Raised Bed Soil Calculator
Soil volume, bag count, and compost share for any raised bed.
What it does
A raised bed eats more soil than it looks. An 8 by 4 ft frame at a foot deep holds 32 cubic feet, which is over twenty store bags or more than a cubic yard in bulk. Guessing at the cart usually means a second trip, so volume first, bags second.
The tool above runs both unit systems. Imperial mode takes feet and inches and prints cubic feet, bag count, and the compost share. Metric mode takes centimeters and prints liters with the same bag and compost plan. Defaults mirror each other: 8 x 4 ft at 12 in matches 243.8 x 121.9 x 30.5 cm at 906 liters.
Two constants carry the page. NIST defines the US gallon as 231 cubic inches and the liter as 1,000 cubic centimeters, which turns any box volume into store units. Clemson Extension frames the garden side: beds 3 to 4 ft wide for reach, compost at 10 to 20 % of the mix, and boxed beds at least 12 to 18 in deep for school gardens.
Depth deserves the last word. Six inches grows lettuce and herbs, while tomatoes and carrots want twelve or more. Soil also settles about 10 % after the first watering, so fill proud of the frame and top up after the first soak.
How to use
- Pick imperial (ft, in) or metric (cm).
- Measure inside length and width of the bed frame.
- Pick soil depth: 6 in for lettuce, 12 in for most vegetables.
- Read volume first for bulk orders, bags for store runs.
- Read the compost share to split the mix.
- Add about 10 % extra depth allowance for settling.
Formula
- L
- Bed length: ft in imperial mode, cm in metric mode.
- W
- Bed width across the short side.
- D
- Soil depth: 6 in for shallow roots, 12 or more for tomatoes.
- 231
- 231 cubic inches per US gallon, exactly by NIST definition.
Keep one unit per calculation; the tool converts inside. Bag math assumes 1.5 ft3 bags and rounds up, since half bags are not sold.
Worked example
Example: 8 x 4 ft bed, 12 in deep
Imperial defaults: 8 ft by 4 ft bed, 12 in of soil.
- Inputs: 8 ft long, 4 ft wide, 12 in deep (1 ft).
- Volume: 8 x 4 x 1 = 32 ft3.
- Yards: 32 / 27 = 1.19 yd3. Liters: 32 x 28.3168 = 906.1 L.
- Bags: 32 / 1.5 = 21.33, so 22 bags of 1.5 ft3.
- Compost at 15 %: 32 x 0.15 = 4.8 ft3. Tool shows 32 ft3, 22 bags, 4.8 ft3 compost.
Reference table
| Bed | 6 in deep | 12 in deep |
|---|---|---|
| 4 x 4 ft | 8.0 ft3 | 16.0 ft3 |
| 4 x 8 ft | 16.0 ft3 | 32.0 ft3 |
| 4 x 12 ft | 24.0 ft3 | 48.0 ft3 |
| 3 x 6 ft | 9.0 ft3 | 18.0 ft3 |
Limits
- Rectangular beds only. Round and tiered beds need their own geometry.
- Box interior assumed. Thick timbers shrink fill versus outside dims.
- Settling of about 10 % sits outside the volume; top up after soaking.
- Compost share is a planning 15 %, not a lab tested recipe.
Common mistakes
- Measuring outside the frame instead of inside.
- Ordering exact volume with no settling allowance.
- Buying by bag count for beds over a yard: bulk is cheaper.
- Filling only 6 in for deep root crops.
- Skipping the compost share and planting in straight topsoil.
Try it yourself
Quick checks. Answers sit tucked under each question.
4 x 4 ft bed, 6 in deep?
Depth 0.5 ft. 4 x 4 x 0.5 = 8 ft3. 8 / 1.5 = 5.33, so 6 bags. Compost 8 x 0.15 = 1.2 ft3.
120 x 80 x 30 cm bed?
120 x 80 x 30 = 288,000 cm3 = 288 L. 288 / 28.3168 = 10.17 ft3, so 7 bags. Compost 288 x 0.15 = 43 L.
Questions
How is bed volume figured?
Length times width times depth in matching units. An 8 x 4 ft bed at 1 ft deep holds 32 ft3.
How many cubic yards is that?
Divide ft3 by 27. The example 32 ft3 is 1.19 yd3, which is how bulk yards are sold.
How many bags of soil do I need?
Divide volume by the bag size and round up. With 1.5 ft3 bags, 32 ft3 needs 22 bags.
How deep should a raised bed be?
Clemson Extension suggests beds 3 to 4 ft wide for reach, with boxed beds at least 12 to 18 in deep for school gardens.
How much compost goes in the mix?
About 10 to 20 % of the mix per Clemson guidance. The tool plans 15 %, which is 4.8 ft3 on a 32 ft3 bed.
Does the soil settle?
Yes, by roughly 10 % after watering. Buy the full computed volume and top up after the first soak.
Sources
- NIST: SI Units, Volume (liter definitions)
- NIST: Gallon definition (231 cubic inches)
- Clemson Extension: Raised Beds