Urban Worm Company Compost Calculator Guide

The Urban Worm Company’s compost calculator helps you estimate the right amount of worm food and bedding for your vermicomposting bin. You input the surface area of your bin (in square feet) and the number of worms you have, and it returns the recommended weekly feeding amount and the amount of bedding needed. This guide explains how that calculator works, why the numbers matter, and how to adapt them to your specific setup.

Formula

The calculator uses two simple rules of thumb that are widely cited in vermicomposting guides:

• Feeding rate: Feed worms about half a pound (0.5 lb) of food per week for every 1,000 adult worms. So the formula is: Weekly feed (lb) = (Number of worms / 1000) × 0.5.
• Bedding depth: Start with about 6 inches (0.5 ft) of bedding in the bin. So the formula is: Bedding volume (ft³) = bin area (ft²) × 0.5.

In the calculator, area is the surface area of the bin in square feet, and worms is the number of worms (often estimated by weight, since 1 pound of worms is roughly 1,000 individuals).

Worked Example

Suppose you have a bin that is 2 feet long and 1 foot wide, giving a surface area of 2 ft², and you start with 1,000 worms.

1. Weekly feed: (1,000 / 1000) × 0.5 = 1 × 0.5 = 0.5 lb of food per week.
2. Bedding volume: 2 ft² × 0.5 = 1 ft³ of bedding (about 7.5 gallons).

That matches the calculator output: 0.50 lb and 1.00 ft³.

Why These Ratios Matter

The half-pound-per-1000-worms rule is a starting point, not a strict law. Worms eat roughly their own body weight per day under ideal conditions, but in a bin they also need bedding (carbon) to balance the nitrogen-rich food scraps (the classic carbon-to-nitrogen ratio of about 25:1). Overfeeding leads to smelly, acidic conditions; underfeeding slows reproduction and composting. The 6-inch bedding depth gives enough material to absorb moisture and provide air space.

How to Use the Calculator for Your Bin

To get useful numbers, you need two pieces of information:

• Bin surface area: Measure the length and width of the bin in feet, multiply them. For a typical 10-gallon storage tote (about 16″ × 12″), that’s roughly 1.33 ft². For a 3-tray commercial bin, the top tray area might be 2 ft².
• Worm population: If you bought worms by weight, 1 lb ≈ 1,000 mature worms. If you’ve had the bin for a while, the population may have doubled, so adjust accordingly.

Once you have those, plug them in. The calculator gives you the weekly feed amount and the initial bedding volume. You can then adjust based on what you observe: if food disappears within 2 days, increase slightly; if it’s still there after a week, cut back.

Common Mistakes and Misconceptions

One common mistake is treating the feeding amount as a daily figure. The calculator outputs a weekly amount. If you feed every day, split that weekly amount into daily portions (e.g., 0.5 lb/week ÷ 7 ≈ 0.07 lb/day). Another error is using the bin’s total volume instead of surface area for the feeding rate – worms live in the top few inches, so surface area is what matters. Also, the “1000 worms per poundâ€� is an estimate for mature Eisenia fetida (red wigglers); juvenile worms are smaller, so you might have more per pound.

A widespread misconception is that you must feed exactly this amount or the worms will starve. In reality, worms can go weeks without food, and they will eat slower if the bin is cold. The number is a guideline to prevent overfeeding, not a strict requirement.

Adjusting for Temperature and Bin Type

Worm activity depends on temperature. In cooler conditions (below 60°F), worms eat less, so reduce feeding by 25–50%. In warmer conditions (above 80°F), they eat more, but be careful of overheating. Also, the type of bin matters: a shallow tray bin has more surface area per volume, so it can handle more food than a deep tote with the same footprint. If you use a continuous flow-through system, you might feed more frequently but in smaller amounts.

Costs and Alternatives

Using the calculator can help you avoid buying too many worms or too much bedding. A 1,000-worm starter (about 1 lb) costs roughly $30–40, and bedding (shredded cardboard, coconut coir, or peat moss) is inexpensive. If you generate more food waste than the calculator suggests, you have options: add more worms, enlarge the bin, or start a second bin. Some people choose to compost outdoors in a pile instead, which doesn’t require the same feeding discipline.

Frequently Asked Questions

How many worms do I need for my bin?
Start with 1,000 worms per square foot of bin surface area. For a typical 10-gallon tote (1.33 ft²), that’s about 1,300 worms. You can always add more later.

What if I feed more than the calculator says?
Overfeeding can cause odors, fruit flies, and acidic conditions. If you accidentally overfeed, stop feeding for a week or two and add more dry bedding to absorb moisture and balance the bin.

Can I use this calculator for a worm tower or outdoor bin?
Yes, the surface area rule applies to any bin, but outdoor bins may have different moisture levels and temperature swings. Adjust feeding based on how quickly food disappears.

How do I know if my worms are eating the right amount?
Check the bin weekly: if the food is mostly gone within 2–3 days, they’re hungry; if it’s still there after a week, reduce feeding. The bin should smell earthy, not sour.

What bedding should I use?
Shredded cardboard, paper, coconut coir, or peat moss all work. Avoid glossy paper and treated wood. Moisten the bedding until it feels like a wrung-out sponge.

Is the 0.5 lb per 1000 worms per week accurate for all worm species?
It’s based on red wigglers (Eisenia fetida), the most common composting worm. Other species, like European nightcrawlers, have similar appetites but may differ slightly. Adjust based on observation.

Related

• Garden Size Calculator: How Big Is My Garden?
• Grow a Garden Calculator: Soil & Compost Volume
• Garden Exp Calculator: Estimate Costs & Yield

in area | Bin surface area | ft² | 2 | 0.1 | 1000
in worms | Worm population | worms | 1000 | 100 | 1000000
out feed | Weekly feed amount | lb | (worms / 1000) * 0.5 | 2
out bedding | Initial bedding volume | ft³ | area * 0.5 | 2