Unlike buying a car, or a house, clothes are more like buying gremlins: they don't stay the same once they meet water. It's an important consideration when trying them on, as the size in the shop is usually not the size you'll live with.
But how much do they actually shrink? As a committed nerd, I wanted numbers and, ideally, decimal points.
So how much do clothes shrink in the wash?
Short answer: about 5% if they're cotton, and nothing if they're polyester.
Long answer: it depends, so strap in.
The first 'it depends' is material. 100% cotton will shrink more than cotton blends. Higher quality cotton with longer fibres (long-staple cotton) tends to shrink less. There are also a number of treatments that manufacturers can apply to cotton in order to limit shrinkage (more about them below).
However, I appreciate 'it depends' is an unsatisfying answer, which is why I decided to take 4 normal t-shirts and give them a 1 hour, 30° cycle. Of course, this is just one wash, and repeat washes and temperature and tumble-drying will increase the shrinkage until it plateaus. But anyway, here are the results:
Cotton t-shirts shrink 4.8% in length.
So about 5%, basically. But it does vary by brand, between 3% and 7%. Also it varies by measurement. This 4.8% is 'length', i.e. the distance from the edge of the collar down to the hem. Sleeves shrink more than 6%, whereas the chest shrinks least, at 2.6%. See useful graphic below.
Why does cotton shrink?
There is tension in cotton fibres which is released during heating. They pull closer together. The reason for less shrinkage in the chest is that t-shirts are made of 'jersey cotton', which has sideways stretch built into the fabric. However, the length is less shrink-resistant.
Can you 'un-shrink' cotton?
Yes, a bit. If you stretch a garment out while damp you can undo some of the shrinkage. Ironing or steaming also helps. But obviously there is a limit, and you can end up distorting the garment if you're too aggressive.
Can you stop cotton shrinking?
Pure, untreated cotton will always shrink, and that's fine - it's a natural material. Obviously the best way to avoid shrinkage is to wash the garment rarely, or use low temperatures. You can also get 'treated' cotton, which will counteract the shrinkage, although usually has some sort of aesthetic side-effect.
Pre-shrinking
Just as it sounds, the garment is pre-shrunk before being sold. Either the fabric can be shrunk, then cut and sewn. Or the garment can be manufactured to a larger size, then shrunk. Usually looks less 'new' and soft, because it isn't.
Mercerisation
A chemical treatment which reduces shrinkage. Often leaves a slightly shiny or lustrous look. Full details on Wikipedia.
Sanforisation
A mechanical stretching and shrinking process. This is often the the technical process which results in 'pre-shrunk' fabric. Full details on Wikipedia, or, if you like wordless videos of large machinery (who doesn't?), see here on Youtube.
Worth noting that all of these methods still result in a bit of shrinkage, but it will be reduced. For instance, I also tested a mercerised t-shirt and it still shrank 3.5% in length.
Anyway, hope that was useful.
Jack, Founder of Ruler of London