Raised Bed Soil Calculator
Enter your bed dimensions and get the exact fill volume in cubic feet, cubic yards, and bags, split by blend component. This calculator accounts for freeboard (the gap below the rim) and first-season settling, which is where most estimates go wrong. The formula, a worked example, and every assumption are documented below.
Your Bed
Inside wall, ground to rim
Adjust assumptions & blend
Rim gap left unfilled
Wood, brush, coarse debris
Blend by volume (%)
Results
Order this much soil
– cubic yards
The blend is mixed, not layered. Component percentages describe the whole soil zone. Weight assumes moist blended soil at ~2,200 lb per cubic yard.
Freeboard
Soil blend
Bottom fill
The formula this calculator uses
2. Net volume: Vnet = L × W × Dfill × N (inside dimensions in feet, N beds)
3. Order volume: Vorder = Vnet × (1 + s) (s = settling allowance)
4. Convert: yd³ = Vorder ÷ 27 | bags = Vorder ÷ bag volume
5. Split blend: Vcomponent = Vorder × pcomponent
The two terms that separate a real number from a bad guess are freeboard and settling. Freeboard is the gap you leave below the rim so water and mulch stay in the bed instead of washing over the edge. Settling is the volume you lose in the first season as the compost fraction decomposes and the mix consolidates under its own weight and irrigation. Skip both and you will over-order by roughly ten percent and still be short by spring.
Worked example: one 4 × 8 ft cedar bed, 12 in tall
No bottom fill layer, standard 60 / 30 / 10 blend, 2 in freeboard, 12% settling.
- Dfill = 12 − 2 − 0 = 10 in = 0.833 ft
- Vnet = 8 × 4 × 0.833 × 1 = 26.67 cu ft
- Vorder = 26.67 × 1.12 = 29.87 cu ft
- Cubic yards = 29.87 ÷ 27 = 1.11 cu yd
- Bags at 1.5 cu ft = 29.87 ÷ 1.5 = 19.9 → 20 bags
- Topsoil 17.9 cu ft · Compost 9.0 cu ft · Aeration 3.0 cu ft
Verdict: order 1.25 cubic yards bulk, or 20 bags at 1.5 cu ft. At roughly 2,200 lb per cubic yard moist, that is about 1.2 tons — more than a half-ton pickup should carry in one trip.
Assumptions, and when to change them
Every default in the calculator is listed here and every one is editable. If your situation differs, change the input rather than adding a fudge factor.
| Assumption | Default | Basis and when to change it |
|---|---|---|
| Freeboard | 2 in | Keeps soil and mulch in the bed during heavy rain and overhead watering. Use 1 in for beds under 8 in tall, 3 in if you top with coarse mulch. |
| Settling allowance | 12% | Consolidation plus first-season decomposition of the organic fraction. Use 8% for a low-compost blend, up to 20% for blends above 40% compost. |
| Blend ratio | 60 / 30 / 10 | Topsoil or screened loam, finished compost, aeration amendment such as perlite, pumice, or coarse bark. Widely used for vegetable beds. Shift toward compost for heavy feeders, toward aeration for poorly draining sites. |
| Bulk density | 2,200 lb/yd³ | Moist blended garden soil. Ranges roughly 1,800–2,600 lb/yd³ depending on moisture and mineral content. Used only for the weight estimate. |
| Unit conversion | 27 cu ft = 1 cu yd | Exact. |
| Bed geometry | Rectangular | For L-shaped or tiered beds, run each rectangle separately and add the results. For circular beds, use πr² for area and enter an equivalent length and width. |
| Dimensions | Inside face | Measure the inside of the walls, not the outside. On a 2x lumber bed that is a 3-inch difference across the width — about 6% of the volume on a 4 ft bed. |
When to call a professional
Most raised beds are a weekend job. These are the cases where they stop being one:
Beds taller than about 24 inches. Retained soil generates lateral earth pressure that increases with the square of the height. Above roughly 2 feet, unbraced timber sides bow and eventually fail — you need cross ties, buttresses, or an engineered wall section.
Beds cut into a slope. Any bed that retains uphill soil is a retaining structure, not a planter, and needs drainage behind it. Many jurisdictions require a permit above 4 feet of retained height. Start with how to grade a yard.
Beds over a septic field, leach line, or buried utility. Added surcharge load and root intrusion are both problems. Call 811 before you dig regardless.
Bulk deliveries over about 3 cubic yards. That is a loaded truck on your driveway. Ask about slab thickness and turning radius before you schedule.
Standing water in the bed footprint after rain. Fix the drainage first or you are building a planter box on a pond — see how to fix standing water in a yard.
Built for planning, not blind ordering. Verify dimensions, supplier conversions, local codes, and site conditions before construction.
Frequently Asked Questions
How much soil do I need for a 4x8 raised bed?
A 4 ft by 8 ft bed that is 12 inches tall needs about 30 cubic feet, which is 1.1 cubic yards or 20 bags at 1.5 cubic feet each. That figure assumes 2 inches of freeboard below the rim and a 12 percent settling allowance. Fill the same bed to the very top with no settling allowance and you would calculate 32 cubic feet, which is why published numbers vary.
Should I fill a raised bed all the way to the top?
No. Leave 1 to 3 inches below the rim. That gap holds irrigation water and mulch in the bed instead of letting it sheet over the sides, and it gives you room to top dress next season without rebuilding the edge. Two inches is a good default for a bed 10 inches or taller.
How many bags of soil fill a raised bed?
Divide your total cubic feet by the bag volume. Bagged garden soil is most often sold in 1.5 or 2.0 cubic foot bags, so a 30 cubic foot bed takes 20 bags at 1.5 cubic feet or 15 bags at 2.0 cubic feet. Always round up. Above roughly 15 bags, bulk delivery is usually cheaper per cubic foot even after the delivery fee.
What is the best soil mix for a raised bed?
A common starting point for vegetable beds is 60 percent topsoil or screened loam, 30 percent finished compost, and 10 percent aeration amendment such as perlite, pumice, or coarse bark fines. The proportions are by volume, not weight. Increase compost for heavy feeders like tomatoes and squash, increase the aeration fraction on sites where the bed sits on clay or drains slowly.
Can I put something in the bottom of a raised bed to use less soil?
Yes, in tall beds. Logs, branches, and coarse woody debris in the bottom third of a bed 18 inches or deeper will cut your purchased soil volume meaningfully. Enter that depth in the bottom fill field and the calculator removes it from the order. Two cautions: the layer will decompose and drop over two to four seasons, so expect to top up, and do not do this in beds under 12 inches deep because you will be left with too little root zone.
Why does my raised bed soil sink every year?
Two causes. The organic fraction is decomposing, which is normal and expected, and the mineral fraction is consolidating under irrigation and its own weight. Losing 1 to 3 inches in the first year is typical. Top dress with compost each spring rather than replacing the whole volume.
Is bagged or bulk soil cheaper?
Bulk is almost always cheaper per cubic foot, often by half, but it comes with a delivery fee and a pile on your driveway that you have to move by wheelbarrow. The crossover is usually somewhere around 1 cubic yard, or roughly 18 bags. Below that, bags are easier to justify. Above 2 cubic yards, bulk is the clear answer.
Do I measure the inside or the outside of the bed?
The inside. On a bed built from 2x lumber, each wall is 1.5 inches thick, so the inside width of a bed you call 4 feet is actually 3 feet 9 inches. On a 4 by 8 bed that is about a 6 percent difference in volume, which is more than a bag.