Should I buy baby clothes by age or by size?

Use age only as a rough starting point. Height and weight are far more reliable, because two babies the same age can differ by several centimetres and a kilogram or more. If you know your baby's height, enter it above — the calculator will use it instead of age for a more accurate match.

What does a size like 'EU 68' or '70' mean?

European, Japanese, and Korean baby sizes are based on the child's height in centimetres. An EU 68 fits a baby up to about 68 cm tall, and a Japanese 70 fits a baby around 70 cm. That's why measuring height gives you the most accurate fit in those systems.

Should I size up for my baby?

Often, yes. Babies grow fast, clothes shrink slightly after washing, and bulky nappies add girth. Choose the "Room to grow" fit above to bump the suggestion up one size, especially if your baby is tall, on a growth spurt, or between sizes.

Why do brands fit so differently?

There's no enforced standard for baby clothing sizes, so each brand sets its own measurements. One brand's 6–9M may run small while another's runs large. Always compare the brand's own size chart — usually given in height and weight ranges — to your baby's measurements.

How do I measure my baby's height for sizing?

Lay your baby down on a flat surface, gently straighten the legs, and measure from the top of the head to the bottom of the heel. Do this every few weeks during the first year, since babies grow quickly and can change sizes in a matter of weeks.

Baby Clothing Size Calculator

Enter your baby's age — and height if you have it — to get a suggested clothing size plus a UK, EU, US, Japan, and Korea conversion table.

How baby clothing sizes work

Baby clothes are usually labelled by age — Newborn, 0–3M, 3–6M, and so on — but those labels are only a loose guide. They're really shorthand for an average height and weight at that age, and no two babies grow on the exact same curve. One four-month-old might already be wearing 6–9M while another comfortably fits 3–6M. That's why the most useful question isn't "how old is my baby?" but "how big is my baby right now?" This calculator lets you answer both: enter age for a quick estimate, or add a height measurement for a result you can actually trust.

When you submit the form, the tool finds the size band your baby falls into, then shows you the equivalent size in five common systems — the United Kingdom, Europe, the United States, Japan, and Korea — so you can shop confidently no matter where the brand is from. It also shows the age range and height range for that size so you can sanity-check the match against what you already know about your baby.

Why height and weight matter more than age alone

Age is convenient, but it's the least precise of the three measurements. Babies have growth spurts and quiet stretches, and centile curves vary widely. A tall, lean baby and a shorter, rounder baby of the same age may need different sizes for length versus fit. Height is the single best predictor of clothing fit because most of a garment's "does it fit?" comes down to how long the body, sleeves, and legs are. Weight is a helpful secondary check: a baby who is heavier than typical for their length may need to size up for comfort around the middle, especially in fitted babygros and trousers.

If you only enter age, the calculator uses the age band. The moment you add a height, it switches to matching by height instead, because that's the more accurate signal. Weight, when you enter it, is shown as informational context rather than the deciding factor — it helps you decide whether to lean toward the next size up.

How the UK, EU, US, Japan, and Korea systems differ

The biggest source of confusion when shopping internationally is that the five systems don't all measure the same thing:

United Kingdom (age-based): Sizes are written out as "Newborn," "0–3 months," "3–6 months," and "2–3 years." The label is an age range, and brands map their own height/weight ranges onto it.
United States (age-based): Very similar to the UK, with sizes like Newborn, 0–3M, 3–6M, up to 24M, then switching to "T" sizes (2T, 3T) for toddlers. The bands line up closely with UK sizes.
Europe / EU (height-based): Sizes are a number in centimetres — 56, 62, 68, 74, 80, and so on — representing the child's height. EU 68 fits a baby up to about 68 cm tall. Because it's a direct height measurement, the EU system is often the most precise once you've measured your baby.
Japan (height-based): Also uses centimetres (50, 60, 70, 80, 90, 95), again keyed to height. Japanese sizes can run a touch slimmer, so the height match is a good anchor.
Korea (height-based): Uses centimetre sizes too, often given as ranges like 70 or 80–90. Like the EU and Japan, it's built around height rather than age.

This is exactly why entering a height makes such a difference: three of the five systems are defined by height, so a measured number converts almost directly.

