A worm bin turns kitchen scraps into rich vermicompost, even in an apartment. The key to a healthy bin is matching the number of worms and the size of the bin to the food you produce. This calculator does that sizing.
How much do worms eat?
Composting worms, mostly red wigglers (Eisenia fetida), eat roughly half their body weight a day in an established bin. New bins, cool temperatures and freshly shipped worms eat less; a warm, mature bin can process up to their full weight.
worms needed = daily scraps ÷ daily ration
For 3.5 lb of scraps a week, or 0.5 lb a day, at half their weight: 0.5 ÷ 0.5 = 1 lb of worms, about 1,000 red wigglers. Worms are sold by the pound; a population doubles in a few months when conditions are good, so you can start smaller and feed less at first.
Bin size
Worms feed near the surface, so the surface area matters more than depth. A common rule is 1 sq ft of surface per pound of scraps per week: 3.5 sq ft for this example, such as a bin of about 2 × 2 ft or 1.5 × 2.5 ft. A depth of 8 to 12 in is enough; deeper bins can turn anaerobic at the bottom.
Bedding
Fill the bin with moist bedding: shredded newspaper or cardboard, coconut coir or aged leaves, damp like a wrung-out sponge. At 10 in deep, the 3.5 sq ft bin needs about 2.9 cu ft of bedding. Add a handful of soil or sand for grit.
What to feed
- Yes: fruit and vegetable scraps, coffee grounds and filters, tea leaves, crushed eggshells, bread in small amounts.
- No: meat, fish, dairy, oils, salty or spicy food, and large amounts of citrus or onions.
Bury food under the bedding and rotate feeding spots. Chopping scraps speeds things up.
Harvest
Worm castings, or vermicompost, build up over 3 to 6 months. As a rough guide, expect about a quarter of the weight of the scraps as finished castings, about 46 lb a year here. Use them as a top dressing or mix them into potting soil.
Troubleshooting
- Bad smell: overfeeding or too wet; stop feeding and add dry bedding.
- Fruit flies: bury food deeper and cover with a layer of dry bedding.
- Worms trying to escape: too wet, too acidic or too hot; keep the bin between 55 and 77 °F.