Square feet times depth in inches, divided by 324, gives cubic yards of topsoil to order.
You can guess and overbuy, then stare at a leftover pile for months. Or you can measure once, do one clean calculation, and order the right amount on the first try.
This piece walks you through the math, the measuring tricks that stop bad numbers, and the real-world ordering details that trip people up (bags vs. bulk, compaction, and uneven beds).
Start with the space you will fill
Topsoil orders go sideways when the area number is wrong. Before you touch a calculator, map the space you will actually fill with soil.
If your garden has edging, boards, stones, or a raised frame, measure the inside footprint. If you plan to leave paths, measure beds only, not the full rectangle of the whole garden zone.
Measure rectangles and squares
Measure length and width in feet. Multiply them to get square feet.
- Square feet = length (ft) × width (ft)
If a bed flares out a bit, measure the narrow end and the wide end, average them, then multiply by the length.
Measure circles and curved beds
For a round bed, measure the radius (center to edge) in feet.
- Square feet = 3.14 × radius × radius
For an oval, measure the longest length and the widest width, then use the rectangle math as a practical estimate. You’ll still land close enough for ordering.
Break odd shapes into simple chunks
L-shaped and wedge-shaped gardens are easier when you split them into two or three rectangles, then add the areas.
A quick sketch on paper helps. Write each section’s length, width, and square feet right on the drawing so you don’t re-measure or mix numbers later.
Pick a depth that matches your goal
Depth is the second lever. The right depth depends on what you’re doing: building a new bed, leveling low spots, or refreshing an established garden.
Common depth ranges that work well
- 1–2 inches: Light top-up on an existing bed where soil level sank.
- 3–4 inches: A solid refresh layer when you’re improving a tired bed surface.
- 6 inches: New bed fill over a prepared base, or a deeper refresh where the existing soil is poor.
- 8–12 inches: Filling a raised bed frame from near-empty.
If you’re filling a brand-new raised bed, don’t default to the full frame height. Many beds are built taller than the soil line you need on day one. You can fill to a working depth now, then top up after the first season settles.
Measure depth the way soil actually sits
Depth in product listings is the poured depth. Once you rake and water, the layer will sit a bit lower. That’s normal.
If you’re spreading topsoil over an existing bed, measure from the current soil surface up to the finished grade you want, not from the ground below the bed.
Use the topsoil volume formula
Bulk topsoil is sold by volume, usually cubic yards. The cleanest way to get cubic yards is to convert your bed area into volume with a single constant.
Square feet and inches to cubic yards
- Cubic yards = (square feet × depth in inches) ÷ 324
That “324” comes from unit conversion. One cubic yard is 27 cubic feet, and one inch is 1/12 of a foot, so the math collapses into a tidy constant.
Length, width, and inches to cubic yards
If you like working from raw measurements:
- Cubic yards = length (ft) × width (ft) × (depth in inches ÷ 12) ÷ 27
Both paths land on the same result. Use the one that feels cleaner for you.
How Much Topsoil Do I Need For A Garden? Use this bed-by-bed method
If you have more than one bed, don’t mash everything into one big area unless the depths match. Do it bed-by-bed. It takes a few extra lines on paper and prevents ordering the wrong amount for the deep beds.
- Measure each bed’s square feet.
- Choose the depth for that bed.
- Compute cubic yards for that bed.
- Add the cubic yards from all beds.
This also makes it easier to stage delivery and spreading. You can finish one bed at a time and avoid dragging soil across paths you want to keep clean.
Sample math with real numbers
Bed: 12 ft × 4 ft = 48 sq ft. Depth: 4 inches.
- Cubic yards = (48 × 4) ÷ 324 = 192 ÷ 324 = 0.59 cubic yards
That’s a little over half a yard. If your supplier sells in half-yard steps, you’d order 0.5 yards for a thin finish, or 1 yard if you want a cushion for low spots and settling.
Round up the way suppliers actually sell
Many bulk yards are delivered in 0.5-yard steps. Some places do 0.25-yard steps. Bags come in fixed sizes like 1 cubic foot or 2 cubic feet.
Rounding is normal. The trick is to round with a reason, not with a shrug. If your beds are uneven, if you expect settling, or if you’re new to spreading soil, add a small buffer. If your bed edges are tall and clean and you’ve done this before, you can order closer to the number.
Coverage cheat sheet for common depths
The formula is fast, but a cheat sheet helps you sanity-check your result before you click “checkout” or schedule delivery.
| Depth (inches) | Square feet covered by 1 cubic yard | Cubic yards for 100 sq ft |
|---|---|---|
| 1 | 324 | 0.31 |
| 2 | 162 | 0.62 |
| 3 | 108 | 0.93 |
| 4 | 81 | 1.23 |
| 5 | 64.8 | 1.54 |
| 6 | 54 | 1.85 |
| 8 | 40.5 | 2.47 |
| 12 | 27 | 3.70 |
If you want a source-backed version of the “divide by 324” method, the UAF Cooperative Extension depth-to-yards formula shows the same conversion in a worked equation.
