I Saw It First Sizing: Does It Run Small, and Should You Size Up?

By Ella Blake – Fashion Stylist  |  Tellar Fashion Hub – Always honest, unbiased & unsponsored

Yes — I Saw It First does run small, and in some styles it runs noticeably small. If you're hovering between sizes, the advice from most shoppers (and from me, after ordering a good few pieces myself) is to size up at least one, sometimes two. That said, it really depends on the garment type, and there are a few things worth knowing before you click checkout.

I Saw It First is one of those brands that feels exciting online — the trend turnaround is fast, the price point is genuinely accessible, and the styling is usually bang on whatever's having a moment. But the sizing? It can be a lottery. I've had a pair of co-ord trousers arrive looking like they were cut for someone four inches shorter and two sizes smaller than me. And I've also had a midi dress fit perfectly straight out of the bag. The trick is knowing what to expect from each category.

How Does I Saw It First Size Their Clothes?

I Saw It First is a Manchester-based online fast fashion brand, launched in 2016 and targeting a primarily young, trend-led audience. Like many brands in this space — think Boohoo, PLT, Nasty Gal — their sizing is based on UK standard measurements, but the actual cut can vary considerably between ranges and even between pieces in the same collection.

They offer sizes from a UK 6 up to a UK 24 across most ranges, which is better than a lot of comparable brands. However, the measurements used for their size chart lean towards the smaller end of the standard UK sizing scale — meaning a size 12 from I Saw It First may measure closer to what you'd expect from a size 10 at M&S or Next.

Quick answer: Order one size up from your usual UK size as a starting point. For bodycon or fitted styles, consider going up two sizes. For floaty, oversized or relaxed-fit pieces, your normal size may work fine.

I Saw It First Size Guide: Approximate Measurements

Here's a rough breakdown of what I Saw It First's size chart typically looks like. These measurements are approximate — always check the specific product page before ordering as they can vary:

UK SizeBust (inches)Waist (inches)Hips (inches)630.523.532.583225341034273612362938143831401640334218423544204437.547

If your measurements sit between two sizes on this chart — which is common — I'd always go with the larger. Stretch fabrics are more forgiving, but structured pieces like blazers, co-ords and jeans tend to be quite unforgiving if they're too tight.

Sizing by Category: What to Expect

DRESSES & TOPS

• Bodycon dresses almost universally run small — size up, and if you carry weight on your hips, size up twice

• Flowy midi and maxi dresses are generally more true-to-size, though shorter in length than you might expect

• Crop tops and co-ord tops are cut tight — this appears to be intentional for the aesthetic, so bear that in mind

• Smocked and elasticated tops are more forgiving across sizes

JEANS & TROUSERS

• Denim is usually cut quite narrow in the waist — I personally had a pair of straight leg jeans that were two full inches tighter in the waist than the size chart suggested

• Wide leg and palazzo styles are more generous and often true-to-size

• Faux leather trousers run small — definitely go up a size

• Inseam lengths tend to suit taller frames; if you're 5'4" or under, factor in a potential hem

KNITWEAR & LOUNGEWEAR

• Knits and loungewear are some of the more reliable categories — these tend to be fairly true-to-size

• Oversized styles are genuinely oversized and great if you want a relaxed fit

• Co-ord sets can vary wildly — the top and the bottoms sometimes feel like different sizes, so treat them as separates

OCCASIONWEAR

• Satin and structured evening dresses run small — size up without hesitation

• If the garment has a zip (especially a back zip), go up one size at minimum; there is little to no ease built in

• Ruched styles are more forgiving than they look and can stretch across a range of sizes

The Returns Situation (Worth Knowing)

One thing I'd flag: I Saw It First's returns policy has changed over the years and it's worth double-checking before you order. At the time of writing they charge for returns, which means getting the size wrong costs you. This makes it even more important to check the measurements carefully and — where in doubt — size up.

The reviews on individual product pages are genuinely useful here. Look specifically for comments on sizing rather than just general ratings. If six people have mentioned something comes up small, trust them.

How Does I Saw It First Compare to Other Brands?

Here's where it gets helpful to have a frame of reference. I Saw It First generally sizes similarly to Boohoo and PrettyLittleThing — i.e., smaller than high street. If you're a UK 12 at the following brands, here's roughly what you'd need at ISAWIT:

• If you're a 12 at Zara, order a 12–14 at I Saw It First

• If you're a 12 at ASOS, order a 12 — ASOS tends to run similarly

• If you're a 12 at H&M, order a 12–14

• If you're a 12 at Next, order a 14 at I Saw It First

• If you're a 12 at River Island, order a 12–14 depending on the style

• If you're a 12 at New Look, you're probably fine with a 12, as they size similarly

• If you're a 12 at Topshop, order a 12–14

Better Alternatives If the Sizing Frustrates You

If you love the trend-forward aesthetic of I Saw It First but keep running into sizing grief, these brands offer a similar vibe with more consistent sizing — and in some cases, better fabric quality too:

HIGH STREET ALTERNATIVES

• ASOS – massive range, sizes well-labelled, detailed size charts per product, plus a Curve range that's genuinely inclusive

• River Island – trend-driven with a younger feel, generally more consistent sizing than ISAWIT and excellent for going-out styles

• New Look – great value, similar price point, and sizing that's reliably true-to-size across most of their range

• Topshop (via ASOS) – the Topshop brand still exists on ASOS with a strong trend-led offer; sizing is consistent and well-documented

• Zara – a step up in quality at a similar or slightly higher price point; sizing runs small so follow the same advice, but the cut is usually more polished

• Urban Outfitters – excellent for going-out pieces and occasion dressing with a more editorial edge; sizing is generally reliable and the fit is often more generous

• Mango – great for occasion dresses, co-ords and summer styles; tends to run slightly small in the waist but the size chart is clear and detailed

• Warehouse – underrated for trend-forward pieces with noticeably better fabric and construction than fast fashion alternatives; sizing is true-to-scale UK sizes

TWO INDEPENDENT PICKS WORTH KNOWING

• Oh Polly – a UK independent brand that has exploded for going-out and occasion dressing; they're transparent about their fit, run small in bodycon styles, but their size guidance per product is excellent and customer photos are genuinely helpful

• Rebellious Fashion – a Midlands-based independent with a wide size range (UK 6–24), competitive prices and a product review section that's particularly reliable for sizing intel. Great for curve-friendly trend pieces

Stop Guessing Your Size — Use Tellar

The whole reason I Saw It First's sizing frustrates people is the same reason sizing frustrates everyone: every brand does it differently, and the numbers don't mean what we think they mean. That's exactly the problem Tellar was built to solve.

Tellar is the UK's leading free sizing tool — enter your measurements once, and instantly get your precise size across 1,500+ brands. No more ordering two sizes and returning one. No more second-guessing. Just your exact size, every time.

• Measure once — bust, waist, hips, or use an existing brand size you trust

• Use the Store Size Lookup tool — matched instantly to I Saw It First, ASOS, Zara, River Island and 1,500+ more

• Always free — works in-browser, no app, no download, no subscription

• The Tellar Fashion Hub — hundreds of honest, unsponsored style guides written by our team of stylists. No ads, no affiliate bias, just straight advice

Visit Tellar.co.ukFind My Size in I Saw It First →

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Tellar Fashion Hub posts are always honest, independent and unsponsored. We are not paid by any brand featured in this article. Some links may be affiliate links — this never influences what we recommend or how we review brands.