Raised Bed Soil Calculator
Work out how much soil your raised bed needs. Enter the bed's inside length, width and fill depth to get cubic yards, cubic feet and bags — pre-set to a 4 × 8 ft bed filled 10 inches deep.
Sets a typical depth — tweak anything below.
For a 8 ft × 4 ft area at 10 in deep, order about 1.09 cubic yards (1.19 US tons) of topsoil.
Estimates only. Densities vary by moisture, compaction and supplier — confirm quantities before ordering.
How much soil does a raised bed need?
Multiply the bed's inside length by its inside width by the fill depth, all in feet, then divide by 27 for cubic yards. The classic 4 × 8 ft bed filled 10 inches deep works out to almost exactly 1 cubic yard — that's the number worth remembering, because it means a single yard of bulk soil fills one standard bed. In bags it's a different story: 27 cubic feet is about 18 bags of the 1.5 cu ft size, which is why bulk wins the moment you're filling more than one bed.
Measure the inside, not the outside
This is the mistake this calculator exists to catch. You fill the space inside the frame, but people measure the frame itself. A bed built from nominal 2-inch lumber has boards that are actually 1.5 inches thick, so a '4 × 8' bed is really 3.75 × 7.75 inside — about 9% less soil than the outside dimensions suggest. On one bed that's a wasted bag or two; on six beds it's most of a yard you paid to have dumped on your driveway. Measure between the boards, and enter that.
How deep should a raised bed be?
10 to 12 inches suits almost everything, and 6 inches is the floor. What actually sets the depth is what's underneath: if your bed sits on open soil, roots can push down past the frame, so 6–8 inches of good soil on top is genuinely enough for lettuce, herbs and most greens. If the bed sits on a patio, gravel, or a solid liner, the soil in the frame is all the root run there is — go 12 inches, and 18 for carrots, potatoes or anything else that roots deep. Deeper beds also dry out more slowly, which matters more than people expect in a hot summer.
What to fill a raised bed with
Not pure topsoil, and not pure compost. Straight topsoil compacts into a brick in a frame and drains badly; straight compost is too rich, holds too much water, and shrinks dramatically as it breaks down — fill a bed with it and you'll be looking at a half-empty bed next spring. The standard mix is roughly 60% topsoil and 40% compost, give or take, and many people add a third part — perlite, coarse sand, or bark fines — for drainage. Work out your total volume here, then split it: a 1 cubic yard bed takes about 0.6 yd³ of topsoil and 0.4 yd³ of compost. This calculator uses a topsoil density; if you want the weight of your actual mix, enter its density in the extra options.
Beds settle — plan for it
Fresh soil is fluffy and it drops. Between watering, rain and the compost continuing to break down, a bed filled level in April is commonly an inch or two down by July, and lower again next season. That's normal and it isn't a mistake — it's why beds get topped up every spring. Fill to the top of the frame rather than trying to hit an exact number, order a little extra rather than exactly enough, and expect to add a bag or two of compost each year. The default 10% overage above covers a fresh fill; if you're filling deep beds with a compost-heavy mix, bump it up.
Common bed sizes (inside dimensions)
| Bed | At 6 in | At 10 in | At 12 in |
|---|---|---|---|
| 2 × 4 ft | 4 cu ft · 3 bags | 6.7 cu ft · 5 bags | 8 cu ft · 6 bags |
| 3 × 6 ft | 9 cu ft · 6 bags | 15 cu ft · 10 bags | 18 cu ft · 12 bags |
| 4 × 4 ft | 8 cu ft · 6 bags | 13.3 cu ft · 9 bags | 16 cu ft · 11 bags |
| 4 × 8 ft | 16 cu ft · 11 bags | ≈ 1 cubic yard | 32 cu ft · 22 bags |
| 4 × 12 ft | 24 cu ft · 16 bags | 40 cu ft · 1.5 cu yd | 48 cu ft · 1.8 cu yd |
Bag counts assume 1.5 cu ft bags. A cubic yard is 27 cu ft — past about one yard, bulk delivery is cheaper than bags.
Worked examples
- 4 ft × 8 ft = 32 sq ft
- 32 × 10 in (0.83 ft) = 26.7 cu ft
- 26.7 ÷ 27 = 0.99 cubic yards
- ≈ 1 cu yd — or 18 bags at 1.5 cu ft
- 2-inch lumber is really 1.5 in thick
- Inside = 3.75 × 7.75 = 29 sq ft
- 29 × 0.83 ft = 24.2 cu ft
- vs 26.7 from outside dims
- 4 × 4 = 16 sq ft each
- 16 × 1 ft = 16 cu ft per bed
- 16 × 3 beds = 48 cu ft
- 48 ÷ 27 = 1.8 cubic yards
Common mistakes to avoid
- Measuring the outside of the frame. You fill the inside. A '4 × 8' bed of 2-inch lumber is 3.75 × 7.75 inside — about 9% less soil than the outside numbers say.
- Filling with pure compost. Too rich, holds too much water, and shrinks hard as it breaks down — the bed is half empty by next spring. Mix roughly 60% topsoil to 40% compost.
- Filling with pure topsoil. On its own it compacts into a brick in a frame and drains badly. It needs compost through it to stay open.
- Going too shallow on a hard surface. 6 inches is fine over open soil because roots continue down. Over a patio or liner, the frame is all the root run there is — go 12 inches, more for carrots or potatoes.
- Ordering exactly enough. Fresh soil settles an inch or two in the first season. Fill to the top of the frame, order a little extra, and plan to top up each spring.
Frequently asked questions
How much soil do I need for a 4x8 raised bed?+
About 1 cubic yard at 10 inches deep — 26.7 cubic feet, or roughly 18 bags at 1.5 cu ft each. At 12 inches it's 32 cubic feet. Measure your bed's inside dimensions and enter them above for the exact figure.
Should I measure the inside or outside of the bed?+
The inside — you're filling the space between the boards. Nominal 2-inch lumber is really 1.5 inches thick, so a 4 × 8 bed is 3.75 × 7.75 inside, about 9% less soil than the outside measurement suggests.
How deep should a raised bed be?+
10–12 inches covers almost everything. Six inches works if the bed sits on open soil, because roots keep going down past the frame. If it's on a patio, gravel or a liner, go 12 inches — and 18 for deep-rooting crops like carrots and potatoes.
What's the best soil mix for a raised bed?+
Roughly 60% topsoil to 40% compost, optionally with perlite or coarse sand for drainage. Don't use either one alone: pure topsoil compacts and drains poorly in a frame, and pure compost is too rich and shrinks badly as it decomposes.
How many bags of soil for a raised bed?+
A 4 × 8 bed at 10 inches needs about 18 bags of 1.5 cu ft soil. That's already the point where bulk is cheaper and far less work — a cubic yard delivered costs less than 18 bags and fills the same bed.
Why does my raised bed soil keep sinking?+
Because it's supposed to. Fresh soil is fluffy, watering settles it, and the compost in the mix keeps breaking down — an inch or two in the first season is normal. Top up with compost each spring rather than trying to fill it once and for all.