Worm Bin Size Calculator

Vermicomposting uses worms, almost always red wigglers (Eisenia fetida), to turn kitchen scraps into worm castings. Castings are among the best natural fertilizers there are, and the process is genuinely odorless when the bin is the right size. Get the size wrong and you get the opposite: food sitting uneaten, a sour smell, and fruit flies.

The Basic Rule: Worms consume roughly half their body weight in food per day. 1 pound of worms (approximately 1,000 worms) processes about 3.5 lbs of food waste per week.

Bin Size Formula: Bin surface area (sq ft) = Pounds of food waste per week / 1

Bedding should be 8 to 12 inches (20 to 30 cm) deep. Worms feed at the surface, so a deep bin buys you nothing: surface area is what decides how much food the bin can work through, and a wide shallow tote beats a tall bucket every time.

Household Food Waste Estimates:

These are the figures the calculator uses. The worm column is the food figure divided by 3.5 and rounded up to the nearest half pound, which is how suppliers sell them.

Buying more worms than this does no harm beyond the cost, and buying fewer is fine too: the population doubles roughly every three months until it matches whatever you are feeding it. What you cannot get away with is a bin that is too small, because surface area is what limits how fast food can be worked through.

Common Bin Sizes:

What Worms Eat vs. Avoid:

Ideal Conditions:

• Temperature: 55–77°F (13–25°C). Worms are most active at 60–70°F (15–21°C). They will die below 40°F (4°C) or above 90°F (32°C).
• Moisture: bedding should feel like a wrung-out sponge, moist but never dripping. Around 60 to 80% moisture.
• pH: Slightly acidic to neutral (6.0–7.0). Crushed eggshells help buffer acidity from fruit scraps.
• Darkness: Worms avoid light. Keep the bin covered.

Carbon-to-Nitrogen Ratio: For best results, maintain a ratio of about 3:1 brown materials (carbon) to green materials (nitrogen) by volume. This prevents odors and provides balanced nutrition for the worms.

Harvesting Timeline: Castings are ready in 3 to 6 months. They look dark and crumbly, smell of nothing much, and hold no recognizable scraps.

There are two ways to get the worms out. The light method: tip the castings into cones on a sheet under a bright lamp, wait twenty minutes while the worms burrow down away from it, then scrape the top off each cone and repeat. The side-feeding method is lazier and works just as well: feed only one half of the bin for a fortnight, and the worms migrate across on their own.

Starting Your Bin: Order red wigglers from a reputable supplier. Start with 1 lb of worms for a new bin and let the population grow. Worms double their population every 3–4 months under good conditions. Prepare bedding first: fill the bin with moistened shredded newspaper or coconut coir, then add worms and a small amount of food.