GardenCalcs.uk
Soil Amendment Calculator — Improve Your Garden Soil
Calculate quantities of grit, sand, or organic matter to improve your soil.
The plan area of the border or bed you are improving — length × width of the ground surface
How deep you will dig the amendment in — 15-30 cm for forking into a border, less for top-dressing
Grit or sharp sand opens up heavy clay for drainage; compost or manure adds organic matter to sandy soil; perlite and vermiculite lighten potting and seed mixes
Percentage of amendment in total mix
Extra material for waste and settling (default 1.0)
How We Calculate This
This calculator works out how much amendment material you need based on the area, working depth, and desired mix ratio.
Formula: Amendment Volume = Area × Depth × (Mix Ratio / 100)
Weight uses typical loose bulk densities: horticultural grit and sharp sand ~1,500 kg/m³, perlite ~100 kg/m³, vermiculite ~120 kg/m³, garden compost ~500 kg/m³, well-rotted manure ~600 kg/m³.
Bag counts assume the usual UK retail pack sizes: grit and sand in 25 kg bags, perlite and vermiculite in 100 L horticultural bags, and compost and manure in 50 L bags.
Frequently Asked Questions
For clay soil, add grit at 20-30% of the total soil volume being worked. Dig it into the top 20-30 cm for best results.
Sharp sand (not builders sand) can help, but grit is better. You need large quantities — at least 25% by volume — to make a real difference.
Well-rotted compost or manure is best for sandy soil, adding organic matter that retains moisture and nutrients.
Dig amendments into the top 15-30 cm of soil. For established borders, top-dress with compost and let worms work it in.
Annual organic matter additions in autumn or spring are ideal. Structural amendments like grit only need adding once. Test your soil every 2-3 years to check nutrient levels.
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Last updated: February 2026
All calculations are estimates based on standard gardening guidelines. Results may vary with soil conditions, climate, and plant varieties.