adidas Ultraboost 4.0 fits true to size for most people — your usual Nike sneaker size, which is about half a size larger in number than your Air Force 1. The Primeknit upper hugs the foot and the Boost midsole compresses slightly with wear. Based on 577 owner-reported pairs in the Feetlot database, the typical wearer takes the same number in Ultraboost 4.0 as in Air Max 1, Air Max 90, or YEEZY 350 V2. If unsure: order true to size. Wide feet should size up half — the Primeknit gives, but the heel cage and Torsion bar don't.
Ultraboost 4.0 Sizing — What 577 Pairs in the Feetlot Database Tell Us
The adidas Ultraboost 4.0 is the most-tracked Ultraboost generation in the Feetlot database (with 1.0, 2.0, and 3.0 close behind). Across 577 owner-reported pairs, the residual variance is tight (standard deviation ≈ 0.23 size units), meaning sizing is consistent across the various 4.0 colorways and the Ultraboost 4.0 DNA reissues. The "Ultraboost runs about half a size small" pattern lines up with what Feetlot data actually shows: the typical wearer sits at their true Nike sneaker size — about half a size higher in number than where they wear AF1.
The reason Ultraboost 4.0 runs at true Nike size — rather than half down like AF1 — is the Primeknit-and-cage construction. The Primeknit upper hugs the foot tighter than a traditional sneaker upper, and the molded heel counter plus Torsion bar in the midsole hold the foot at length. The Boost foam compresses by 1–2 mm at the heel after the first 10 hours of wear but doesn't change length. Going below true Nike size pinches the toes against the heel counter.
Should You Size Up or Down in Ultraboost 4.0?
Standard fit (most people)
Order true to size — your usual Nike sneaker size. The Primeknit upper accommodates moderate foot widths through its stretch, and the heel cage anchors the back of the foot. Don't bring your "half down from AF1" adjustment to Ultraboost — the knit-and-cage construction doesn't leave AF1's heel slack.
Wide feet
Size up half. The Primeknit upper gives at the forefoot, but the rigid heel cage and Torsion bar limit how much width the knit can absorb before the rest of the upper presses in. Width isn't offered separately on most Ultraboost colorways. Half a size up is the standard wide-foot adjustment in the Feetlot database.
Narrow feet
True to size works for most narrow feet because the Primeknit hugs the foot regardless of width. Going half down can occasionally fit very narrow feet but tends to make the heel feel like it's slipping forward in the cage.
Ultraboost 4.0 vs Ultraboost DNA vs Ultraboost 21
The standard Ultraboost 4.0 (2018 release) and the various Ultraboost 4.0 DNA reissues use the same last and the same length advice — true to size. The Ultraboost 21 and Ultraboost 22 use a redesigned upper with a slightly more accommodating midfoot cage; the length you'd buy is the same as the 4.0, but the 21/22 feels marginally less restrictive at the throat. Sizing advice on this page is calibrated to the 4.0 — same advice applies to 1.0 through 4.0 within a quarter size.
How Ultraboost 4.0 Compares to Other Sneakers
The Ultraboost 4.0 fits at the same numerical size as adidas NMD R1, adidas YEEZY Boost 350 V2, Air Jordan 1, Air Jordan 3, Air Jordan 4, Air Jordan 5, Air Jordan 11, Nike Air Max 1, Air Max 90, Air Max 95, Air Max 97, Air Max 270, Nike Dunk Low, Nike Dunk High, SB Dunk Low, Nike SB Janoski, Nike Blazer Mid '77, Vans Authentic, Vans Old Skool, and New Balance 574. According to Feetlot data, all of these round to the same size in 0.5 increments — same number you wear in Ultraboost.
The shoes that run larger in number than Ultraboost 4.0: Nike Air Force 1, Converse Chuck Taylor (Low and Hi), adidas Stan Smith, adidas Superstar, adidas Gazelle, adidas Samba OG, and Sperry Authentic Original all run about half a size larger than Ultraboost — so if you wear Ultraboost in 10, you'd take 9.5 in those. Boot-style models (Red Wing Iron Ranger, Wolverine 1000 Mile) run a full size smaller than Ultraboost in number — size down a full size from Ultraboost there.
Sign in to Feetlot and add a few of your other sneakers to get a personal Ultraboost size recommendation calibrated to your actual foot.
adidas Ultraboost 4.0 Size Chart (US / EU / UK)
| US Men's | US Women's | UK | EU |
|---|---|---|---|
| 7 | 8 | 6.5 | 40 |
| 7.5 | 8.5 | 7 | 40.5 |
| 8 | 9 | 7.5 | 41.5 |
| 8.5 | 9.5 | 8 | 42 |
| 9 | 10 | 8.5 | 43 |
| 9.5 | 10.5 | 9 | 43.5 |
| 10 | 11 | 9.5 | 44 |
| 10.5 | 11.5 | 10 | 44.5 |
| 11 | 12 | 10.5 | 45.5 |
| 11.5 | 12.5 | 11 | 46 |
| 12 | 13 | 11.5 | 47 |
| 13 | 14 | 12.5 | 48 |
Common Sizing Mistakes
- Sizing up half because "Ultraboost runs small". They run at true Nike size, which feels small only if you're benchmarking against AF1 (which itself runs roomy). True to your Nike sneaker size is correct — don't size up an extra half thinking the Primeknit needs more room.
- Buying Ultraboost in your AF1 size. AF1 runs about half a size larger in number than Ultraboost. If your AF1 fits half down from true Nike, your Ultraboost should be true to your Nike size — not also half down.
- Expecting the heel cage to break in. The plastic cage at the heel doesn't flex meaningfully. If true-to-size feels narrow at the heel, the issue is the cage geometry, not the size. The Ultraboost 21/22 redesigned cage may help.
- Ignoring the Boost compression. Boost midsoles compress by 1–2 mm at the heel after the first 10 hours of wear, which can make the shoe feel slightly looser. Length doesn't actually change.
- Carrying YEEZY 350 sizing directly. YEEZY 350 V2 and Ultraboost fit nearly the same in raw numbers, but YEEZY's stretchy Primeknit accommodates wider feet better than Ultraboost's heel cage. If you size up half in YEEZY for width, you may still need that half up in Ultraboost — but not more.
How Feetlot Computes These Numbers
Every Ultraboost sizing recommendation on Feetlot is the output of a global offset model fit to over 100,000 owner-reported shoe records. Each shoe gets a single number — its "size offset" — that captures how its sizing drifts relative to a reference shoe (the Nike Air Force 1). When a Feetlot user provides their size in any tracked shoe, the model recovers their true foot baseline and recommends the matching Ultraboost size.
This works better than the pairwise approach you'll see on most sizing blogs because Feetlot uses the entire wardrobe graph. A YEEZY 350 owner contributes data about how YEEZY fits relative to AF1 owners (who often own both), which links back to Ultraboost owners through any shared model. Even when two users share zero shoes directly, the chain of users in between transmits a consistent recommendation. The result: sizing advice that holds up no matter how unusual a wardrobe is.