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Formula
The volume of topsoil, compost, or fill needed for a rectangular area is:
Volume (yd³) = Length (ft) × Width (ft) × Depth (in) / 324
For cubic feet: Volume (ft³) = Length (ft) × Width (ft) × Depth (in) / 12
Where:
- Length – the longer side of the area, in feet.
- Width – the shorter side, in feet.
- Depth – how deep you want the material, in inches.
- 324 – conversion factor: 1 cubic yard = 27 cubic feet, and 1 foot = 12 inches, so 27 × 12 = 324.
Worked Example
Using the default values from the calculator: length = 40 ft, width = 25 ft, depth = 3 in.
- Convert depth to feet: 3 in ÷ 12 = 0.25 ft.
- Calculate volume in cubic feet: 40 × 25 × 0.25 = 250 ft³.
- Convert to cubic yards: 250 ÷ 27 = 9.26 yd³ (rounded to two decimals).
The calculator’s first output shows 9.26 yd³, matching the manual calculation. The second output shows 250 ft³, which is the unrounded cubic feet value.
When to Use This Calculator
Use this calculator when you need to order soil, compost, mulch, or gravel for a garden bed, raised planter, or landscaping project. It works for any rectangular area with a uniform depth. For irregular shapes, divide the area into rectangles, calculate each, and sum the results.
The result is the volume you need to fill the space. If you are buying bagged material, check the bag’s volume (usually in cubic feet) and divide the total cubic feet by that number. If you are ordering bulk material, suppliers typically sell by the cubic yard, so use the first output.
Common Mistakes
- Mixing units: Length and width are in feet, depth in inches. The calculator handles this, but if you do it manually, convert everything to the same unit first.
- Forgetting compaction: Soil settles after watering. For a 3-inch layer, order about 10–15% extra to account for compaction.
- Rounding too early: If you round the cubic feet to 250 and then divide by 27, you get 9.26. Rounding the depth to 0.3 ft instead of 0.25 would give 300 ft³, a 20% error.
- Ignoring the shape: This formula assumes a flat, uniform depth. For sloped areas, measure depth at several points and average.
Limits of the Formula
This is a geometric volume calculation. It does not account for:
- Material density: Weight varies by material. A cubic yard of dry topsoil weighs about 2,000 lb, while compost may be lighter. Check with your supplier for weight limits on trucks.
- Settling: Organic materials decompose and settle over time. You may need to top up after a few months.
- Irregular depth: If your bed is deeper at one end, the estimate will be off. Use an average depth.
For large projects, always confirm with your supplier—they may have delivery minimums or recommend a specific overage.
Frequently Asked Questions
How many bags of soil do I need?
Divide the total cubic feet (the second output) by the volume of one bag. Most retail bags are 1.5 or 2 cubic feet. For example, 250 ft³ ÷ 1.5 ft³ per bag = 167 bags. Round up to the nearest whole bag.
Should I order extra soil?
Yes, order about 10–15% extra for compaction and settling. If you are filling a raised bed that will hold plants, the roots will displace some soil, so you may not need as much. For a 3-inch top dressing, 10% extra is a safe margin.
Can I use this for mulch?
Yes, mulch is sold by volume. The same formula works. However, mulch is often lighter than soil, so the weight will be lower. A cubic yard of wood mulch weighs about 400–600 lb, depending on moisture.
What if my area is not rectangular?
Break it into rectangles, calculate each with the calculator, and add the volumes. For circular beds, use a different formula (π × radius² × depth). For very irregular shapes, you can approximate with a rectangle that covers the whole area, but you will overestimate.
How deep should I make the layer?
For new garden beds, 6–8 inches of soil is common. For top dressing an existing lawn, 1–2 inches is enough. For mulch, 2–3 inches is typical. Deeper layers can smother plants or cause drainage issues.