How to read the conversion table

After you calculate, the result card shows a two-column table: the left side lists each size system, and the right side shows the matching size for your baby's band. The bottom two rows give the age range and the height range (in both centimetres and inches) that the matched size is designed for. Use those ranges as a gut check — if your baby's real height sits comfortably inside the listed range, you've got a good match. If your baby is near the top of the range, consider sizing up so they don't outgrow the clothes in a couple of weeks.

Age to UK to EU quick-reference table

Here's a summary you can scan at a glance. These are typical ranges; brands will vary.

Age	UK size	EU size (cm)	Height
Newborn (0–1 mo)	Newborn	56	46–56 cm (18–22 in)
1–3 months	0–3M	62	56–61 cm (22–24 in)
3–6 months	3–6M	68	61–67 cm (24–26 in)
6–9 months	6–9M	74	67–72 cm (26–28 in)
9–12 months	9–12M	80	72–78 cm (28–31 in)
12–18 months	12–18M	86	78–83 cm (31–33 in)
18–24 months	18–24M	92	83–89 cm (33–35 in)
2–3 years	2–3Y	98	89–96 cm (35–38 in)

Fit tips: when to size up

The "Room to grow" option bumps the suggestion up one size, which is the right call more often than parents expect. Reach for it when:

Your baby is tall for their age. Length runs out before width on most babies, so a longer size buys weeks of wear.
You use reusable nappies. Cloth adds noticeable bulk around the bottom; trousers and babygros that fit over them often need to be a size larger.
The clothes will be washed often. Cotton baby clothes can shrink a little in hot water and the tumble dryer, so a snug "just right" pick can become tight after a few washes.
You're buying ahead — for a gift, a season change, or a sale. A size up means the outfit still fits when the occasion arrives.
Your baby is mid-growth-spurt. If they've just jumped a centile, today's perfect fit won't last.

On the flip side, stick with "Just right" for newborn photos, snug sleep sacks where fit matters for safety, and any item where a baggy fit would be a hazard or simply look swamped.

Why brand sizing varies so much

There is no legally enforced standard for baby clothing sizes the way there is for, say, food labelling. Each manufacturer decides what measurements its "6–9M" represents. A European-leaning brand may cut slimmer; a high-street value brand may cut generously; a premium brand may build in extra length. Fabric matters too — stretchy knits forgive a size mismatch that stiff woven cotton won't. The practical takeaway: treat every brand as its own system. Most reputable brands publish a size chart with the actual height and weight ranges for each label. Compare those numbers to your baby's measurements, and trust the numbers over the label.

Limitations of this calculator

This tool gives a well-informed starting point, not a guarantee. It maps your baby to a general size band and converts that band across five systems, but it can't account for a particular brand's quirks, an unusual body proportion (long torso, chunky thighs), or the cut of a specific garment. It also assumes typical growth; premature babies, multiples, and babies at the edges of the growth chart may fit differently. Always double-check against the brand's own chart and your baby's current height and weight, and when you're between sizes, size up — babies grow into clothes far faster than they shrink out of them.

Frequently asked questions

Should I buy baby clothes by age or by size?: Use age only as a rough starting point. Height and weight are far more reliable, because two babies the same age can differ by several centimetres and a kilogram or more. If you know your baby's height, enter it above — the calculator will use it instead of age for a more accurate match.
What does a size like 'EU 68' or '70' mean?: European, Japanese, and Korean baby sizes are based on the child's height in centimetres. An EU 68 fits a baby up to about 68 cm tall, and a Japanese 70 fits a baby around 70 cm. That's why measuring height gives you the most accurate fit in those systems.
Should I size up for my baby?: Often, yes. Babies grow fast, clothes shrink slightly after washing, and bulky nappies add girth. Choose the "Room to grow" fit above to bump the suggestion up one size, especially if your baby is tall, on a growth spurt, or between sizes.
Why do brands fit so differently?: There's no enforced standard for baby clothing sizes, so each brand sets its own measurements. One brand's 6–9M may run small while another's runs large. Always compare the brand's own size chart — usually given in height and weight ranges — to your baby's measurements.
How do I measure my baby's height for sizing?: Lay your baby down on a flat surface, gently straighten the legs, and measure from the top of the head to the bottom of the heel. Do this every few weeks during the first year, since babies grow quickly and can change sizes in a matter of weeks.