Happy Pi Approximation Day 22/7!

Happy Pi Approximation Day 2013!

If you don’t know what the heck I’m talking about, then allow me to explain.  In mathematics, science & engineering, π is regularly used in descriptions of circular motion.  This includes electrical waveforms and A.C. theory.  Today is the 22/7, and mathematicians commonly use the fraction 22/7 as an approximation of π.  Put it into your calculator and you’ll get roughly 3.14.
The true (exact) value of π is unknown – it cannot be expressed as an exact decimal number.  The key here is ‘decimal’ number, because after all our decimal number system that we choose to use is actually arbitrary! Computers have had a fair crack at it and they’ve managed to compute π to thousands of millions of decimal places using various types of algorithms! But, ultimately, it’s not possible to express it as an exact number.

π Approximation Day, held each year on 22/7, is an opportunity for number nerds all over the world to celebrate their favourite irrational number.

Behold the Pi Pie!

The traditional way to celebrate π approximation day is to bake a π pie.  That’s my kind of celebration, so this year Nerys John and I collaborated on a batch of scrumptious mixed berry pi pies.  Here’s how we did it:

What you’ll need

What you'll need
What you’ll need


  • A roll of shortcrust pastry (amount depends on how many pies you want to make)
  • A package of fruits – we chose mixed berries (Tesco Summer Fruits)
  • Apples (you can add apples or not – your choice!)
  • A jar of granulated sweetener
  • Flour
  • A small amount of milk (for brushing onto the tops of the pies for browning)
  • A small jar of cinnamon


  • A rolling pin
  • A light brush
  • A small pie baking tray
  • Small foil pie cases
  • A large dish
  • A sharp knife
  • A large pie cutting stencil (for making the pie bottoms)
  • A small pie cutting stencil (for making the pie tops)
  • A suitable π stencil.  We used a home-made π stencil that we cut out of cardboard.

First, the berries

Pouring the filler

The first thing we need to do is pour the berries into a mixing dish.  If you’re adding apple, you’ll also want to slice the apple up into small pieces and add them into the dish as well.

Add the cinnamon – with flippancy

Once you’ve added your chosen filling to the mixing dish you need to sprinkle some cinnamon onto the top of them and mix.  Personally I’d like to be able to offer you exact amounts here, because that’s how I like to work, but apparently when it comes to baking it’s better to be flippant with your measurements – much to my dismay.  Add an arbitrary amount of cinnamon such that your bowl resembles that of the photo on your right, and then mix.

Sugar coated! Mmmm

Next up, it’s time for my favourite bit – the sugar.  Again, the Engineer in me wants to give you specific measurements here but unfortunately a good cook has to be skilled in flippancy so you’ll need to throw an arbitrary amount of the sweetener onto your mixed berries such that they vaguely resemble the photo on your left.  If you’re thinking that baking is a lot like being tortured right now, then you’re not alone.  My solution was to find someone experienced in flippancy to cast the ingredients – I recommend you do the same.  Somehow certain humans have managed to build a resistance to this kind of chaos (shrug).

Mixed Berry Filling

Right then, now it’s time to mix everything together.  Be careful here; you want everything to be nicely mixed, but you don’t want to pummel the berries into oblivion.  If you do that you’ll end up with a soppy, watery mix and apparently that’s not good for the final result.  The best way to do this is to find a nice large spoon and just turn the mix over and over gently until the ingredients blend together nicely.

Once you’ve mixed the berries and the flavouring you need to heat the filling up in the microwave for a short while.  How long? Well, it’s a chef’s measurement again I’m afraid.  Just try them in for a minute or so, stir, then try again until eventually you’ve got a pretty warm and well mixed filling.  It’s normal for the filling to go a bit more watery during this process – the point is for the filling to become a bit more gooey (all fruit pies are gooey, right?) but not too gooey that the whole thing turns to water.  You still want the filling to be stodgy with recognisable berries in the mix.

Now the pastry.

