Few brand pairs pull in opposite directions like these two budget giants. Old Navy uses US alpha sizing and runs famously generous — garments frequently measure a size or more over the label, and independent tests have found pants measuring inches larger than marked. Primark sizes in UK numerics and has the opposite reputation: pieces tend to run small, sharpest in denim and anything fitted, with garment-to-garment consistency loose enough that Primark revised its whole chart in 2021 after complaints.
Because one brand uses US letters and the other UK numbers, you can't convert by label at all — you have to go through body measurements. Below, both charts are aligned by bust so you can find your Primark number from your Old Navy letter (and vice versa), plus the fit adjustments each brand earns once the chart has done its part.
The short answer
Primark runs smaller — and Old Navy runs larger than its own chart, so expect a bigger jump than the numbers suggest
Aligned by body measurement, the charts themselves convert cleanly: an Old Navy S body (34–35" bust) is roughly a Primark UK 10, an Old Navy M is a UK 12, an L spans UK 14–16, and an XL spans UK 18–20. The men's charts use the same letters but Old Navy's sit higher — its XL covers the same 47–49" chests as Primark's 2XL.
Then the real-world tendencies stack on top, in opposite directions. Old Navy garments often measure over the label, so the chart understates how roomy they fit; Primark trends smaller than its labels, especially in jeans, fitted dresses, and tailored shirts — even Primark staff are known to recommend sizing up in denim. Practically, that means a shopper moving from Old Navy to Primark should convert by measurement and then still lean a size up in anything fitted.
Old Navy vs Primark at a glance
| Old Navy | Primark | |
|---|---|---|
| Sizing system | US alpha sizes (XS–XXL) | UK numeric sizes (a UK 12 is a US 8) |
| Overall fit | Runs large — garments often measure over the label | Runs small — sharpest in denim and fitted styles |
| Where to adjust | Size down for a fitted look in tops, dresses, jeans | Size up in jeans, fitted dresses, and formal shirts |
| Consistency | Basics true to size; fashion pieces carry the excess | Famously loose — identical items can fit differently |
| Chart history | Stable published chart | Chart revised in 2021 after runs-small complaints |
Size-label mapping (women)
Aligned by body bust measurement, not by label — Old Navy uses US alpha sizing and Primark uses UK numerics, so labels can't be compared directly. US equivalents are from Primark's own conversion column.
| Old Navy | Old Navy bust | Primark UK (US) |
|---|---|---|
| XS | 31–33" · 78.5–84 cm | UK 6–8 (US 2–4) |
| S | 34–35" · 86.5–89 cm | UK 10 (US 6) |
| M | 36–37" · 91.5–94 cm | UK 12 (US 8) |
| L | 38.5–40.5" · 98–103 cm | UK 14–16 (US 10–12) |
| XL | 42.5–45" · 108–114.5 cm | UK 18–20 (US 14–16) |
Women's bust and waist side by side
Rows are matched on bust; Old Navy publishes inches first, Primark centimetres first. Waists don't align as neatly because the brands grade proportions differently — another reason to check both measurements.
| Match | Old Navy bust | Primark bust | Old Navy waist | Primark waist |
|---|---|---|---|---|
| S ↔ UK 10 | 34–35" · 86.5–89 cm | 87 cm · 34.5" | 26–27" · 66–68.5 cm | 70 cm · 27.5" |
| M ↔ UK 12 | 36–37" · 91.5–94 cm | 91 cm · 36" | 28–29" · 71–73.5 cm | 74 cm · 29" |
| L ↔ UK 16 | 38.5–40.5" · 98–103 cm | 102 cm · 40" | 30.5–32.5" · 77.5–82.5 cm | 85 cm · 33.5" |
| XL ↔ UK 18 | 42.5–45" · 108–114.5 cm | 109 cm · 43" | 34.5–37" · 87.5–94 cm | 94 cm · 37" |
Men's tops side by side
Body chest measurements at the same letter (Primark states its men's sizes are the same across UK, EU, and US). The letters look shared, but from M up Old Navy's chart sits one to three inches higher — its XL covers the same chests as Primark's 2XL.
| Size | Old Navy chest | Primark chest |
|---|---|---|
| S | 37–38" · 94–96.5 cm | 36–38" · 91–96 cm |
| M | 39–41" · 99–104 cm | 38–40" · 97–102 cm |
| L | 43–45" · 109–114.5 cm | 41–43" · 103–108 cm |
| XL | 47–49" · 119.5–124.5 cm | 44–46" · 112–117 cm |
Switching between Old Navy and Primark
- Never convert by label between these two — go from your body measurements to Primark's UK number (an Old Navy M body is roughly a UK 12), then lean a size up in anything fitted.
- In denim the brands are mirror images: take the smaller size at Old Navy (stretch relaxes with wear), and size up at Primark — even its staff recommend it.
- Moving from Primark to Old Navy, your converted letter will likely fit roomy; drop one size in fitted tops, dresses, and jeans, and keep your size only in basics.
- Primark's garment-to-garment consistency is loose, so when a piece matters, check the measurements on the label rather than trusting the size — the chart is a starting point, not a guarantee.
Measurements are based on each brand's published size charts and may vary by garment, fabric, and region. For the full charts and fit notes, see the Old Navy size guide and the Primark size guide.
Old Navy vs Primark sizing FAQs
Which runs smaller, Old Navy or Primark?
Primark, decisively. It trends smaller than its labels — worst in jeans, fitted dresses, and tailored shirts — while Old Navy is one of the most generously sized mainstream brands, with garments that often measure over the label. Between the two, expect the same body to need a visibly "bigger" size at Primark.
What Primark size am I if I wear a medium at Old Navy?
By the charts, a UK 12: Old Navy's M fits a 36–37" bust and Primark's UK 12 fits 91 cm (36"). But since Old Navy runs big and Primark runs small in fitted styles, try the UK 12 first in basics and consider a UK 14 for jeans or a fitted dress.
Are Old Navy and Primark sizes labelled the same way?
No. Old Navy sells women's clothing in US alpha sizes (XS–XXL), while Primark labels in UK numerics — and a UK size is four numbers higher than its US equivalent, so a Primark UK 12 is a US 8. Men's tops use letters at both, but Old Navy's letters cover larger chests from M up.
Do men's sizes match between Old Navy and Primark?
Only at S. Old Navy's chart sits higher from there: its M covers a 39–41" chest against Primark's 38–40", its L covers 43–45" against Primark's 41–43", and its XL (47–49") matches Primark's 2XL band. Most men wear one letter higher at Primark than at Old Navy.
Why does Primark fit so inconsistently compared to Old Navy?
Primark's garment-to-garment consistency is famously loose — manufacturing tolerances vary across factories and batches, so two copies of the same piece can measure differently, and the brand revised its chart in 2021 after runs-small complaints. Old Navy's quirk is different: it's consistent, just consistently generous.
