GardenCalcs.uk
Worm Composting Calculator — Wormery Size Guide
Calculate wormery size and worm quantities for your food waste.
Number of people whose kitchen scraps will feed the wormery
Compostable food scraps per person each week. UK average is roughly 1.7-3.5 kg/person; count only worm-safe waste (no meat, dairy, citrus or onion)
At the half-body-weight feeding rule and ~0.5 g per worm, 1 kg of daily waste needs ~2 kg of worms = 4,000 worms (default)
Conservative beginner stocking density: ~1 cm² per worm equals about 5 kg/m². Going below 1 cm² crowds the bed (10 kg/m² is the practical maximum)
Finished castings as a fraction of fresh input weight. Kitchen waste is 80-90% water, so realistic yield by mass is well under the 0.6 default — lower it for a more cautious estimate
How We Calculate This
Composting worms eat roughly half their body weight in food waste per day, and an average red wiggler (Eisenia fetida) weighs about 0.5 g. So to process 1 kg of food a day you need about 2 kg of worms — that is 2 ÷ 0.0005 ≈ 4,000 worms per kg of daily food waste. For example, 1 kg of worms (about 2,000 worms) processes roughly 0.5 kg of waste daily.
Formula: Worms needed = Daily waste (kg) × 4,000
Bin volume is sized from food throughput: a 75-litre UK wormery handles about 2.5 kg of food per week (≈ 25–30 litres per kg of weekly waste). Bed surface area uses a conservative starting density of about 1 cm² per worm (≈ 5 kg/m²). Worm castings output is taken as approximately 60% of input weight, though much of the wet input is lost as water and CO₂ during digestion.
Frequently Asked Questions
Composting worms eat about half their body weight in food each day, so to process 1 kg of food daily you need roughly 2 kg of worms — about 4,000 worms per kg of daily food waste (each red wiggler weighs around 0.5 g). A household of 2 producing about 0.85 kg/day therefore needs roughly 3,400 worms, weighing about 1.7 kg.
Use composting worms (Eisenia fetida or Dendrobaena veneta), not earthworms. They are surface feeders that thrive in decomposing organic matter.
Composting worms can double their population every 3-4 months under ideal conditions. Start with fewer worms and the colony will grow to match food supply.
Fruit and veg scraps, tea bags, coffee grounds, cardboard, and paper. Avoid citrus, onion, meat, dairy, and oily foods.
Brandling worms (Eisenia fetida) are best for wormeries. They are red/tiger striped and available from garden centres. Do not use common earthworms as they prefer deeper soil.
Related Calculators
Soil Amendment Calculator
Calculate quantities of grit, sand, or organic matter to improve soil.
Compost Ratio Calculator
Calculate compost C:N ratios and get mix recommendations for faster composting.
Raised Bed Soil Calculator
Calculate how much soil you need to fill a raised bed of any size.
Compost Bin Size Calculator
Work out the ideal compost bin size for your garden waste output.
Topsoil Calculator
Calculate topsoil volume needed in cubic metres, bulk bags, or tonnes.
Was this calculator helpful?
One tap. Votes are anonymous and carry the page address, your device type, screen size and country, the numbers in the form and the results on screen, so we can check the answer you saw.
Last updated: February 2026
All calculations are estimates based on standard gardening guidelines. Results may vary with soil conditions, climate, and plant varieties.