Soil Calculator
Find out exactly how much soil a garden bed needs, then break it into a three-part mix, count the bags, estimate weight, and price the job. Works for rectangular, circular and triangular beds in metric or imperial units.
Formula
Worked example
An 8 ft × 4 ft rectangular bed, 6 in deep, with 10% settling: area = 32 ft², volume per bed = 32 × 0.5 = 16 ft³, add 10% = 17.6 ft³ (0.65 cu yd). With 1.5 cu ft bags: 17.6 ÷ 1.5 = 11.7, so buy 12 bags.
How the soil volume is calculated
A garden bed holds a volume equal to its base area times the fill depth. For a rectangular bed that is length times width; for a circular planter it is pi times the radius squared; for a triangular bed it is half the base times the perpendicular height. The depth, usually measured in inches or centimetres while the horizontal dimensions are in feet or metres, is converted to the same unit before multiplying. The result in cubic feet is divided by 27 to reach cubic yards, the unit most bulk soil suppliers price by. In metric the result is already in cubic metres, which equal 1,000 litres each, matching how bagged potting compost is sold across the UK and Europe.
Settling, bag counts and the soil mix
Fresh soil and compost are fluffy and full of air, so they compact noticeably after the first few waterings, often by an inch or two in a typical raised bed. The settling allowance (default 10%) adds that buffer automatically so you do not have to make a second shopping trip. The bag count is the total volume with settling divided by the bag volume, rounded up, because partial bags cannot be returned. US bagged soil is sold in cubic feet (1, 1.5 and 2 cubic foot bags are most common) while EU/UK potting compost is sold by the litre (30, 50 and 60 litre bags). The soil mix breakdown lets you split the order into topsoil, compost and aeration material such as perlite or coarse grit, using the classic 60/30/10 raised-bed blend or any ratio that suits your plants.
Shape modes, multiple beds and cost
Choosing a circular shape calculates area from the outer diameter using the circle area formula, which is the correct approach for round stock-tank planters. The triangle mode suits corner beds or keyhole plots. If you are building several identical beds at once, enter the count and the total is multiplied before bagging. The optional cost estimate works in two modes: per bag at retail price, or per cubic yard for bulk delivery. Bulk delivery is almost always cheaper once you need more than about half a cubic yard, so the calculator shows both the bagged total and the cubic yard figure so you can compare. The weight estimate uses a typical moist topsoil density of around 100 pounds per cubic foot (1,600 kg/m3) as a planning figure for delivery logistics or deck-load checks.
Buying soil and what to expect
When you buy bagged soil, the volume on the bag is measured loose in the bag, so the same volume will compress once placed and watered, which is why the settling buffer exists. For large projects where bulk delivery is cheaper, compare the cubic yard price: one cubic yard equals 27 cubic feet or roughly 765 litres, so a bed needing more than about 18 of the common 1.5-cubic-foot bags is worth pricing as a bulk order. When mixing your own blend, premix the components in a wheelbarrow or tarp before filling so the ratios stay consistent throughout the bed depth. Organic matter such as compost breaks down over the growing season and beds will sink a little each year, so topping up with compost each spring is normal and keeps fertility high.
Bags of soil for common raised bed sizes
| Bed size | Fill depth | Net volume | With 10% settling | 1.5 cu ft bags |
|---|---|---|---|---|
| 4 × 4 ft | 6 in | 8 cu ft | 8.8 cu ft | 6 |
| 4 × 4 ft | 12 in | 16 cu ft | 17.6 cu ft | 12 |
| 8 × 4 ft | 6 in | 16 cu ft | 17.6 cu ft | 12 |
| 8 × 4 ft | 12 in | 32 cu ft | 35.2 cu ft | 24 |
| 10 × 4 ft | 8 in | 26.7 cu ft | 29.3 cu ft | 20 |
| 12 × 4 ft | 12 in | 48 cu ft | 52.8 cu ft | 36 |
| 4 ft circle (dia) | 12 in | 12.6 cu ft | 13.8 cu ft | 10 |
Using 1.5 cu ft (approx 42 L) bags with a 10% settling allowance. Actual bag count depends on bag size and soil density.
Frequently asked questions
How much soil do I need for a 4 × 8 foot raised bed?
An 8 ft × 4 ft bed filled 6 inches deep needs 16 cubic feet of soil before settling. Adding a 10% settling buffer brings that to about 17.6 cubic feet, or roughly 12 bags of the common 1.5-cubic-foot size. Doubling the depth to 12 inches doubles everything to 35.2 cubic feet with settling, or about 24 bags.
How many bags of soil are in a cubic yard?
A cubic yard is 27 cubic feet, so it takes 18 of the 1.5-cubic-foot bags, 27 of the 1-cubic-foot bags, or about 14 of the 2-cubic-foot bags to make one cubic yard. In metric, one cubic yard is roughly 765 litres, or about 15 standard 50-litre bags.
What is a good soil mix ratio for a raised bed?
A popular starting blend is 60% topsoil, 30% compost and 10% coarse aeration material such as perlite or grit. Topsoil provides weight and mineral content, compost adds fertility and moisture retention, and the aeration component opens the mix so roots and water can move freely. Heavy-feeding vegetables do well with more compost (up to 40-50%), while drought-tolerant herbs prefer more grit.
Should I add extra soil for settling?
Yes. Fresh topsoil and compost are airy and can settle by an inch or two after the first few waterings. Adding 10% extra is a practical buffer that avoids a second trip to the garden centre. Beds tend to sink a little each growing season as organic matter breaks down, so an annual spring top-up with compost is normal maintenance.
How do I calculate soil for a circular or triangular bed?
For a circular bed, use the circle area formula: pi times the radius squared (that is 3.14159 times half the diameter, squared), then multiply by the fill depth. For a triangle, multiply 0.5 times the base times the perpendicular height, then by the depth. This calculator handles both shapes automatically when you select the shape at the top.
When is bulk delivery cheaper than bagged soil?
As a rule of thumb, once you need more than about half a cubic yard (13-14 of the 1.5-cubic-foot bags), a bulk delivery from a landscape supplier is usually cheaper per cubic yard, though there may be a minimum-order quantity and a delivery charge to factor in. Use the cost estimate toggle to compare the per-bag retail total against the bulk cubic-yard price from your supplier.