When parents say a brand "runs big" or "runs small," they almost always mean the same thing: their child fit one brand's size 5 perfectly, then the next brand's size 5 was completely different. That experience is real. But the explanation most parents land on — that one brand cuts its clothes differently — is usually wrong.
We pulled the published size chart measurements for 30+ major kids' clothing brands, simulated 1,000 children at realistic height distributions, and compared what each brand's garments actually measure at each height. The result: for the vast majority of brands, the size chart measurements are within 1–2cm of the cross-brand average. That's less than the stretch in an elastic waistband.
There are a small number of genuine exceptions — brands where the data really does show a consistent difference. We'll cover those. But first, the more useful explanation: the thing that actually makes sizing feel inconsistent across brands. For the full brand-by-brand directory and side-by-side comparison at every height, see our complete kids clothing size chart covering 36+ brands.
The Bracket Problem: Why Sizing Feels Inconsistent
Every brand divides the height range for a given size into a bracket. Cat & Jack's size 5 might cover 43–47 inches. Carter's size 5 might cover 44–48 inches. The brackets are different widths and they start at different points.
Now imagine a child who is exactly 47 inches tall. In Cat & Jack, they're at the very top of the size 5 bracket — the garment is cut for someone at the ceiling of that range, which means it fits snugly. In Carter's, the same child is near the middle of the size 5 bracket — more room. In a third brand, they might fall at the very bottom of a size 6 bracket and be pushed into a larger size entirely.
The parent's experience: the same child needs a different size in three different shops. The conclusion they draw: "Carter's runs big" or "Brand X runs small." The actual explanation: the brackets don't align, and the child landed at a different position within each one.
The fix is simple in theory: match your child's exact height to each brand's bracket, not the size label. That's what the size finder below does — and why it gives different sizes for different brands even for the same child.
Find Your Child's Size Across All Brands at Once
Enter your child's height and we'll show the correct size for Cat & Jack, Carter's, Gap, Old Navy, H&M and 25+ other brands — side by side, so you can see exactly why the sizes differ.
Find your child's size
Select a brand, enter your child's measurements, and we'll show the right size — plus how long it will last.
Size recommendations are based on official brand size charts and are provided as a guide only. Fit may vary by item, style, and individual child. Outgrow is not responsible for sizing decisions made based on these recommendations.
What the Size Chart Data Actually Shows
To answer this question with data rather than anecdote, we built a simulation. We generated 1,000 children with heights spanning each age range, matched each child to the correct size bracket in each brand, and interpolated the actual garment measurements proportionally within each bracket. Then we compared: how does each brand's garment measure at any given height, relative to the cross-brand average at that same height?
The result is a coverage-neutral comparison — brands with baby sizes are compared against other brands with baby sizes, and brands that only cover older kids are compared against each other. No brand gets penalised or rewarded for its size range.
Brand-by-Brand Verdict
| Brand | Verdict | What the data says |
|---|---|---|
| Cat & Jack | True to size | Measurements average across all age bands. Chest slightly generous in older girls, but within normal range. |
| Carter's | True to size | Waist is 1–2cm generous in baby and toddler range, average at older sizes. Does not run small. |
| Gap Kids | True to size | Chest, waist and hip all average across the full height range. One of the most consistent brands in the dataset. |
| Old Navy | Hip runs slim | Chest and waist are average, but hip in bottoms runs consistently slim across all age groups. Real finding, high confidence. |
| The Children's Place | True to size | Average across all measurements and age groups. No meaningful deviation in either direction. |
| H&M | True to size | Fully average across chest, waist and hip at all age ranges. H&M uses age-based sizes (not US numbers) — that's the only real complication. |
| Zara Kids | True to size | Average across measurements. The perception that Zara runs small is likely a European-vs-US size label difference, not cutting. |
| Next Kids | True to size | Average across all measurements. Girls' waist is slightly slim in older sizes but within normal range. |
| OshKosh B'Gosh | True to size | Waist slightly generous in baby and toddler range — similar to Carter's. Average at older sizes. |
| Nike Kids | True to size | Broadly average. Girls' chest runs slightly generous in kids and older kids sizes — minor. |
| Adidas Kids | True to size | Average measurements. Use height to match bracket, not age label — the European age system can cause confusion. |
| Marks & Spencer | Boys waist runs generous | M&S boys waist deviates +5.54cm above the cross-brand mean — the largest generous finding in our entire database. Girls runs average through kids range and slightly slim in the older sizes. |
| Primark | True to size | Fully average. Primark uses age-based sizing — always match on height. |
| Uniqlo Kids | Waist runs slim | One of the few genuine exceptions. Waist runs consistently slim in both boys and girls from toddler through older kids. Chest also slightly slim. |
| Decathlon Kids | Runs slim | The clearest outlier in the dataset. Slim in chest, waist and hip across all four age bands. If your child is lean, size up. |
| Reima | Hip runs generous | Hip in bottoms is genuinely generous from baby through kids range — by design, to allow layering under ski pants and outerwear. |
| Mayoral | Hip runs generous | Consistent with Reima. Hip in bottoms generous across baby through kids. Mediterranean brand sizing tends to allow more seat room. |
Verdicts based on Monte Carlo simulation of 1,000 children per age band using published size chart measurements. "True to size" means within 1σ (~1.5cm) of cross-brand mean at equivalent heights.
