There is a school of thought that says that an academic is a device for turning coffee into research.  Some of my colleagues would disagree – they are fuelled solely by tea.  I am a hybrid model, running on both – but that, as the saying goes, is another story.  If you are a regular reader here at ‘Envelope’, you’ll know that many of my ponderings start when two neurons, that should not be in the same vicinity, collide: random thoughts come together and inspire me to undertake some spurious maths.

At the moment, I’m trying to have a blitz on the garden, which has gotten desperately out of control*:


*image for illustrative purposes only.  Actual garden more overgrown and with less homely statuary.

I’m restraining my desire to buy lots of new plants, although I have had to plug a few gaps created by over enthusiastic ‘weeding’.  The weather in the UK, or at least the bit where I live, has been a bit weird of late, and it felt like April and May were swapped round.  April was warm, dry and sunny and (the first half of) May…was not.  But it is now very much a time to get mulches down to protect plants from dehydration and that nearly exposed earth from getting too scorched.

There are many mulches that the avid gardener will bore you to death with while regaling the relative merits of each and where on absolute no account should you use it.  You will be pleased to know that I am not such a gardner, and will simply mention that coffee grounds are popular: usually free, disliked by slugs (supposedly), with an interesting texture and, when freshly laid, with a lingering aroma of the café…

If you don’t frequent coffee shops, then it may be news to you that some dispose of the used grounds by putting them back into the bags the beans come in, and then giving them away to any patrons who want them.  At sometimes there is quite a pile, at others it is rather like watching two people at the sales trying to get the last heavily discounted espresso machine in the shop.  I picked up a couple of bags the other day, and duly put them on the garden, which should have been the end of the matter.  But no.  I started thinking about how many bags of coffee grounds I’d need to cover the garden and how many cups of coffee this would represent.  I’ll be honest, I’m not really sure how big my garden is, and for a number of reasons it would be hard to work this out with any pretence at accuracy.  Then you’ve got the patio and the lawn to take account – and all in all, it might be better to think in terms of requirements per square metre.  That probably makes more sense for you in terms of your garden as well.

So, as a starting point, we probably want at least a 2 cm depth of mulch, so that means we need 0.02 cubic metres of coffee grounds.  The refilled bags normally give you about 300 x 140 x 40 cubic mm or 0.00168 of a cubic metre – eeep! We’re going 119 bags of coffee grounds per cubic metre! I’d better get more decisive at the coffee shop – at a bag a day it’s going to take me four months to get enough for a small fraction of the garden.  Buying more (ground covering) plants is starting to seem like a better option.

Academics are not the only people who like their coffee of course.  Many cultural icons are intrinsically linked with this form of caffeine addiction.  Reputedly Beethoven ground exactly 60 beans everyday for his morning cup.  (The internet being what it is, you could easily lose an hour or so with the various discussions about the likelihood of the great man counting out the exact numbe of beans everyday, how he would have brewed his coffee and changes in the kinds of bean available then and now.  I’m pleased to report I managed to resist the temptation – this time at least).  I mention this, because the other part of my meandering thoughts were regarding the number of cups of coffee that would be required to provide the grounds.

There is a part of me that would quite like to work from first principles: how many beans in a bag (typically a kg, quick web search suggests ∼0.13 g per bean = ∼7700 beans per bag; I’m really not sure whether this feels right or not, but it does tell us that Beethoven used less than 10 g of coffee per cup – but then he wasn’t making espresso).  But we don’t need to do that.  I did weigh the bag I bought home – 1.4 kg – but again, we don’t really need this information: it does perhaps remind us that things are going to get complicated because not only is the volume nw more efficiently utilised (because we’ve ground down those inefficiently packed beans), but the grounds are of course wetted (technically term, not poor English, I promise!).

One of the key differences between proper, trained-barista brewed coffee and just about every other kind of coffee is that baristas use a “porta filter”, which is that little cup that the grounds are tamped into – I’m simplyfying a great deal,of course.  But this process means that the same amount of coffe is used every time, in terms of both mass and volume (and it can be done without having to weigh the coffee every time).  There is a great deal of variation in the size of the porta filter, but 58 mm o.d. (outer diamter) seems to be becoming the standard.  This leads to a slightly shallower basket – I can’t find the exact dimensions online, and have not found a quiet enough time/the courage to enquire of my local caffeine dealers.

One of the hallmarks of a back of the envelope calculation is that we keep things simple, so lets assume that the volume filled with grounds is a disc 50 mm in diameter and 10 mm deep – this gives us ∼20,000 cubic mm or 0.00002 of a cubic metre.  So we’re going to get about 84 cups in a bag. Given the assumptions we’ve, we can easily round off our calculations to say we’re going to need 10,000 cups of coffee worth of coffee grounds.

Anyone fancy going for a coffee?


2 thoughts on “Coffee

  1. Mulching the entire garden with grounds? No wonder you’re having to turn to the local coffee shop! I limited myself to simply the herb garden but, as I hadn’t heard of its efficacy with slugs, also surrounded my pots with some ‘left-over from the builders’ sharp sand. I’m afraid I just couldn’t bring myself to go the route of crunched up egg shells …

    Liked by 1 person

    1. Yeah…There is a definite sustainability problem here! I think if I tried to do the entire garden in one hit (assuming I could source enough grounds), I’d probably be knocked unconsious/throw up from the smell. It’s a surprisingly unfriendly aroma compared to the coffee-shop itself.


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