How to choose the right adult onesie size: the complete guide

Which adult onesie size should you choose? The answer comes down to two measurements: your height in centimetres and your chest. Your height sets the length of the onesie, your chest confirms the width. For loungewear you'll wear all winter, aim for roomy and, between two sizes, always take the bigger one. Here's the complete guide to measuring accurately, understanding the cuts, dealing with in-between sizes, giving one as a gift without getting it wrong and caring for your onesie so it lasts.

Why onesie sizing works differently

A onesie isn't a pair of jeans. It's a single piece that covers the body from neck to ankles, so if just one measurement is wrong, everything pulls. With trousers, if the length isn't right, you roll them up. With a onesie, a leg that's too short drags on the shoulders every time you lift your arms.

That's why height — how tall you are standing up, in centimetres — becomes the number one reference point. Lots of people think "I'm a medium", but two people in M can be ten centimetres apart. On a one-piece, those ten centimetres change everything: comfort at the crotch, ease at the shoulders, the overall drape.

How to measure yourself, step by step

Two measurements are enough to choose an adult onesie. No need for fancy equipment: a dressmaker's tape measure and a wall will do. Measure yourself in your underwear, no cheating, and write the figures down before checking them against the brand's size guide.

Height (the key measurement)

Put your heels, bottom and shoulders against a wall, looking straight ahead. Lay a flat ruler on top of your head, mark the wall, then measure from the floor to the mark. That's your height. On most onesie size guides, this is the figure that decides your base size, before any adjustment.

Chest and hips

Wrap the tape measure horizontally around the fullest part of your chest, without pulling it tight, then do the same at your hips. These two measurements confirm the width. If your chest puts you one size above your height, follow your chest: it's easier to roll up sleeves than to widen fabric.

Roomy or fitted cut: which should you go for?

For a lounging onesie, roomy wins almost every time. A good onesie should let you sit cross-legged, curl up in a ball on the sofa and slip a jumper underneath without anything pulling. A fitted cut looks better in a photo, but soon feels heavy going after two hours slumped in front of a series.

There's a real difference between "your size" and "skin-tight". A onesie in your size follows your shape without squeezing. A fitted onesie tightens at the shoulders and crotch as soon as you move. For long-haul cosiness, ease beats style every time. And honestly, wrapped up in thick flannel, nobody's judging the cut.

Does the fabric change which size to choose?

Yes, and it's often overlooked. A thick fabric with little stretch, like fleece flannel, doesn't forgive a style that's too snug: it won't stretch to make up the missing centimetres. On the other hand, a thin cotton jersey keeps a bit of give and copes better with being in between.

Here's how the fabric affects your choice.

• Thick fleece flannel: not very stretchy, warm, it needs a roomy cut. If in doubt, size up.
• Thin microfleece: a little more supple, but it pills and loosens, so don't overcompensate by going too big.
• Cotton jersey: stretchy and breathable, it forgives being slightly between sizes.
• Sherpa / teddy fleece: very thick, it takes up room inside; keep some leeway at the shoulders.

The Monster Pyjamas go all in on thick flannel with a deliberately loose cut, designed for moving freely. The result: the fabric and the cut pull in the same direction, and choosing your size comes down to your height alone.

Between two sizes: what should you do?

Go for the size up. It's the most useful tip in this guide, and it applies to almost every one-piece. A onesie that's a little big stays snug and easy to live in. A onesie that's too snug pulls at the shoulders, rides up at the crotch and ruins the fun from the very first evening.

A few special cases are worth a look:

• Tall and slim: follow your height, even if it means a bit of extra room across the width. Length is what pulls first.
• Short and curvy: follow your chest or hip measurement, extra length is easier to deal with.
• Sensitive crotch area: if you're straddling two sizes, the bigger one avoids that "too short" feeling that drags downwards.

With no built-in feet, as on the Monster Pyjamas, the question of shoe size disappears completely. You only choose on height and width, which cuts the risk of getting it wrong even further. That's a real advantage when you're ordering online.

Giving a onesie without knowing their measurements

Giving a onesie without knowing the person's measurements is the classic end-of-year scenario. Good news: a roomy onesie is far more forgiving than a fitted shirt. The trick is to aim generous rather than snug, because a style that's a little big stays wearable, whereas one that's too small ends up at the back of the wardrobe.

To get it right:

• Go by the person's usual top size (S, M, L) and go up one if in doubt.
• Discreetly work out their height: taller than you, shorter, about the same? That's often enough.
• Choose a style without feet to avoid the shoe-size trap.
• Keep the returns policy in mind: a onesie that's easy to exchange takes all the pressure off.

For more ideas, have a look at our guide to funny gift ideas for adults: the one-piece has pride of place in it.

Care: keeping the right size over time

Choosing the right size also means keeping it, wash after wash. Thick fabrics like flannel mostly dislike heat: a wash at 30 °C on a delicate cycle and air drying protect the brushed fabric and prevent shrinking. A hot tumble dryer, on the other hand, tightens the fibres and makes the surface pill.

A few simple habits extend the life of your onesie:

• Do up the zip before washing to protect the fabric and your other clothes.
• Turn the onesie inside out to protect the hood and the embroidery.
• Go easy on the fabric softener, which clogs fleece and reduces its softness.
• Dry flat or on a wide hanger rather than hanging it by the shoulders.

The Monster Pyjamas follow exactly this logic: machine wash at 30 °C, air dry, and they come out good as new. A well-cared-for onesie keeps its original cut and roominess, so the size you chose stays the right one season after season.

The key takeaways

• Measure your height and your chest: these are the two reference points that really count.
• Aim for roomy for loungewear, never tight and fitted.
• Between two sizes, take the bigger one, especially in thick flannel.
• When giving one as a gift, go up a size and choose a style without feet.
• Wash at 30 °C and air dry to keep the cut over time.

Once you've found the right size, all that's left is to enjoy it. You can discover the roomy cut and thick flannel of the Monster Pyjamas, or keep reading with our guide to a grown-up pyjama party to give your new onesie its moment.

Cover photo: Siora Photography / Unsplash