as much as a third of their original volume within the first season. Buying a little extra, or planning a follow-up top-off, keeps your project from looking thin a few months down the line.
Common Soil Calculation Mistakes
Using the same depth for all three materials. Compost is applied far thinner than topsoil, and mulch always sits on the surface rather than being worked in.
Mixing up inches and feet mid-calculation. Depth is almost always measured in inches while length and width are in feet — convert everything to the same unit before multiplying.
Applying compost too thick on an existing lawn. More than about 1/2 inch at a time can smother grass instead of feeding it — thin, repeated applications work better than one heavy layer.
Not accounting for settling. Skipping this step is the most common reason people come up short a few weeks after finishing a project.
Buying bags when bulk would’ve been cheaper. Past roughly 2 cubic yards, bulk delivery almost always beats the per-bag price, even after a delivery fee.
Not sure your exact numbers yet? Use the Garden Soil Calculator above to get cubic feet, cubic yards, and bag counts for topsoil, compost, or mulch — with depth automatically suggested for your project type.
Why trust this guide: the depth guidelines and volume formulas above reflect figures commonly used across landscaping and gardening references. Every yard and every bag brand is a little different, so treat these as a strong starting point and adjust based on what you see once material is delivered.
So don’t overthink it too much — get your rough numbers, grab a little extra for settling, and get started. If you’re working on a project that mixes several materials at once, drop a comment below and I’ll help you work through the numbers.
Frequently Asked Questions
At the typical 6-inch depth for a new garden bed, a 100 sq ft area needs about 50 cubic feet of topsoil, which is roughly 1.85 cubic yards.
Topsoil is the base growing medium that gives roots structure. Compost is decomposed organic matter that feeds the soil and improves fertility. Mulch sits on top of the soil surface to retain moisture, block weeds, and moderate temperature — it isn’t dug into the ground the way topsoil and compost are.
A thin layer of about 1/4 to 1/2 inch is standard for top-dressing an existing lawn. Going much deeper can smother the grass instead of helping it.
Most flower beds and borders do well with 2-3 inches of mulch. Going deeper than 3-4 inches can actually restrict oxygen and moisture from reaching plant roots.
Under about 2 cubic yards, bagged material is usually more convenient. Past that point, bulk delivery is typically cheaper per cubic foot, even after accounting for a delivery fee.
Yes. Freshly spread topsoil can settle and compact over time, sometimes losing up to a third of its initial volume. It’s smart to buy slightly more than your exact calculation and account for a follow-up top-off.
New sod installations typically need 3-5 inches of quality topsoil underneath to give roots room to establish. A new seeded lawn can often get by with slightly less, around 4-6 inches worked into the existing ground.
Yes — many projects use all three. Calculate each material separately using its own recommended depth for your project type, since they usually aren’t applied at the same depth or in the same layer.
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