A coat bought in the second week of September is still on the hook in March. Six months. A three year old keeps growing through all six of those months, and the sweater under the coat gets thicker as the weeks go on. So the question isn't whether to size up. It's what happens to the sleeves and the hem if you do, and whether the child can still use the arms.
Start from height, not age
European coats follow height. A tag that prints 98 fits a child up to 98 centimeters tall, about three years old, and a 104 fits up to 104, about four. A tag that prints an age sits on the same table: 3Y is 92 to 98, 4Y is 98 to 104. So a child measuring 97 in September is at the top of the 98 and is likely past it by Christmas. That child takes the 104.
A child measuring 93 in September has a real choice. The 98 fits now with five centimeters to spare. The 104 fits in spring and hangs a bit this autumn. Which one is right depends on what goes under it and what the child does in it, and that's the rest of this page. The full height table by age is on our size guide, and the centimeter system is explained on what size is 110 in kids clothes.
What goes under it decides the size
A coat is cut with room for a normal layer underneath, a tee with a sweatshirt, or a shirt with a cardigan. That's what the size assumes. Put a chunky wool knit under a slim European coat in the exact size and the shoulders pull, the arms won't bend all the way, and the child fights the zipper.
Two ways out. Size the coat up one and keep the knits chunky. Or keep the coat in size and keep the layers thin: a long sleeve tee, then a fine sweatshirt or a cotton cardigan, which is how most of the autumn layers shelf is built. Thin layers trap more warmth than one thick one anyway, and they come off in an overheated classroom.
Down and padded pieces are the exception. A puffer or a quilted vest already carries its warmth. Under it goes one thin layer, so it sits closer to the size the child is now. The vests and gilets on our coats and jackets shelf are the most forgiving pieces there: no sleeves to be too long, a hem that can be a bit longer without getting in the way.
Three checks that settle it
Sleeves, with the arm bent. Have the child bend the elbow and reach forward, the way they grab a railing. The cuff should sit at the wrist bone, not over the hand. A cuff over the hand in September means the child can't grip the climbing frame and will have wet sleeves by October. One size up adds a bit of sleeve, and a turned cuff hides that for a couple of months. Two sizes up doesn't hide anywhere.
Hem, when the child sits. A coat that's the right length standing should not bunch under a child sitting on a scooter or in a car seat. One size up on a coat that already reaches mid thigh gets in the way of knees. On a bomber or a short jacket the extra length lands at the hip and nobody notices.
Shoulders. On a coat with a set in sleeve, the seam sits on the shoulder bone. If it sits down the arm, the coat is too big now and the sleeves will swallow the hands. This is the one measurement that can't be fixed with a turned cuff or a chunky knit. An oversize cut or a raglan sleeve is built differently, so judge those by sleeve length and movement instead.
Piece by piece
Wool and check coats. Structured, slim, cut with room for a shirt and a knit. These are the pieces on our shelf where one size up works best: the sleeves turn, and a hem that runs a little longer on a coat reads as intended. A three year old at the top of 98 takes the 4Y coat. The Tocoto Vintage check coats with furry lining and the trench coats on the coats shelf are this type.
Bomber jackets. Short by design, elastic at the hem and cuff. The elastic cuff does the work a turned cuff does on a coat, so a bomber one size up looks intended, not borrowed. The quilted and sherpa bombers from Konges Sløjd sit here.
Puffers and down. Warmth is in the fill, not in the layers under it. Size these to the child now, with one thin layer under. A down jacket one size up loses shape at the shoulders and lets cold in at the neck. The Colmar down jackets are the case for buying the size the child is.
Vests and gilets. The autumn piece for a child who runs hot. Over a sweatshirt in October, under a coat in January. A vest forgives a size up, there are no sleeves to get wrong, but it still has to close over the chest without gaping at the arms.
Baby coats, 12 and 18 months. A baby in a stroller isn't gripping a railing, so a sleeve that runs a little long is warmth, as long as the hands still move. A baby who started walking last month is a different case, and the sleeve rule comes back. The wool hooded baby coats from 1 + in the Family come in 12 and 18 months.
Hoods and drawstrings
US safety rules ban hood and neck drawstrings on children's outerwear in sizes 2T through 12, so most kids coats sold here have a fixed hood or a snap on hood. A hood that fits a bigger head is fine. A hood that falls over the eyes on a child who's sized up is the one thing to watch for on a coat bought a year ahead.
What it costs to get it wrong
Coats on our shelf run $40 to $120, vests and gilets $45 to $82. Most are single pieces from small runs and we rarely restock, so the size you see in the selector is the one that exists. If it comes back to us, return shipping is on you, which is why we'd rather measure first. Send the child's height and the coat you're looking at, and we'll measure the actual garment: shoulder to cuff, shoulder seam to hem, across the chest with the coat closed. That takes an email and saves the return.
The knits that go under the coat, cardigans and sweatshirts, are sized the ordinary way, in the size the child is now, on the cardigan shelf and the long sleeves and sweatshirts shelf. How each label on our shelf writes its sizes is on European kids brands size chart. And if you're building the whole autumn wardrobe for a two to four year old at once, the toddler autumn capsule is six pieces that go together.