GardenCalcs.uk
Container Compost Calculator — Compost for Pots
Work out how much compost you need to fill your containers.
How many containers of this size you are filling. Group pots of similar volume into one calculation.
Volume per pot in litres. Rough guide: 30cm round pot ~15-20 L, hanging basket ~12 L, growbag ~33 L, window box ~10 L. Don't fill to the rim — leave a 2-3cm watering gap.
Shop price of one 40L bag (UK multi-purpose is typically £4-£8). Leave at 0 to skip the cost estimate.
Turn on only if you add crocks or gravel at the base (RHS suggests a 2-3cm layer in deep pots). Reduces the compost figure by the Efficiency Factor below (85% = a 15% cut). Off by default since many modern pots don't need it.
Percentage of pot volume still filled with compost after the drainage layer. Only applied when the drainage toggle is on. 85% means a 15% reduction for crocks/gravel; set to 100% for no reduction.
How We Calculate This
Total compost = Number of pots × Average pot volume.
If drainage layers are included, volume is reduced by 15% to account for crocks or gravel at the bottom.
Results in 40L and 70L bag quantities for easy purchasing.
Frequently Asked Questions
Multi-purpose peat-free compost for most plants. Ericaceous compost for acid-loving plants. John Innes for long-term plantings.
Only if you actually add crocks or gravel. If you do, reduce compost volume by about 15%. The drainage toggle is off by default, so the headline figure is the full pot volume — switch it on if you line the base of deep pots.
Annual plants: fresh compost each year. Perennials: top-dress or repot every 2-3 years. Refresh the top 5 cm annually.
Yes, mix old compost with fresh at 50/50. Remove old roots and add slow-release fertiliser. Do not reuse compost from diseased plants.
Annual containers (bedding, veg) need fresh compost each year. Permanent shrubs in pots benefit from annual top dressing and full replacement every 3-4 years.
Related Calculators
Hanging Basket Calculator
Work out compost, liner, and plant quantities for hanging baskets.
Drainage Layer Calculator
Work out crocks or gravel needed for container drainage layers.
Grow Bag Calculator
Calculate how many grow bags you need and plants per bag.
Window Box Calculator
Calculate compost volume and plant quantities for window boxes.
Pot Volume Calculator
Calculate the volume of round, square, or tapered plant pots.
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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.