I love worms, and I am geeky about numbers, so when someone asks a question on a worm composting forum that involves math, I am usually the one who answers.
And questions come up, like, “How many pounds of worms do I need if I want to sell 100 pounds of worms every week?” I’ll give you a hint: it is a big number. And the math gets a little complex.
When a question like this was posted to a forum I participate in, one of the moderators removed the post. It isn’t a commercial grower’s forum, and the question was considered one that is only relevant to commercial growers.
But maybe not. Maybe someone wants to know how often they can split their bins, or how often they can give a pound of worms away and still maintain their worm population.
Any answer you come up with will be a rough estimate, and will be based on some assumptions, one of which is that the total worm biomass will double every 90 days. Worms can actually breed faster than that, but in practice, this is a good rule of thumb, even if it is not always accurate.
It isn’t the total number of worms that doubles. You will have a lot more than double the total number of worms, since most of the worms that make up the increase in biomass will be babies and juveniles. Some of the increase will be these babies and juveniles growing into adults.
And there are a lot of factors that come into play, such as temperature, worm density, available food, etc.
But total biomass doubling every 90 days gives us a place to start, and our numbers are likely to be somewhat accurate.
The simplest question to answer is how many worms will I have in so many days? In 90 days we can expect to have twice the biomass. In 180 days, four times. In 270 days, 8 times.
If our starting mass is Ms, and our ending mass is Me, then
Me = Ms × 2(D / 90)
where D is the number of days. So if our starting mass is one pound, in a year we should have 1 × 2(365 / 90) pounds of worms. That works out to roughly 16 ½ pounds.
From there, we can work out some more complex answers.
How long will it take my current worm population to grow to a certain biomass? The equation is
D = log2(Me / Ms) × 90
What does that mean? I told you the math would get complex. Do you remember learning about logarithms in high school math class? Well, you finally have a use for them.
We take our ending biomass, divide by the starting biomass, then take the base 2 logarithm of that answer and multiply it by 90.
So what is a logarithm? It is a way to undo exponentiation, much like subtraction undoes addition, or division undoes multiplication. So if ab = c, then loga(c) = b. If you are not a math geek, that explanation probably didn’t make sense. And if you are a math geek, that explanation probably wasn’t necessary. Just remember what my daughter told me when she had to review logarithms in a college math class. “Dad, logarithms are evil!” And they are. And you probably don’t have a base 2 logarithm key on your calculator. There are workarounds, but I don’t have time to discuss them here. If you are not a math geek, you won’t understand them. If you are a math geek, you have already Googled them by now.
So, how long will it take for my 1 pound of worms to become 10 pounds of worms?
D = log2(10 / 1) × 90
It will take about 300 days for your 1 pound of worms to become 10 pounds.
But don’t go shouting to the world that you have 10 pounds of worms 300 days after you set up your bin. Remember, these are rough estimates, and there are a lot of other factors to consider.
So, now you are feeling generous, and you want to start giving worms away. Or you are feeling entrepreneurial, and you want to start selling them. How many pounds of worms do you need to be able to give some away, and maintain your worm population?
Pn = Pr / 2(D / 90) – 1
Where Pn = pounds needed, Pr = pounds removed and D = days. So, if you want to give away 5 pounds of worms every week:
Pn = 5 / 2(7 / 90) – 1
You will need about 90 pounds of worms.
And, finally, if I have a certain population of worms, how often can I give worms away?
D = 90 × log2(Ms / (Ms – Pr))
Where D is the number of days, Ms is the starting biomass and Pr is the number of pounds removed. (And there’s those pesky logarithms again!)
So, now that I have roughly 100 pounds of worms, how often can I give 5 pounds away?
D = 90 × log2(100 / (100 – 5))
I can give 5 pounds away about every 7 days.
That is a lot of math. But not to worry. I have done the math for you. I have a worm calculator posted on line at scottbryce.com/worm_calculator/cgi-bin/worm_calculator.pl.
So, if you have just purchased your first pound of worms, and you want to know when you can start earning 6 figures selling their offspring, you have a place to crunch some numbers. And maybe to get totally discouraged. Raising that many worms takes a lot of time, effort, food and space. Unless you are Brian Paley, but that may be a topic for another blog post.