Once you've made the decision to begin vermicomposting, a pretty logical question is  “how many worms do I need for a worm bin?”

If you have already purchased your worm bin, you have already decided how many worms you need, within a certain range anyways. You have also decided how much waste you'll be able to turn into worm castings as well.

But if you are taking a more deliberate approach and trying to recycle a known quantity of organic waste in your home, office, or classroom as part of a Zero Waste initiative or to simply reduce your contribution to the waste stream, then we attack the question a little differently.

This article is part of a “Vermicomposting 101” series of posts aimed at helping the beginning vermicomposter. The read other “VC101” articles on how to start a worm bin, how to choose worm food, how to maintain moisture, and the differences between composting and vermicomposting, please visit the Vermicomposting 101 section of this site!“

Case 1: I Have a Worm Bin Already. How Many Worms Do I Need For It?

To be quite honest, this is how most people go about vermicomposting. They figure out what bin they want based on the space they have available, or how the bin looks, THEN wonder what quantity of composting worms to buy.

Reasonable worm stocking densities will range from about 1/2 – 1 1/2 lbs per square foot with 2lbs being the absolute maximum density I would suggest to start with. To find the square footage of a square or rectangular bin, multiply the width of the surface area (in inches) by the length of the surface area (in inches) and divide by 144.

(For a circular bin like the Can O Worms, square the radius and multiply times pi (3.14). Then divide by 144!)

The Urban Worm Bag has a max available surface area of 24 x 24 inches. (24W x 24L)/144= 4 square feet.

The Hungry Bin has a similar surface area, but for over 3x the price!

Note: Both the Urban Worm Bag and the Hungry Bin have a tapering bottom, so if you are starting a new bin, then the effective surface area is lower towards the bottom.

Once you measure your available surface area, then you can decide your stocking density, roughly adhering to the rules of thumb below.

Play It Safe, Start Slow: 1/2 lb per square foot

If you are starting a new worm bin from scratch,  I normally recommend playing it safe. The risk isn't that the worms won't like the density itself. The risk is that they may hate their new surroundings and try to leave the premises.

Worms aren't cheap, so I'd much rather see a newbie start a new worm bin with about 1/2 lb per square foot.

For Red Wigglers, this is 2 lbs, or about 1600-2000 worms. 

For European Nightcrawlers, 2 lbs will equal about 600-800 worms.

Step It Up: 1 lb per square foot

If you're starting a worm bin with an existing quantity of vermicompost or you're confident your worm bin will be immediately hospitable to your worms, then I think you're safe to stock your new worm bin at 1lb per square foot.

There are two advantages to this density.

• Your bin's waste processing capacity will be higher.
• Reproduction in the bin is theoretically higher as worms are more likely to encounter each other and make a love connection.

But for a larger home system like an Urban Worm Bag, 4 lbs of composting worms may cost as much as the worm bin itself.

Tournament Level Player: 2 lbs per square foot

OK, vermicomposting ninjas, if you're highly confident in your skills and ability to maintain optimal conditions, then by all means, max perform your bin by stocking at 2 lbs per square foot.

Keep in mind that worms are excellent self-regulators and will stop reproducing once they sense that space is getting restricted (or that conditions aren't otherwise optimal).

While this is 4x as dense as I recommend beginners start with, it's also going to get closer to the maximum processing capacity of your worm bin.

Case 2: How Many Worms Do I Need to Recycle My Waste?

Instead of deciding on a worm bin and then working forward to the answer. Let's start with the end and work backwards to find the number of worms AND the size of the bin required to house them.

If you're looking to turn your household food scraps into worm castings, the math is fairly straightforward.

American adults, on average, create one pound of compostable fruit and vegetable waste each day. So a typical American household will create between 3-4 pounds daily.

Composting worms can consume 25-33% of their own weight daily, according to conservative estimates.

So using the most conservative estimate, you need 4 times more weight in worms than you create in food waste daily.

For a family of 4 who eats at home often, this means you would need roughly 16 lbs of worms to consume your compostable food waste each day. That's a lot of worms! For a frame of reference, you would need two Urban Worm Bags to house this quantity of worms.

Keep in mind, though, that worms will still consume the carbon-rich worm bedding that you started your bin with and will be periodically adding to your bin. So it's not like you need to supply your worms with a preset amount of food each day.

They won't starve, trust me!

How Many Worms Are In Your Worm Bin?

What's your best estimation of the number of worms in your bin? And how much are you processing?

Did you start with a worm bin then figure it out or did you select a bin or amount of worms based on the amount of food waste you create?

If you liked what you read here, I invite you to join my e-mail list below and read the rest of my Vermicomposting 101 Series.