Soil Amendment Calculator

How to use the soil amendment calculator

Enter bed length and width in feet, the depth of existing soil you are amending in inches, and the target amendment percentage in the finished mix.

The calculator returns cubic yards of amendment to blend in — not the total bed volume.

Amendment mixing formula

Existing volume (cu ft) = Length × Width × (Existing depth in ÷ 12). Amendment volume = Existing volume × (Amendment % ÷ (100 − Amendment %)). Cubic yards = Amendment cu ft ÷ 27.

The ratio formula accounts for amendment as a fraction of the final blended volume, not just the original soil.

• Existing cu ft = L × W × (depth / 12)
• Amendment cu ft = existing × (pct / (100 − pct))
• 25% target on existing soil is a common vegetable bed starting point
• Cubic yards = amendment cu ft / 27

Worked example

A 10 ft × 4 ft bed with 6 inches of existing soil, targeting 25% compost in the final mix: Existing volume = 40 × 0.5 = 20 cu ft.

Amendment needed = 20 × (25 ÷ 75) = 6.67 cu ft ≈ 0.25 cubic yards of compost.

Choosing amendment percentage

Twenty to thirty percent compost improves organic matter and nutrient retention in most garden soils. Sandy soils may benefit from higher compost or coir; heavy clay may need compost plus gypsum or coarse sand for structure.

Do not exceed 40–50% compost in permanent beds — balance with native soil or topsoil for structure.

Blending best practices

Mix amendments thoroughly into the full tillage depth. Re-test pH and nutrients after major amendments before fertilizing aggressively.

• Use our compost calculator if you know depth rather than percentage.
• Pair with soil pH adjustment before adding lime or sulfur.
• Gypsum helps clay structure but does not replace organic matter.
• Allow mixed beds to settle 1–2 weeks before precision seeding.