Raised Bed Soil Calculator

Find out exactly how much soil you need to fill a raised garden bed — in cubic feet, cubic yards, litres, and number of bags. Supports rectangular, square, circular, and custom shapes with mix ratios and settling buffer.

The Raised Bed Soil Calculator finds exactly how much soil you need to fill or top up a raised garden bed. Results are displayed in cubic feet, cubic yards, litres, and bag count so you can shop directly without manual conversion. The calculator supports rectangular, square, circular, and custom shapes in both metric and imperial units.

Volume estimates are based on the dimensions entered and are accurate for planning and purchasing purposes. Actual soil needs may vary due to settling and measurement rounding — the advanced options include a settling buffer to account for this. For professionally built or lined beds, measure the inner dimensions of the frame.

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Top Up: add soil on top of what is already there

Advanced options (settling, mix ratio, bag size)
% extra to buy (soil settles over time)
You need — cubic feet of soil
Cubic Feet
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Cubic Yards
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Litres
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Bags Needed
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How This Calculator Works

The calculator converts your bed dimensions into a total soil volume, then breaks that volume into litres and a bag count based on the bag size you select. Three adjustments are available in the advanced options: a settling buffer, a soil mix ratio, and a custom bag size.

Bed Volume

Volume is calculated using standard geometric formulas for each bed shape. The result is first computed in cubic centimetres, then converted to cubic feet, cubic yards, and litres. If you enter multiple beds, the per-bed volume is multiplied by the bed count to give a combined total.

Soil Settling and Wastage

Freshly filled raised beds typically settle 10–20% in the first growing season as organic material in the compost and peat compresses under its own weight. The settling buffer field adds a percentage on top of the calculated volume so your order covers that shrinkage. The default is 10%; increase to 15–20% if your mix is heavy in compost or peat moss.

Soil Mix Ratios

When a mix ratio is selected, the total volume is divided between components proportionally. For example, selecting 80% topsoil and 20% compost on a 906-litre bed returns 725 litres of topsoil and 181 litres of compost — each shown as a separate bag count so you can purchase the right amount of each product.

Bag Count

The bag count is the total litres divided by the bag size, rounded up to the nearest whole bag. Rounding up is intentional — it is better to have one spare bag than to run short mid-fill. Common bag sizes (20 L, 40 L, 50 L, 1 cu ft, 2 cu ft) are preloaded; you can also enter a custom size for bulk bags or local products.

Raised Bed Soil Formula

The volume formula varies by bed shape. In each case the result is in the same unit as the dimensions entered, then converted internally.

Rectangular bed: Volume = Length × Width × Depth

Square bed: Volume = Side × Side × Depth

Circular bed: Volume = π × (Diameter ÷ 2)² × Depth

Custom area: Volume = Area × Depth

For example, a rectangular bed 4 ft long, 8 ft wide, and 12 inches deep:

Volume = 4 × 8 × 1 = 32 cubic feet (906 litres, 23 bags of 40 L)

With a 10% settling buffer: 32 × 1.10 = 35.2 cubic feet to purchase (about 25 bags).

The calculator handles all unit conversions automatically — the formulas are shown here for transparency.

How to Measure Your Raised Bed

Accurate measurements give you an accurate soil order. Here is how to measure each dimension:

  • Length and Width: Measure the inside dimensions of the bed frame, not the outside. The inside measurement is the actual growing area that needs to be filled.
  • Depth: Measure from the bottom of the frame (or the ground surface if the bed has no base) to the top edge. For a top-up calculation, also measure the depth of existing soil so the calculator can find the difference.
  • Circular beds: Measure the diameter (the widest point across the centre), not the radius. The calculator divides the diameter by two to get the radius and then applies the circle-area formula.

Soil Volumes for Common Raised Bed Sizes

Use this quick-reference table to estimate soil needs for the most popular raised bed dimensions. All calculations assume a full fill with no settling buffer.

