How Many Worms Should I Buy for a Worm Compost Bin?

Once you’ve made the decision to begin vermicomposting, a pretty logical question is  “how many worms should I buy 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.

Case 1: I Have a Worm Bin Already. How Many Worms Should I Buy 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.

Americans create 1lb of compostable food waste per day

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 read the rest of my Vermicomposting 101 Series.

Need Some Red Wigglers? Or European or African Nightcrawlers?

As you may know, since the onset of COVID-19, composting worms have been in very short supply. The largest online reseller in the country is often out of stock.

I’d be thrilled if you want through us for red wigglers, their larger cousins, the European nightcrawlers, or the ginormous African Nightcrawlers!

20 thoughts on “How Many Worms Should I Buy for a Worm Compost Bin?

  1. Thanks Steve for another great article. Do your estimations apply to all composting worms, no matter the species, or is particular to a certain species? I ‘m asking because the composting worm that seems to be the most popular in the USA (from what’s on the internet) is Eisenia foetida. And this species is not common or is almost non-existent here in the Fiji Islands.

    1. Hi Siteri,
      Thanks for the kind words! Yes, in general, this would apply. I am not sure what you are composting with on the Fiji Islands, but the numbers in the article shouldn’t stray too far from what you would experience.

  2. For my first worm bag I have added 2 lbs of red worms, 1/4 pound of European nightcrawlers, 1 lb of african nightcrawlers, 1 lb of Indian blue worms, 1/2 lb of alabama jumpers, and probably at least another pound at least of whatever worms I happened to have found in my yard.

    I thought maybe the worms different eating habits might complement eachother, I know there is some research that has shown red wigglers and European nightcrawlers for example of are a good pairing because the reds stay on the surface while the nightcrawlers burrow more deeply. At any rate I guess we will see how my multispecies worm bag turns out.

    I also just wanted a really voracious bag of worms so I can feed them all our scraps since we tend to produce a lot as we eat at home all the time.

  3. I started with .98 sq ft stacking bin system (that I engineered myself, to emulate commercially produced systems) and 100 worms. I am a newbie, obviously, and didn’t want to overpopulate. Based on what I’ve read here, I may have *under* populated, though I gather they will reproduce quickly enough. My question is, approximately how long will it take the worms to populate the space?

    1. Hi Babs,
      I don’t know if I have enough information to tell you that. You’re at 1/10th of a pound per square foot, so what I would do is do small, concentrated feedings in one area of the bin to draw the worms together. This will help accelerate the process!

    1. Hi Richard,
      I’m not sure I have enough information to know, but I can give it a stab. If you’re using continuous flow (like with the Michigan SoilWorks CFT) you may get better efficiency.) There are a few rules of thumb you can follow to get to a reasonable answer. Assume worms will eat 25% of their own weight each day. Assume, say, 75% of that will exit the worm in the form of castings. So 100 lbs of worms will eat 25 lbs and produce 19 lbs of castings daily, 133 lbs weekly. If I do some high school algebra, that gets me to 75lbs of worms needed to make 100 lbs in a given week.

      Your mileage may vary though!

  4. Newbie here starting small with 1/2lb red wigglers in UWB. I am thinking of filling with shredded paper/cardboard and aged sawdust 1/4 full. Let it sit for several days then introduce the worms and food. I will separate it in a corner each time it is added to measure how much is being eaten. Weekly feedings.

    What adjustment to this plan will help it function better? Would adding casting be helpful?

    1. Thanks for the order Jani! I would also add some food before the worms to help with microbe growth. And yes, adding castings can help introduce biology to your worm bin. I wouldn’t waste money on store-bought castings to do it though.
      Cheers!
      Steve

  5. I am starting to keep worms as a working pet for my kids. Is it bad for the Vermicomposting if the worm population starting is less than 1/2 lb per square foot?

  6. I’m getting ready to start my own worm farm, I have 100 gallon round, two and a half feet deep, stainless steel milk tank, that I was going to use, approximately how many pounds of nightcrawlers would be safe to start with? And what should I use for the bottom layer?

  7. Totally confused… I am a newbie. I have my bin set up and am ready to order my worms. i have a VermiHut Plus 5-Tray Worm Compost Bin. It’s 15″ x 15″ (225 sq in divided by 144 is just over 1.5 sq ft), so “start a new worm bin with about 1/2 lb per square foot” but then “For Red Wigglers, this is 2 lbs, or about 1600-2000 worms”. What am I missing? Are you recommending 1/2 lb per square foot or 2 lbs per square foot? I was also wondering why depth isn’t a factor, like, why aren’t we calculating cubic feet instead of square feet? Another question: I have read that nightcrawlers aren’t recommended for composting. Can’t remember exactly why, but something to do with their preferred depth in the soil? Just curious about that. Thanks in advance for your assistance.

    1. Yes, yes! I’d like to see answers to Vicki’s questions. I fear no one is following the page now. Looks like Steve was last here in March 2021

  8. Hi Vicki and Kat….I’m still here! This one just got through the cracks, I would imagine Vicki has ordered her worms by now.
    My guidance for a new vermicomposter is 1/2lb per square foot. Pros can probably start with up to 2lbs per square foot. This has everything to do with cost and the risk of losing your worms if you don’t create a hospitable habitat.

    Vermicomposting depends on surface area, not volume. Composting worms tend to inhabit the top 8-12 inches of their habitat rather than burrowing down multiple feet. So, for instance, if you chose 1 lb of worms for a 1 square foot bin that measures 12 inches deep, tripling the depth does not triple the amount of worms that bin could host.

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