Raised Bed Soil Calculator
How much soil mix fills a raised bed, measured inside its walls, split into compost, topsoil and aeration, with bags to buy and room for settling.
Result
Buy 29.06 ft³Estimate
Measured inside the walls. Add your own allowance for settling.
- Compost
- 9.69 ft³ (0.36 yd³)Estimate
- Topsoil
- 9.69 ft³ (0.36 yd³)Estimate
- Aeration
- 9.69 ft³ (0.36 yd³)Estimate
- Bags, if buying bagged
- 39Estimate
- Topsoil weight
- 786 lb (357 kg)Estimate
- Lost to the walls
- 10%
https://calcgallery.com/landscaping/raised-bed-soil-calculator/
Results are estimates for planning only. Actual needs vary with site conditions, products and workmanship. Always confirm quantities with your supplier and follow local building codes and manufacturer instructions. Read the full disclaimer
How to use the Raised Bed Soil Calculator
- Enter the bed's outside length and width, the depth of soil you want, and the thickness of its walls.
- Set the mix in parts. Equal parts compost, topsoil and an aeration material is a common starting point, not a rule.
- Add an allowance for settling if you like, and read the volume to buy, each part of the mix, and the bags.
The arithmetic
inside volume = (length − 2 × wall) × (width − 2 × wall) × depthto buy = inside volume × (1 + settling allowance)each part = to buy × its parts ÷ all the partsbags = to buy ÷ bag volume, rounded upBags round up, since a part bag is still a bag to buy. The topsoil’s weight uses a typical density for screened topsoil, which varies a lot with moisture; it is there to judge whether a load will fit in a car, not to order by.
With your numbers
- Soil = (length − 2 walls) × (width − 2 walls) × depth × (1 + settling)
Quick reference
Measured inside the walls. Add your own allowance for settling.
| Job | Soil mix to buy |
|---|---|
| 8 × 4 ft, 12 in deep, 2-by walls | 29.06 ft³ |
| Same, 10% for settling | 31.97 ft³ |
| 8 × 4 ft, measured outside only | 32 ft³ |
| 4 × 4 ft, 10 in deep | 11.72 ft³ |
| 2 parts compost, 1 topsoil | 29.06 ft³ |
| 2.4 × 1.2 m, 30 cm | 779.52 L |
Where the numbers come from
The fixed figures this calculator relies on, and the published source each one was checked against. How the calculators are built and checked
1 source
US Army FM 5-434, Earthmoving Operations (15 June 2000), Table 1-2 "Approximate material weights", loose column, copy at https://vulcanhammer.net/wp-content/uploads/2017/01/fm5_434.pdf: earth (loam or silt), dry, 1,900 to 2,200 lb/yd³; Woodland Earth, Screened Topsoil: about 2,200 lb/yd³, https://www.woodlandearth.com/topsoil-compost/p/screened-topsoil
Used for
- Bulk density of screened topsoil, about 2200 lb/yd³ or 1.1 US tons/yd³. Varies widely with moisture and composition.
Common questions about the Raised Bed Soil Calculator
- How many bags of soil do I need for a raised bed?
- 39 bags of 0.75 cubic feet, or 30 bags of 1 cubic foot, for a bed 8 by 4 feet outside and 12 inches deep. With 1.5-inch walls the inside is 7.75 by 3.75 feet, which holds 29.06 cubic feet, and 29.06 ÷ 0.75 = 38.75, rounded up to 39. Any settling allowance you enter is added before the bags are counted.
- How much soil do I need for a 4 by 8 raised bed?
- Less than 4 × 8 × depth suggests, because the walls take up room. With 1.5-inch boards, a bed 8 by 4 feet outside is 7.75 by 3.75 feet inside, and at 12 inches deep it holds about 29 cubic feet, not 32: 9% less. Measure inside the walls, or let the page subtract them.
- What soil mix should I use?
- A widely used mix is equal parts compost, topsoil and something to keep it open, such as perlite, vermiculite or fine bark. It is a convention rather than a standard, so the page lets you set any proportions: two parts compost to one of topsoil, for example.
- Should I buy extra for settling?
- Often, yes. Fresh mixes rich in compost settle noticeably in the first season as the compost breaks down and the soil firms. How much depends on the mix, so the page takes your own allowance rather than assuming one, and it can be topped up later.
- Bags or bulk?
- For a single small bed, bags are convenient. For several beds, bulk delivery by the cubic yard or cubic metre is usually much cheaper per unit, so the page gives the volume in both.