The rolled pastry
The rolled pastry

Cover your work surface with a dusting of flour so that the pastry can be moved around on the work top without binding and sticking.  I’ve always been taught to use WD40 in this kind of scenario, but apparently that’s a bad idea here so I guess we just stick to the flour.  Roll the pastry out to a flat sheet as shown in the photo on your left, whilst being mindful that the thickness of the rolled pastry will be proportional to the thickness of your pie casing.  You want it thin, but not too thin.  If in doubt then use the force, Luke.  Let go.
Once the pastry is rolled it’s time to cut out your pie tops, pie bottoms, and π symbols.  Using the larger pie stencil for the bottoms and the smaller pie stencil for the tops, cut as many pie bottoms and tops as you can afford to make whilst at the same time allowing enough room to make your π symbols.  We’ll worry about the pie symbols in a moment, but for now just cut out your pie tops and pie bottoms, then peel them away from the pastry and put them to one side.

The Pi Symbols

Making Pi Symbols!
Making Pi Symbols!

It would be really cool if we had a π shaped pastry cutter, like the ones you’ve used to cut the pie tops and bottoms.  Unfortunately I didn’t have anything of the sort.  I considered ordering some thin sheet metal (maybe copper) and making my own π-shaped pastry cutter, but perhaps that’s an improvement for next year.  For this year we simply drew a π shape onto cardboard (crap artist? Me too – get someone

Don't cut your fingers!
Don’t cut your fingers!

skilled to do it), then we cut out the card board shape, and used it as a stencil from which to cut around the pastry with a sharp knife.  Something like that shown on your right.  Depending on the amount of pies you’re intending on making this part can be a bit laborious and that’s where a decent home made metal (cutting) π stencil would come in handy.  But persevere and the end result will be worth it 🙂

Preparing the bottoms, the filling, and the tops

Preparing the pie bottoms
Preparing the pie bottoms

Nearly there – it’s time to prepare your pies for baking.  Carefully press your pie bottoms into the pie foils as shown in the photo on your left.  You want a snug fit but don’t force the pastry too much or you’ll never get them out of the foil in one piece afterwards.

A filled pie!
A filled pie!

Once the bottoms are in, it’s filling time.  Spoon a generous amount of filling into the pie bottom (drain off any watery content beforehand) but be aware that you need to be able to comfortably fit the pie top on afterwards.  If you fill them up too much then the filling will breach your pie tops a little bit when they’re in the oven.  In my experience some of them breached and some of them didn’t.  That’s what happens when you’re flippant, I reckon.  No consistency.  But good luck trying to persuade a chef to optimise their process! I think Engineers are banned from most kitchens.

Press fitted pi pies!
Press fitted pi pies!

Finally you’ll need to press-fit the pie tops.  Just drape the pie top over the filling such that it’s centralised, and then pinch all around the edges of the pastry where top meets bottom (if top

Brushing milk so that the pie browns.
Brushing milk so that the pie browns.

doesn’t meet bottom, you’ve screwed up) so that the finished pie is sealed all the way around.  Once you’re happy with the sealing you can place your π symbol on the top (you don’t need to press it on, it’ll adhere during the baking process) and then you just need to brush a small amount of milk over the pie for browning.


The hard bit is over, but this is perhaps the most crucial part of the job in terms of getting the best possible result.  Set your oven to 180 degrees centigrade (Fahrenheit? Pah – use proper units) and make sure it’s nice and hot before you put the pies inside.  Once the oven has thoroughly warmed through then you can put your finished pies in for baking.  It’s tempting to leave them in until they are nice and brown (like you do with a meat pie, for example) but in the case of fruit pies it’s better to watch them carefully and bring them out when they’re just lightly browned.  If you leave them in too long the filling gets really hot and then you’ll end up with a breach of the warp core.  It’s all over after that, and not even Geordie La Forge will be able to help you.  So basically just be careful.


And here’s the finished result.  They were bloody lovely too! Thanks to Nerys John for directing the kitchen process.  I could never have been this skilled on my own, and that would have meant that I’d never have managed a decent result.  In some rare cases, Engineering isn’t the answer.  This might be the only time I ever publically admit that!


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