Bed Size Depth Cubic Feet Litres 40 L Bags
4 × 4 ft 12 in 16 cu ft 453 L 12 bags
4 × 8 ft 12 in 32 cu ft 906 L 23 bags
4 × 4 ft 6 in 8 cu ft 227 L 6 bags
8 × 8 ft 12 in 64 cu ft 1,813 L 46 bags

Soil Mix Guide

100% Topsoil or Raised-Bed Mix: The simplest approach. Good-quality bagged raised-bed mix drains well and contains enough nutrition for the first season. Best for gardeners who want an easy start without blending. Refresh with compost each spring.

80% Topsoil / 20% Compost: A popular and cost-effective mix. The compost adds organic matter, beneficial microbes, and slow-release nutrients. This ratio works well for vegetables, herbs, and flowers and is easy to source from any garden centre.

Mel’s Mix (one-third each): Blended compost, peat moss or coconut coir, and coarse vermiculite in equal parts. The vermiculite improves drainage and aeration; the coir holds moisture without waterlogging. Widely used in square-foot gardening and produces excellent yields, though it costs more to set up initially.

Frequently Asked Questions

Multiply the length by the width by the depth of the bed. For a 4 × 8 ft bed that is 12 inches (1 ft) deep: 4 × 8 × 1 = 32 cubic feet, or about 906 litres. For a circular bed use Volume = π × (diameter ÷ 2)² × depth. Enter your dimensions above and the calculator converts and totals everything automatically.
Topsoil is garden-grade mineral soil, typically heavy and inexpensive, used as the bulk of a fill. Raised-bed mix is a lighter, pre-blended product formulated for drainage and root growth — a better default if budget allows. Potting soil is the lightest and most expensive, blended for containers; it can become waterlogged in large wooden beds. Most gardeners use raised-bed mix as the base and blend in 20–40% compost for nutrition.
A widely used option is Mel’s Mix: equal thirds of blended compost, peat moss or coconut coir, and coarse vermiculite. For a simpler approach, 60% topsoil or raised-bed mix with 40% compost gives excellent results for most vegetables, herbs, and flowers. Use the Soil Mix selector in the advanced options to split your order by component automatically.
Yes. Select the shape that best matches your container — Rectangular for window boxes and planter troughs, Circular for round pots, or Custom Area for irregular shapes. For grow bags with a stated volume (e.g., 200 L), you do not need the calculator — just check the bag label. The Top-Up fill mode is also useful for refreshing an existing container with a layer of new compost.
At least 6 inches suits shallow-rooted crops such as lettuce and herbs. 12 inches is the standard recommendation for most vegetables. 18–24 inches suits deep-rooted crops like tomatoes, carrots, and parsnips, or beds placed on concrete or paving where roots cannot extend into native soil below.
The calculator is geometrically exact for the dimensions entered. Accuracy in practice depends on how precisely you measured the bed: a 5% error in one dimension produces a 5% error in volume. The main real-world variable is settling — fresh soil rich in compost or peat can compress 10–20% in the first season. Use the settling buffer in the advanced options to add a purchasing cushion.
Yes. Soil typically settles 10–20% in the first season, especially mixes rich in compost or peat. Buy 10–15% more than the calculated volume to allow for this. The settling factor in the advanced options adds a percentage buffer to the bag count automatically so you do not need to calculate it manually.
Top up with 2–4 inches of fresh compost each spring before planting. Compost replenishes nutrients that crops removed in the previous season and feeds the soil microbiome. Avoid tilling deeply — raised bed soil is meant to stay loose and undisturbed. Use the Top-Up mode in this calculator to find exactly how many bags of compost you need to reach your target depth after settling.

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Calculator Category

This tool belongs to Garden Calculators. Browse similar tools to help plan and maintain your garden.

Results are estimates based on the dimensions entered. Actual soil needs may vary due to settling, compaction, and measurement rounding. Always round up when purchasing bags.