How to buy a t-shirt as a gift without knowing their size

The good news is you don't need to ask them outright. There are a few sneaky ways to work it out, and once you understand how sizing actually works, you can hedge your bets sensibly.

The sneaky ways to find out

The easiest trick is to go and look in their wardrobe or laundry basket, if you have access. Find a t-shirt they actually wear, not one shoved at the back that still has a tag on. Check the label. Write down the size and, ideally, the brand too, because a Uniqlo medium and a Fruit of the Loom medium are not the same animal.

If you can't rifle through their drawers, try asking someone who can. A partner, a sibling, a flatmate, a parent. People love being brought in on a surprise and they'll usually be more accurate than the person themselves, who may be working off a mental image from 2014.

Failing that, there's the casual question. "I saw this amazing jumper in the sale, what size are you these days?" works surprisingly well because it's about something else entirely. You can also play the "I'm sorting through my wardrobe and wondered if any of this would fit you" card. People will tell you their size without suspecting a thing.

If none of that is possible, you're going to have to estimate. Which brings us to the bit that actually matters.

Why the letter on the label is almost meaningless

S, M, L, XL. These letters are not a standard. They're a vague suggestion that varies wildly between brands, between countries, and between men's and women's ranges. A women's large from one high street shop can be smaller than a men's small from another. It's chaos.

What actually matters is the chest measurement. Every decent size guide will give you a chest width in inches or centimetres, and that number is the only one worth paying attention to. If you know the person wears roughly a 40 inch chest, you can look at any size guide anywhere and find the right size, regardless of what letter is printed on it.

So if you're guessing, guess in inches. A rough rule of thumb for adults:

• Petite build: around 34-36 inch chest
• Average build: around 38-40 inch chest
• Broader build: around 42-44 inch chest
• Larger build: 46 inch and up

Then match that number to the size guide on the product page. Don't assume "he's a big guy, so XL." One brand's XL is another brand's medium.

Unisex sizing, and why it runs big on women

Most printed t-shirts, including the ones we use, are unisex. Unisex is a polite way of saying "cut like a men's t-shirt." The shoulders are wider, the body is straighter, and the sleeves are a bit longer than a fitted women's tee.

For a man buying his usual size, this is fine. A men's medium and a unisex medium will feel roughly the same.

For a woman, it's different. If she normally wears a women's medium in a fitted tee, a unisex medium will feel loose and boxy. That's not automatically a bad thing. Plenty of women prefer that oversized look, especially for weekend wear, pyjamas, or anything with a slogan where a slouchier fit works better anyway. But if she likes her t-shirts fitted, you'll want to size down one from her usual women's size. A woman who wears a women's large will probably want a unisex medium.

The exception is if she specifically wants something to tuck in or layer under a cardigan. Then stick to her normal size and accept it'll be a bit roomy.

When to size up (and when not to)

Size up if:

• They're between sizes and you genuinely don't know which way they lean
• The shirt is 100% cotton and likely to shrink a touch in a hot wash (most do, by about 3-5%)
• They tend to wear things loose, or layer t-shirts under shirts and jumpers
• It's for someone older who probably isn't trying to show off their abs

Don't size up if:

• They're known for liking a close fit
• It's a women's cut and she wears it fitted
• The design is positioned centrally on the chest and would look odd floating in the middle of a tent

For a retirement gift, I'd nearly always lean slightly roomy. Comfort wins over flattery once you've handed in your laptop, and a shirt that says Moved my 2pm to the sofa is a weekend favourite rather than something for a formal dinner.

For a golfing dad or uncle, think about what they wear on the course. Polo shirts tend to run slightly fitted, so if you know his polo size, go one up for a t-shirt. Something like Raked it. Hit it. Raked it again. is the kind of thing that'll end up as a go-to weekend shirt, so you want it comfortable rather than clinging.

A word on returns and the quiet backup plan

Even with all this, you might get it wrong. That's fine. Check the returns policy before you order and keep the packaging intact until they've tried it on. Most small shops, ours included, will swap sizes without any drama as long as the shirt hasn't been washed or worn.

And if you really can't work out a size and don't want the stress, a mug is a safer bet. It fits everyone. A mug that simply says Not lost. Just rehomed. will land just as well as a t-shirt would, and nobody's ever returned one because the handle was too small.

But if you've got a size, even a rough one, a t-shirt is still one of the nicest gifts you can give. People wear them. They remember who gave them. And every time they pull it on, they think of you, which is really the whole point.