A guide for parents
UK children's clothing sizes by age
What each size actually means in height and weight, how long your child will wear it, and why the label is only ever a starting point.
What's in here
- The size chart
- How long each size is actually worn
- The label is when it stops fitting, not when it starts
- Go by height, not age
- Boys, girls, and one chart for both
- Why sizes differ so much between brands
- How many of each size do you actually need?
UK children's clothes are labelled by age, which sounds helpful right up until you've got an actual baby in front of you who is nothing like the average.
Two babies born the same week can be two sizes apart. Every brand cuts differently. And the label tells you absolutely nothing about how long the thing will fit, which is the bit you actually want to know.
So here's the lot, including the column nobody else bothers with.
The size chart
| UK size | Height | Weight | EU size | Worn for about |
|---|---|---|---|---|
| Newborn | up to 56cm | up to 4.5kg | 50-56 | 2 to 4 weeks |
| 0-3 months | 56-62cm | 4.5-5.7kg | 62 | 6 to 8 weeks |
| 3-6 months | 62-68cm | 5.7-7.5kg | 68 | 2 to 3 months |
| 6-9 months | 68-74cm | 7.5-9kg | 74 | 3 months |
| 9-12 months | 74-80cm | 9-10kg | 80 | 3 months |
| 12-18 months | 80-86cm | 10-12kg | 86 | 4 to 6 months |
| 18-24 months | 86-92cm | 12-13.5kg | 92 | 6 months |
| 2-3 years | 92-98cm | 13.5-15kg | 98 | about a year |
| 3-4 years | 98-104cm | 15-17kg | 104 | about a year |
| 4-5 years | 104-110cm | 17-19kg | 110 | about a year |
| 5-6 years | 110-116cm | 19-21kg | 116 | about a year |
Indicative ranges based on the charts major UK retailers publish. Every brand differs, sometimes by a whole size, so check the specific shop's chart before buying several of anything.
How long each size is actually worn
That last column is the one that changes everything. A newborn babygro gets about three weeks. A 0-3 month sleepsuit, six to eight. By 2-3 years, the same size lasts a whole year.
It also decides how much to buy, and the honest answer is less than most checklists suggest. The first year is where almost all the waste happens. A baby goes through six sizes before their first birthday and one size between three and four. Buying twelve newborn vests makes sense on the shelf and none at all six weeks later.
The single most useful rule
Buy few, buy big. Three or four of anything is plenty in that first year, and when you're between sizes, always go up. A slightly loose sleepsuit is comfy and lasts weeks longer. A snug one is done by the weekend.
The label is when it stops fitting, not when it starts
This is the one that gets everybody at least once, usually with a birthday present.
A garment labelled "3-4 years" fits a child up to four. It's outgrown around the fourth birthday, not bought for it. So a child who has just turned four wants 4-5, not 3-4.
The same applies where a single year is used. "Age 4" generally means up to four, so it's the same garment as 3-4 years. That convention is widely followed but not universal, and Next in particular is often reported as running a year ahead, so a Next "age 4" may fit a child closer to five.
So read the label as the age they'll grow out of it, not the age they'll grow into it. And if it really matters, check the height on that brand's own chart.
Go by height, not age
The age on the label describes an average child, and most children aren't average. A five-year-old can be 104cm or 116cm, which is two sizes apart wearing the same number.
European brands size by height in centimetres for exactly this reason, which is why you see 74, 86, 98 on labels. It's a better system. If you know your child's height, use the EU column above and ignore the age entirely.
For babies, weight matters as much as height, because a long slim baby and a compact solid one of the same age genuinely need different clothes.
Boys, girls, and one chart for both
The chart above covers both, because that's how UK retailers publish them. Children's clothing is sized the same for boys and girls up to around six or seven, when brands start splitting into separate lines with different cuts.
Children themselves do differ. Boys are on average slightly longer and heavier than girls from birth, which is why the NHS growth charts used by health visitors are separate for each. But the gap at these ages is small enough that no retailer sizes for it, and it widens mainly through puberty, well outside the range here.
