Soil Amendment Calculator
Calculate compost, peat moss, perlite, or other amendments for any garden bed.
Enter dimensions and amendment ratio to get cubic feet or bags needed.
Amendment Needed
How Soil Amendment Quantities Are Calculated
Soil amendments improve structure, drainage, nutrients, or pH. The correct quantity depends on the amendment type, current soil condition, and the volume of soil being treated.
Amendment Volume Formula:
Amendment Volume = Bed Area (m²) × Amendment Depth (m)
For working amendments into the top 20–30 cm of soil, you typically incorporate 20–30% amendment by volume.
Worked Example: Raised bed: 2m × 4m = 8 m², improving top 25 cm of soil with compost (25% by volume):
- Soil volume = 8 × 0.25 = 2.0 m³
- Compost needed = 2.0 × 0.25 = 0.5 m³ (500 liters)
- Bags (40L each) = 500 / 40 = 12.5 → 13 bags
Common Amendment Rates (per 10 m² bed, worked to 20 cm):
| Amendment | Typical Rate | Purpose |
|---|---|---|
| Compost | 80–100 L | Nutrition, water retention |
| Perlite | 40–60 L | Drainage, aeration |
| Vermiculite | 30–50 L | Water retention, seedlings |
| Sand (coarse) | 100–200 L | Break up clay |
| Gypsum | 1–2 kg | Clay flocculation |
| Biochar | 20–40 L | Long-term carbon, CEC |
| Worm castings | 20–40 L | Soluble nutrients, biology |
Common Mistakes:
- Adding sand to clay without organic matter creates concrete-like soil
- Over-liming can raise pH too high and cause nutrient lock-out
- Too much nitrogen-rich compost burns seedling roots