Raised Bed Soil Calculator
How much soil to fill a raised garden bed
Filling a raised garden bed? Enter the bed length, width and height (as the depth) to get the soil needed in cubic feet, cubic yards and bags.
How much soil for raised bed projects
Garden bed soil is bought by volume, so the raised bed soil mix you choose only changes what goes into that volume, not how much of it you need.
Enter the bed's inside dimensions and its inside height. Beds settle noticeably over the first season as organic matter breaks down, so fill right to the top and expect to top up next spring.
| Bed | Area | Cubic feet | Cubic yards | 1.5 ft³ bags |
|---|---|---|---|---|
| 2 × 4 ft | 8 ft² | 8 ft³ | 0.30 yd³ | 6 |
| 4 × 4 ft | 16 ft² | 16 ft³ | 0.59 yd³ | 11 |
| 3 × 6 ft | 18 ft² | 18 ft³ | 0.67 yd³ | 12 |
| 4 × 8 ft | 32 ft² | 32 ft³ | 1.19 yd³ | 22 |
| 4 × 12 ft | 48 ft² | 48 ft³ | 1.78 yd³ | 32 |
The classic 4 × 8 bed at 12 inches needs 22 bags, and at $6 to $10 a bag that is $130 to $220 — usually more than the timber cost. Above about 15 bags, bulk delivery is cheaper, and two beds is already past that point.
| Depth | Cubic feet | Cubic yards | Bags |
|---|---|---|---|
| 6 in | 16 ft³ | 0.59 yd³ | 11 |
| 10 in | 26.7 ft³ | 0.99 yd³ | 18 |
| 12 in | 32 ft³ | 1.19 yd³ | 22 |
| 18 in | 48 ft³ | 1.78 yd³ | 32 |
| 24 in | 64 ft³ | 2.37 yd³ | 43 |
Depth is where the cost lives. Doubling from 12 to 24 inches doubles the fill, and for most vegetables it buys nothing — which is what the next section is about.
How deep does a raised bed need to be?
10 to 12 inches suits almost everything, and the reason is that a bed sitting on open ground is not the only soil available. Roots pass straight through the bottom into the ground beneath, so the bed depth is a head start rather than a hard limit.
| Situation | Depth | Notes |
|---|---|---|
| On open soil, most vegetables | 10 to 12 in | Roots continue into the ground |
| Salad, herbs, shallow roots | 6 to 8 in | |
| Carrots, parsnips, potatoes | 12 to 18 in | Or loosen the soil beneath |
| On a deck, patio or concrete | 18 to 24 in | No soil below to borrow |
| Accessible height, no bending | 24 to 30 in | Fill the base cheaply |
The exception is a bed on a hard surface, where the full root zone has to be inside the box. Everything else is a comfort decision, and a tall bed built for back-friendly access does not need premium mix all the way down.
What to fill it with
Do not use plain garden soil or bagged "topsoil" alone — both compact hard in a container and drain poorly. Do not use pure compost either; it is too rich, holds too much water and slumps by roughly a third in the first season.
| Mix | Recipe | Per component |
|---|---|---|
| 50/50 (simplest) | Half screened topsoil, half compost | 16 ft³ each, 11 bags each |
| Mel's Mix | ⅓ compost, ⅓ peat or coir, ⅓ vermiculite | 10.7 ft³ each |
| Bulk garden mix | Pre-blended, sold by the yard | 1.19 yd³ |
The 50/50 blend is the sensible default: cheap, widely available, and it performs nearly as well as anything more elaborate. Mel's Mix is excellent and expensive, with vermiculite usually the costliest single ingredient. Many landscape yards sell a pre-blended "garden" or "raised bed" mix by the cubic yard, which for anything above one bed is the easiest route.
Compost quantities on their own are covered by the compost calculator, and topsoil has its own page.
Filling a deep bed without filling it with money
For a tall bed, only the top 10 to 12 inches does the work. Fill the bottom with something cheap or free:
- Hügelkultur: untreated logs and branches in the base. They rot down slowly, hold water like a sponge and feed the bed for years. Expect real settling in year one.
- Leaves, straw, grass clippings and cardboard, layered and watered.
- Coarse compost or fill dirt for the lower third.
- Not gravel or rocks. The old advice to add drainage stone at the bottom of a container is wrong — it creates a perched water table that makes drainage worse, not better.
- Nothing that will not break down predictably. No treated timber, no plastic, no rubble.
On open ground, prep is simple: loosen the soil beneath with a fork so roots can get through, and lay plain cardboard over grass to smother it. It rots away within a season. Skip landscape fabric — it eventually blocks the root access that makes a bed on soil work in the first place.
How this raised bed soil calculator works out its numbers
Volume is inside length × inside width × inside depth, reported in cubic feet, cubic yards and bags. Bag counts assume the standard 1.5 ft³ retail bag, so 18 bags to the cubic yard.
- Waste defaults to 0. A bed is a closed box with nothing to cut or spill, so the calculated volume is the volume. Settling is handled by topping up next season rather than by over-ordering.
- Density is 60 lb/ft³ for a typical soil-and-compost blend, between compost at 40 and screened topsoil at 90. It affects weight only, not volume.
- Measure inside dimensions, not outside. On a bed framed in 2 in timber the difference is 4 inches on each axis, which is around 8% of the fill on a 4 ft wide bed.
- Mix ratios are proportions of the total volume: divide the result by two for 50/50, or by three for Mel's Mix.
- Depth guidance assumes a bed on open soil, where roots pass into the ground beneath. Beds on hard surfaces need the full root depth inside the box.
Reviewed September 2026.
Figures reviewed . Every worked example on this page is checked against the calculator above.
Raised Bed Soil Calculator: frequently asked questions
How much soil for a 4x8 raised bed?
A 4 × 8 ft bed 12 inches deep needs 32 cubic feet, which is 1.19 cubic yards or 22 standard 1.5 ft³ bags. At $6 to $10 a bag that usually costs more than the timber did.
How much dirt to fill a raised bed?
Multiply length × width × depth in feet. A 4 × 4 bed at 12 inches is 16 ft³ (11 bags), a 3 × 6 is 18 ft³ (12 bags), and a 4 × 12 is 48 ft³ (32 bags).
How deep should a raised bed be?
10 to 12 inches for most vegetables on open ground, since roots continue down into the soil beneath. Go to 18 to 24 inches only if the bed sits on a deck, patio or concrete where there is no soil to borrow.
Can I just use topsoil in a raised bed?
No. Plain topsoil compacts hard in a container and drains poorly. Blend it roughly half and half with compost. Equally, do not fill with pure compost, which is too rich, holds too much water and slumps by about a third in the first season.
What is the best soil mix for a raised bed?
A 50/50 blend of screened topsoil and compost is the sensible default, which for a 4 × 8 × 12 in bed is 16 ft³ of each. Mel's Mix — equal thirds compost, peat or coir, and vermiculite — is excellent but costs considerably more.
Should I put rocks in the bottom of a raised bed?
No. It is common advice and it is wrong: a coarse layer beneath finer soil creates a perched water table that makes drainage worse. If you want to save fill in a deep bed, use logs, leaves and coarse compost instead.
Should I put anything in the bottom of a raised bed?
On open soil, loosen the ground with a fork and lay plain cardboard to smother grass. In a deep bed, fill the lower portion with untreated logs and leaves — hügelkultur — which saves mix and feeds the bed as it breaks down.
How much does raised bed soil settle?
Noticeably in the first season, particularly with a high compost fraction or a hügelkultur base, since organic matter breaks down and consolidates. Fill right to the top and plan to top up each spring.