In practice it's one more reason the age on the label is approximate. If you've a tall girl and a compact boy of the same age, they will wear different sizes, and the height column is the one to trust.
Why sizes differ so much between brands
There's no legal standard for children's sizing in the UK, so every retailer sets its own. Two things follow.
Some brands run small and some run generous, and it's consistent enough that parents learn it. Next and M&S are usually true to size. Some supermarket ranges run small. Scandinavian brands often run long.
And the same label means different things in different departments. Sleepwear is usually cut roomier than daywear, and anything designed to go over a nappy has extra room where you wouldn't expect it.
The practical version: when you find a brand that fits your child, buy that brand.
How many of each size do you actually need?
For the first year, when sizes last weeks rather than months, roughly:
Newborn and 0-3 months. Four to six sleepsuits, six to eight vests, and very little else. You'll be given more than this and you'll still not need it.
3-6 and 6-12 months. Six to eight sleepsuits or outfits, since this is where weaning and crawling start producing laundry.
Past a year. A normal wardrobe, because the size now lasts long enough to justify one.
Around 300 garments between birth and starting school works out somewhere near £2,500, and the arithmetic is here. Almost all of it's outgrown rather than worn out.
I bought twelve newborn vests. Twelve! He was in them about three weeks and I have never let myself forget it.
🌱 Which is the problem we built Swapaboo for
The first year produces six sizes of barely-worn clothes and there's nowhere sensible for them to go. So we made a swap: photograph what your children have outgrown, we check and value each piece, and that value becomes credits you spend on the next size up from another family.
Every piece is checked and graded before it lists, and the condition is shown before you claim it, so nothing is a surprise when the parcel opens. You pick every item yourself. The only money that changes hands is the delivery or collection, from £1.99, around 85% less than buying the same clothes new.
We're opening in small batches and it's invitation only for now.
Common questions
What size clothes should a 6 month old wear?
Usually 3-6 months or 6-9 months, depending on height and weight rather than age. A 6-month-old is typically 65 to 70cm and 7 to 8kg. If your baby is near the top of that, go up: 6-9 month clothes fit 68-74cm. Height is a better guide than the number of months.
Does age 4 mean a four-year-old?
Usually not. UK age labels describe the age a garment is outgrown rather than the age it starts fitting, so "3-4 years" fits up to four and a child who has just turned four wants 4-5. Where a single year is used, "age 4" generally means the same as 3-4. It is a convention rather than a rule, and some retailers run a size ahead, so check the height on the brand's chart if it matters.
How long do babies wear each size?
Newborn lasts two to four weeks, 0-3 months about six to eight weeks, and each size through the first year roughly two to three months. From two years, a size lasts about a year. Most babies go through six sizes before their first birthday.
What does EU size 80 mean in UK sizes?
European sizes are the child's height in centimetres, so size 80 fits a child around 74-80cm, which is roughly 9-12 months in UK sizing. Size 68 is about 3-6 months, 86 is about 12-18 months, and 98 is about 2-3 years.
Should I size up in baby clothes?
Generally yes, if you are between sizes. Babies grow fast and a slightly loose garment is comfortable and lasts weeks longer, where a snug one is outgrown almost immediately. The exception is sleeping bags, which must fit properly for safety.
Are boys' and girls' clothing sizes different?
Not for babies and young children. UK retailers use the same size chart for both up to around six or seven, when brands begin splitting into separate lines with different cuts. Boys are on average slightly longer and heavier from birth, which is why NHS growth charts are separate by sex, but the difference at these ages is too small for clothing sizes to reflect it.
How many baby clothes do I actually need?
Fewer than you think in the first months. Four to six sleepsuits and six to eight vests covers newborn and 0-3 months, because those sizes are worn for weeks rather than months. It is worth buying sparingly until you know how fast your baby is growing.