For larger areas and quick lookup, Cornell Cooperative Extension shares a printable chart that maps square footage and depth to cubic yards in one grid: Cornell Cooperative Extension volume chart.
Depth coverage numbers also show up in extension material for compost applications, using the same yard-to-area relationship: NC State Extension cubic-yard coverage note.
Convert cubic yards to bags and loads
Once you have cubic yards, you can match that number to what you can buy and carry.
Bulk to cubic feet
- 1 cubic yard = 27 cubic feet
If you need 1.5 cubic yards, that’s 40.5 cubic feet of soil.
Cubic feet to bags
Bag size is printed on the bag in cubic feet. Divide your cubic feet by the bag size to get bag count.
- Bags needed = total cubic feet ÷ bag size (cubic feet)
Sample: 40.5 cubic feet ÷ 1.5 cubic feet per bag = 27 bags.
Bags work well for small beds, tight access, or when you can’t get a bulk drop where you need it. Bulk is often cheaper per unit volume when you have room for delivery and a way to move soil from the pile to the beds.
Match the order to your hauling setup
If you plan to pick up soil, check your vehicle’s manual for payload limits and stay under them. Wet soil weighs more than dry soil. A small trailer can be a safer bet than loading a truck bed past its limit.
If you’re getting delivery, ask where the pile will land and how close it can get to your beds. Ten extra feet of wheelbarrow time per load adds up fast.
Choose a depth by garden task
The same yard of soil can be spread thin over a large area or deep over a small one. Matching depth to task is what keeps the garden level, the paths clean, and the plants happy.
| Garden task | Typical topsoil depth | Notes for ordering |
|---|---|---|
| Level low spots in a bed | 1–3 inches | Mark low zones first, then spread the rest as a thin finish. |
| Refresh an existing veggie bed | 3–4 inches | Plan a light buffer for settling after watering and raking. |
| New in-ground bed after clearing grass | 4–6 inches | Measure only the bed footprint, not paths or borders. |
| Fill a raised bed that is half empty | 6–10 inches | Measure inside walls; don’t use outside frame size. |
| Top up a raised bed after a season | 1–3 inches | Check depth at several points; beds often sink unevenly. |
| Create a gentle mound for drainage | 4–8 inches | Use string lines to keep the crown consistent across the run. |
| Blend into a new planting strip along a fence | 3–6 inches | Account for tapering edges where soil feathers into grass. |
Order smarter by checking three real-world traps
Trap 1: Measuring the outside of edging
If a bed has thick timber walls, stone borders, or blocks, outside measurements inflate area. Measure inside dimensions where soil will sit. That one detail can change the order by a noticeable amount on long beds.
Trap 2: Ignoring tapering edges
Many garden beds slope at the edges so the soil line meets the surrounding ground. That taper uses soil, but it’s hard to see on paper.
If your bed edges feather out, plan a small buffer. If the bed is boxed with vertical sides, you can order closer to the raw math.
Trap 3: Mixing “topsoil” with “screened topsoil” and “garden mix”
Suppliers use names that sound alike. Ask what the product actually is. Plain topsoil can be mineral-heavy. Screened topsoil is passed through a screen for more consistent texture. “Garden mix” often blends topsoil with compost.
For new beds, many gardeners prefer a mix that has organic matter built in. If you’re only topping an existing bed that already has compost worked in, screened topsoil can be enough. Match the product to the job, then measure and order by volume the same way.
Spreading tips that keep your depth honest
You can order the perfect amount and still end up short if the soil gets spread unevenly. A simple layout routine prevents that.
- Drop small piles across the bed first, spaced like a grid.
- Rake the piles into a flat layer.
- Check depth at several points with a ruler or a marked stick.
- Water lightly, then re-check and touch up low spots.
If you’re filling a raised bed, stop an inch below the top edge, water, then check again the next day. Settling is normal, and topping up later is easier than scraping spilled soil off the outside walls.
One-page checklist before you buy
- Inside bed dimensions recorded in feet
- Total square feet per bed written on a sketch
- Depth chosen per bed in inches
- Cubic yards per bed computed with (sq ft × inches) ÷ 324
- All bed cubic yards added into one total
- Purchase format picked: bulk yards or bags
- Drop spot planned so moving soil stays easy
Run that list once and you’ll order with calm confidence. No guessing. No panic re-order. No leftover mountain.
References & Sources
- University of Alaska Fairbanks (UAF) Cooperative Extension Service.“Establishing and Maintaining a Lawn in Southeast Alaska.”Provides the square-feet-and-inches-to-cubic-yards conversion using the ÷324 method.
- Cornell Cooperative Extension.“Calculating Volume Needed of Topsoil, Compost, Mulch” (PDF).Chart-based lookup for cubic yards based on area and depth.
- North Carolina State University (NC State) Extension.“Organic Lawn Care: Composting and Spreaders (Appendices).”States cubic-yard coverage by depth that matches the 324 sq ft at 1 inch relationship.