How many worms do I need?

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.

What worms should I use?

Use composting worms (Eisenia fetida or Dendrobaena veneta), not earthworms. They are surface feeders that thrive in decomposing organic matter.

How fast do composting worms multiply?

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.

What can I feed composting worms?

Fruit and veg scraps, tea bags, coffee grounds, cardboard, and paper. Avoid citrus, onion, meat, dairy, and oily foods.

What kind of worms do I need?

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.

GardenCalcs

Worm Composting Calculator — Wormery Size Guide

Calculate wormery size and worm quantities for your food waste.

Household Size

Number of people whose kitchen scraps will feed the wormery

Weekly Waste per Person

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)

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

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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.