How many spiders are in your home? (Part I)

28 Mar, 2026

Have you ever wondered how many spiders are in your home? Me too. Ever since moving into my unit I have discovered there are quite a lot of spiders crawling around (or floating as they do on their webs). The one in my shower is a female Black House Spider Badumna insignis. I call her Susan or Suz the shower spider.

I was wondering about it all and thinking about what variables/factors would increase or decrease the spider population in my home (or at any given home). I decided I would have a crack at estimating this myself, through my own mathematical function.

To make it easier, from hereon I am going to use the term 'home' to refer to all the buildings that humans live in; the units, flats, apartments, houses, palaces, everything.

There have been numerous studies investigating what factors are important for spider abundance in urban settings. Ideally I would conduct my own research; I would go into everyones home, record the spider population, along with all the various other attributes of the home. I would then make a generalized linear model and determine what are the best predictors for the abundance of spiders. The problem is, I currently don't have the time to do this.

Some factors to consider include:

• Age of home
• Size of home
• Prey abundance
• Fecundity, gestation period, survivorship and life expectancy of the average house spider
• Time of year (seasonality)
• Total number of windows
• Behaviour of human inhabitants
• Predation by other animals
• Predation from other spiders (including cannibalism)
• Time since last human occupancy

Without empirical evidence it is hard to know exactly what factors/variables are most important or are best at predicting the total abundance of spiders. It's also hard to estimate as I am sure many spiders are not easily observable, and hide in the roof, under floorboards etc.

My function provides you with an estimate so you don't have to go around counting them all in your home. It must be stressed that I am no mathematician (please do get in contact if you happen to be a mathematician, I would love to know your thoughts).

I am in the process of writing up the thought process behind it all, detailing how all the constants and variables came to into existence. So I will not try to justify my thoughts at this moment. I will save this for another future post.

Once done, a link to the future post will be located here: How many spiders are in your home? (part II)

Initially the idea for this blog post was more a joke, just a silly thing to do. But I ended up getting really invested into it. I contemplated long and hard about it. I do hope there is some legitimacy to my function. I have tried to keep it as simple as possible, with only three variables. Please note the function is most intended for Australian homes in temperate climates.

Without further delay, here is the function:

$$\begin{array}{c}f(x)=0.20\sqrt{m(x+t)}+0.35(w)\\ \\ \mathrm{W}\mathrm{h}\mathrm{e}\mathrm{r}\mathrm{e}\mathrm{t}\mathrm{h}\mathrm{e}\mathrm{d}\mathrm{o}\mathrm{m}\mathrm{a}\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{s}\mathrm{a}\mathrm{l}\mathrm{l}\mathrm{r}\mathrm{e}\mathrm{a}\mathrm{l}\mathrm{x}\mathrm{\ge }\mathrm{-}\mathrm{t}\mathrm{a}\mathrm{n}\mathrm{d}\mathrm{r}\mathrm{a}\mathrm{n}\mathrm{g}\mathrm{e}\mathrm{i}\mathrm{s}\mathrm{a}\mathrm{l}\mathrm{l}\mathrm{r}\mathrm{e}\mathrm{a}\mathrm{l}\mathrm{y}\mathrm{\ge }0.\\ \\ \mathrm{L}\mathrm{e}\mathrm{t}\mathrm{m}\mathrm{=}\mathrm{t}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l}\mathrm{s}\mathrm{q}\mathrm{u}\mathrm{a}\mathrm{r}\mathrm{e}\mathrm{m}\mathrm{e}\mathrm{t}\mathrm{r}\mathrm{e}\mathrm{s}\mathrm{o}\mathrm{f}\mathrm{y}\mathrm{o}\mathrm{u}\mathrm{r}\mathrm{h}\mathrm{o}\mathrm{m}\mathrm{e}(\mathrm{i}\mathrm{f}\mathrm{m}\mathrm{u}\mathrm{l}\mathrm{t}\mathrm{i}\mathrm{s}\mathrm{t}\mathrm{o}\mathrm{r}\mathrm{y}\mathrm{t}\mathrm{h}\mathrm{e}\mathrm{n}\mathrm{s}\mathrm{u}\mathrm{m}\mathrm{t}\mathrm{o}\mathrm{g}\mathrm{e}\mathrm{t}\mathrm{h}\mathrm{e}\mathrm{r}\mathrm{f}\mathrm{l}\mathrm{o}\mathrm{o}\mathrm{r}\mathrm{s})\mathrm{,}\\ \mathrm{t}\mathrm{=}\mathrm{a}\mathrm{g}\mathrm{e}\mathrm{o}\mathrm{f}\mathrm{b}\mathrm{u}\mathrm{i}\mathrm{l}\mathrm{d}\mathrm{i}\mathrm{n}\mathrm{g}\mathrm{,}\mathrm{t}\mathrm{h}\mathrm{e}\mathrm{t}\mathrm{i}\mathrm{m}\mathrm{e}(\mathrm{n}\mathrm{o}\mathrm{.}\mathrm{o}\mathrm{f}\mathrm{d}\mathrm{a}\mathrm{y}\mathrm{s})\mathrm{s}\mathrm{i}\mathrm{n}\mathrm{c}\mathrm{e}\mathrm{c}\mathrm{o}\mathrm{m}\mathrm{p}\mathrm{l}\mathrm{e}\mathrm{t}\mathrm{e}\mathrm{d}\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{s}\mathrm{t}\mathrm{r}\mathrm{u}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\mathrm{,}\\ \mathrm{w}\mathrm{=}\mathrm{n}\mathrm{o}\mathrm{.}\mathrm{o}\mathrm{f}\mathrm{w}\mathrm{i}\mathrm{n}\mathrm{d}\mathrm{o}\mathrm{w}\mathrm{s}\mathrm{i}\mathrm{n}\mathrm{y}\mathrm{o}\mathrm{u}\mathrm{r}\mathrm{h}\mathrm{o}\mathrm{m}\mathrm{e}.\end{array}$$

The function I must admit is a little vibes based, but I read numerous papers and can justify all parts of the function. There is a component of this function that is derived from real world data too. I will elaborate further in the future post.

I decided I wanted to create a little calculator to do the maths for me! Of course you may not know exactly all the information, just have a guess. For example if your flat is around 10 years old, just enter 365x10=3650 days for the t variable.

The spider calculator

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