The Brands That Actually Do Cut Differently
For most brands, the data lands on "true to size" — but four findings are robust enough to act on.
Marks & Spencer boys runs the most generous waist of any UK brand
The single most pronounced finding in our entire database. M&S boys waist deviates +5.54cm above the cross-brand mean — consistent across toddler, kids and older kids. At 112cm, M&S boys standard waist is 61–62cm versus 54–55cm at H&M, Next, Zara and Tu. M&S girls is a different product entirely — runs average through the kids range and trends slim in the older sizes. Full M&S kids size chart and methodology →
Decathlon runs slim — across the board
This is the strongest finding in the dataset. Decathlon cuts slim in chest, waist and hip consistently from baby through older kids. In practical terms: if your child is on the lean side, a Decathlon size will feel snug. If they have an average or broader build, it will fit as expected. The difference is around 3cm in waist — real enough to matter for a slim child, but unlikely to affect an average build.
Uniqlo runs slim in the waist
Uniqlo's waist runs consistently slim from toddler through older kids — around 4–6cm narrower than average, which is the largest waist deviation in the dataset. The effect is most pronounced in girls. This is Japanese baseline sizing, applied consistently. If your child needs Uniqlo, sizing up one in bottoms is usually right.
Old Navy hip runs slim in bottoms
Chest and waist are average, but the hip measurement in Old Navy bottoms is consistently narrower than other brands across all age groups. For children with broader hips this can cause fit issues even when the waist is fine. The difference is around 2–3cm.
How We Did This Research
Previous attempts to compare brand sizing — including our own early work — fell into a common trap: picking fixed reference heights and reading the measurement for that height from each brand's chart. The problem is that a reference height of, say, 116cm lands at different positions within different brands' size brackets. A brand whose 116cm reference sits at the bottom of its bracket looks generous compared to one where it sits at the top.
Our simulation fixes this. For each of 1,000 simulated children at randomised heights within an age band, we find the matching bracket in every brand and interpolate the garment measurement proportionally within that bracket. Then we compute each brand's deviation from the cross-brand mean at that specific height — not the global mean. A brand is only ever compared against other brands at the same height, so coverage range can't distort the result.
The threshold for flagging a brand as genuinely generous or slim is 1 standard deviation from the cross-brand mean. For context, 1 standard deviation in kids' waist at the 108–140cm range is approximately 1.6cm. Findings marked as confirmed appeared consistently across at least two age bands with sufficient coverage.
Frequently Asked Questions
Does Cat and Jack run big or small?
Neither. Cat & Jack size chart measurements are within normal range of comparable brands. If it felt big on your child, they likely fell at the bottom of the bracket — and any brand's same size would have felt similar at that height. Use the size finder above with your child's actual height for an accurate recommendation.
Does Carter's run small?
No — if anything, Carter's is slightly generous in the waist in baby and toddler sizes, which is the opposite of running small. The perception that Carter's runs small often comes from comparing it to brands with different bracket boundaries.
Does Gap Kids run big or small?
True to size. Gap Kids is one of the most consistent brands in our dataset — chest, waist and hip all sit close to the cross-brand average. If a Gap size felt different to another brand's equivalent, bracket position is almost certainly the explanation.
Does Old Navy run big or small?
Mostly true to size, with one real exception: the hip measurement in Old Navy bottoms runs slim across all age groups. If your child has broader hips they may find Old Navy bottoms snug even when the waist fits. Tops and waist measurements are average.
Does The Children's Place run small?
No. The Children's Place measurements are average across all age bands. This is a consistently true-to-size brand by the data.
Does Next Kids run big or small?
True to size overall. Next uses age-based sizing (Age 4–5, Age 5–6 etc.) rather than US number sizes, which can cause confusion when comparing labels. The garment measurements themselves are average.
Does Decathlon kids clothing run small?
Yes — this is one of the few brands where the data supports a genuine 'runs slim' verdict. Decathlon measures slim in chest, waist and hip across all age ranges. If your child is lean, Decathlon will fit as expected. If they have an average or broader build, sizing up one is worth considering for the best fit.
Why does my child need a different size in every brand?
Almost certainly because of bracket misalignment, not because the clothes are cut differently. Each brand sets its own bracket boundaries — where a size starts and ends in height. A child at 116cm might be at the top of one brand's size 5 bracket and at the bottom of another's size 6. The garment measurements are similar, but the label is different. The size finder above accounts for this by matching your child's exact height to each brand's bracket individually.