TerraEvidence
Evidence-based raised-bed planning

How much soil do I need for a raised garden bed?

For a rectangular bed, multiply the inside length by the inside width by the fill depth. Convert every measurement to one unit first, then convert the result to the unit sold by your supplier.

Use the free soil calculatorSee a worked example
The short answer

Soil volume = inside length × inside width × fill depth. In imperial units, divide cubic feet by 27 to obtain cubic yards. For bags, divide the adjusted volume by the volume printed on one bag and round up to the next whole bag.

Measure before buying

Five steps that keep the estimate honest

The arithmetic is simple. Most purchasing errors come from measuring the wrong space, mixing units, or treating settlement as a fixed number.

1. Measure the inside planting dimensions

Use the space the material will actually occupy, not the outside dimensions of boards or blocks. Measure each bed separately if dimensions differ.

2. Choose the intended fill depth

Fill depth is the thickness of material you plan to add. It is not automatically the full wall height, especially when existing suitable soil already occupies part of the bed or you want space for mulch.

3. Put every dimension in the same unit

Convert inches to feet before multiplying imperial dimensions. For example, 10 inches is 10 ÷ 12, or about 0.833 feet. A metric calculation can use metres for cubic metres or centimetres converted consistently before expressing the result in litres.

4. Calculate the base geometric volume

Multiply length × width × depth. This describes a rectangular space. Rounded, tapered, sloped, or irregular beds require a shape-specific measurement or a cautious estimate.

5. Convert to the purchase unit and round containers up

Bulk material is often sold by cubic yard or cubic metre; bagged material by cubic feet or litres. Divide by the stated container volume and round up because a fraction of a bag is not normally purchasable.

Worked example

An 8 ft × 4 ft bed filled 12 in deep

Base volume

Convert 12 inches to 1 foot, then calculate:

8 ft × 4 ft × 1 ft = 32 ft³

Convert to bulk cubic yards:

32 ÷ 27 = about 1.19 yd³

Whole bags

If each bag contains 1.5 ft³:

32 ÷ 1.5 = 21.33 bags

Round up to 22 whole bags before adding any user-chosen allowance. With a 10% allowance, the calculator returns 35.2 ft³ and 24 whole bags.

Why the purchase amount can differ

Geometry is exact; the material estimate has limits

Fresh fill can settle

Raised-bed fill may settle as loose material rearranges and organic matter decomposes. University guidance documents settling, but the amount varies with the material, moisture, handling, and time. Treat an extra-material percentage as a planning choice—not a guaranteed correction.

Mulch needs headspace

If mulch will sit inside the frame, do not automatically fill soil to the top. Iowa State Extension recommends allowing 1–2 inches for mulch and notes that settling may create a useful lip.

Volume is not weight

Loose and compacted materials have different bulk densities. Do not convert calculated volume to tons or pounds unless the supplier provides a suitable density for that specific material and condition.

Volume is not a soil recipe

A volume calculation does not establish the right proportion of topsoil, compost, mineral material, or soilless ingredients. Extension recommendations vary by site and intended use. Follow a suitable regional source, product information, and soil test rather than assuming the entire volume should be compost.

Existing soil changes the fill depth

If the bed is open to acceptable native soil and you are adding only a surface layer, calculate only the depth being added. If contaminated or unsuitable soil is a concern, seek local guidance before deciding bed design or imported material.

More beds require separate totals

Calculate each distinct rectangular bed, then add the volumes. Multiplying one result by the number of beds is safe only when their inside dimensions and fill depths are identical.

Before you order

  • Confirm inside length, width, and intended fill depth.
  • Check whether the supplier sells by volume or weight.
  • Read the actual bag volume; do not assume a standard bag size.
  • Choose an allowance only for a stated reason.
  • Leave room for mulch if it will remain inside the frame.
  • Ask the supplier what its “yard” or bulk quantity represents.
Decision boundary

This guide calculates volume; it does not certify soil quality, identify contamination, select a crop-specific rooting depth, or prescribe fertilizer, lime, compost, irrigation, or another amendment. Soil composition, drainage, crop needs, climate, source quality, and local requirements still matter.

Reviewed 22 August 2026

University and Extension sources used

The calculation method is stable geometry. The planning cautions preserve the scope and differences in these sources instead of presenting one fill recipe as universal.

University of Georgia Extension

Raised-bed cubic-foot calculation using length × width × height and conversion from cubic feet to cubic yards by dividing by 27.

Read Raised Bed Soils

Texas A&M AgriLife Extension

Conversion between cubic yards and cubic feet, division by the bag volume, and rounding a fractional bag count up to a whole bag.

Read How Much Compost, Soil or Mulch?

University of Florida IFAS Extension

Raised-bed volume calculation and the observation that fresh fill may settle; its example is context-specific rather than a universal settlement allowance.

Read Gardening in Raised Beds

Iowa State University Extension

Raised-bed filling guidance, space for 1–2 inches of mulch, and the role of settling in creating a lip below the frame.

Read Creating Raised Bed Planters

University of Illinois Extension

Why raised-bed fill settles over time as organic matter decomposes and why replenishment is an ongoing observation rather than a one-time exact prediction.

Read the Raised-Bed Soil Guidance