Original GreenCalc gardening illustration for the raised-bed soil volume guide, linked to the Soil Calculator. The scene is illustrative, not a soil-layering specification.
A garden bed’s outside dimensions do not tell you exactly how much soil to buy. The useful measurements are the space inside the walls and the depth that still needs filling. Once those are known, the calculation is geometry. Choosing a suitable growing mix is a separate decision.
The Soil Calculator converts those measurements into volume, whole bags and an estimated bag purchase cost. It supports rectangular and circular beds, metric and imperial units, and an optional extra allowance that you choose.
Start with the empty space you actually need to fill
Measure the inside length and width of a rectangular bed. For a circular bed, measure the inside diameter across the center. Then measure vertically from the existing soil surface to your intended fill level. Do not automatically enter the full height of the bed walls if there is already usable material inside.
If the surface is uneven, measure the required depth in several places. A single uniform depth is a simplification. For a sharply sloping or irregular space, divide it into simpler sections and calculate their volumes separately.
A metric example from measurements to bags
Suppose a rectangular bed has an inside length of 2 m, width of 1 m and fill depth of 30 cm. Convert the depth to meters: 30 ÷ 100 = 0.30 m. The volume is 2 × 1 × 0.30 = 0.60 m³, or 600 liters.
With 40-liter bags and no extra allowance, the order requires 600 ÷ 40 = 15 bags. If you deliberately choose a 10% allowance, the planning volume becomes 660 liters and the result rounds up to 17 whole bags. The extra percentage is an ordering assumption, not a measured settling rate.
An imperial example and the common depth mistake
A bed measuring 4 ft by 8 ft with a 6 in fill depth contains 4 × 8 × (6 ÷ 12) = 16 ft³. Divide by 27 to obtain approximately 0.593 yd³. With 2 ft³ bags, that is eight bags before any chosen allowance.
Multiplying 4 × 8 × 6 without converting inches to feet gives the wrong volume. Units are part of the calculation, not labels to add afterward. The tool converts depths and displays the order volume in both measurement systems.
Use diameter correctly for a circular bed
A circular bed with a diameter of 1 m has a radius of 0.5 m. Its area is π × 0.5². At a fill depth of 20 cm, its volume is approximately 0.157 m³, or 157 liters. Four 40-liter bags provide 160 liters, so the purchase count is four.
Entering the radius as the diameter would make the calculated area four times too small. Check that the measurement passes through the center from one inside edge to the opposite edge.
Round bags after calculating the complete order
When several beds are identical, enter the number of beds so the tool combines their volume before rounding. Rounding each small bed independently can create unnecessary extra bags. For beds with different dimensions, add their unrounded volumes before dividing by the chosen bag size.
Bag volume matters just as much as bed volume. A 40-liter bag is not the same as a 40 kg bag. Volume cannot be recovered from a mass label without an appropriate bulk density. Do not substitute a liquid’s density or assume all soil mixes have the same mass per liter.
Volume does not choose the soil recipe
University extension guidance distinguishes filling a raised bed with a growing mix from simply adding compost. GreenCalc estimates quantity; it does not prescribe a compost percentage, nutrient balance or crop-specific rooting depth. Use local growing guidance and the supplier’s product information to choose a suitable material.
The cost output multiplies the whole-bag count by the price you enter. It excludes delivery, tax and bulk-order minimums. If a supplier sells by cubic yard, use the volume result to request a quote rather than treating the bag-cost field as a bulk price.
Connect filling with planting
After checking the fill quantity with the soil volume tool, use the Seed Calculator to explore planting layout questions. The shared Biology category keeps the gardening tools together while leaving pet-care calculations in their own related group.
By GreenCalc Editorial Team · Updated 2026-10-19
Frequently asked questions
Should I measure outside the bed walls?
Use inside dimensions and only the depth still needing fill. Wall thickness and existing material reduce the space to fill.
Why are bags rounded upward?
Bags are purchased whole. The calculator divides total order volume by bag volume and rounds the final count upward.
Does every order need ten percent extra?
No. Extra allowance defaults to zero and is your own planning choice, not a universal settling or compaction rate.