How the calculator works
soil = inside length × inside width × fill depth × beds × (1 + settling %)
bags = soil ÷ bag size (rounded up)
Bagged soil is sold by volume: litres in the UK and Australia, cubic feet in the US. So the bag count only depends on the bag size printed on the label, not on how heavy the soil is.
Worked examples
US: one 8 × 4 ft bed, 12 in deep: 32 ft³ + 15% = 36.8 ft³ (1.36 yd³), or 25 bags of 1.5 cu ft.
UK: two 2.4 × 1.2 m beds, 30 cm deep: 1.728 m³ + 15% = 1.99 m³ (1,987 litres), or 40 bags of 50 L. That many bags is a good reason to compare with a bulk delivery.
Filling a bed with Mel's Mix
Square Foot Gardening's Mel's Mix is equal parts by volume of blended compost, peat moss (or coir), coarse vermiculite. Pick it above and the calculator splits the total into the three parts. Peat moss is often sold in compressed bales that expand when opened, so use the loose or expanded volume on the label when you work out how many bales to buy.
The assumptions, in plain words
- Rectangular beds, all the same size. For different sizes, work out each bed and add it to your project.
- Bag sizes are real products listed in the sources below. If yours is different, choose "Other bag".
- Settling is applied to the whole volume.
Common mistakes to avoid
- Measuring the outside of the bed. Use the inside length and width: the boards take up room.
- Forgetting settling. New soil sinks, so add 10–20%.
- Assuming every bag is the same. Bags range from 25 L to 1.5 cu ft (about 42 L) and more. Use the size printed on yours.
Measuring an awkward shape? See how to measure an irregular area.
Frequently asked questions
How much soil do I need for a 4 × 8 raised bed?
A 4 × 8 ft bed filled 12 in deep holds 32 cubic feet. With 15% extra for settling, that's 36.8 cubic feet (1.36 cubic yards): 25 bags of 1.5 cu ft.
Why add extra soil for settling?
Fresh soil and compost sink once they are watered and the air pockets close up. Guides suggest adding 10–20%, so we use 15%. Without it, beds often end up a few inches below the rim.
What is Mel's Mix?
Mel's Mix is the raised bed recipe from Square Foot Gardening: one third blended compost, one third peat moss (many people use coir instead) and one third coarse vermiculite, mixed by volume. Choose it in the calculator and you'll get the volume of each part.
Are bags or bulk soil cheaper for raised beds?
For one small bed, bags are often simpler. For several beds, bulk soil by the cubic metre or cubic yard is often cheaper per litre, but delivery charges vary. Open "Bags or bulk?" and type your local prices to see which is cheaper for you. We never guess prices.
Can I check this against a bag label?
Yes. Scotts says two 1.5 cu ft bags of Miracle-Gro Organic Raised Bed & Garden Soil fill a 2 × 2 ft bed 6 in deep. That bed holds 2 cubic feet, and 2 ÷ 1.5 rounds up to 2 bags, which matches.
Sources
- Scotts Miracle-Gro: Organic Raised Bed & Garden Soil (1.5 cu ft; 2 bags fill 2 × 2 ft × 6 in)
- Ace Hardware: Miracle-Gro Performance Organics Raised Bed Soil 1.3 cu ft
- Tractor Supply: Miracle-Gro Raised Bed & Garden Soil 1 cu ft
- Webbs: Westland Raised Bed & Vegetable Soil 40 L (UK)
- Homebase: J. Arthur Bower’s Multi-Purpose Compost 50 L (UK)
- Bunnings: Seasol 25 L Garden Soil Mix (Australia)
- Square Foot Gardening Foundation: Mel’s Mix
- Eartheasy: Soil calculator (settling)
Last updated 11 November 2026. Found a mistake? Tell us.