Match sizes by foot length (cm), then check model fit — US labels alone can mislead across brands.
Chart match · verify brand charts(paid link)
Full converter (model + shoes you own) · Reverse: Keen → Altra
Static conversion table (crawlable). Interactive tool above uses the same cm matching.
| Altra US | cm | Keen US | Keen EU |
|---|---|---|---|
| 6.0 | 23.6 | 6.0 | 38.5 |
| 6.5 | 24.1 | 6.5 | 39.0 |
| 7.0 | 24.6 | 7.0 | 40.0 |
| 7.5 | 25.1 | 8.0 | 41.0 |
| 8.0 | 25.4 | 8.0 | 41.0 |
| 8.5 | 25.9 | 8.5 | 42.0 |
| 9.0 | 26.2 | 9.0 | 42.5 |
| 9.5 | 26.8 | 9.5 | 43.0 |
| 10.0 | 27.2 | 10.0 | 44.0 |
| 10.5 | 27.6 | 11.0 | 45.0 |
| 11.0 | 27.9 | 11.0 | 45.0 |
| 11.5 | 28.4 | 11.5 | 45.5 |
| 12.0 | 28.9 | 12.0 | 46.0 |
| 13.0 | 29.8 | 13.0 | 47.5 |
Altra US 10 is 27.2 cm here; Keen’s nearest US is 10 (EU 44). At US 10.5 the table jumps: Altra 10.5 is 27.6 cm and the nearest Keen becomes US 11 (EU 45). The converter’s “Altra 10 ≈ Keen 10” is true at that row; a half-size up on the Altra side does not stay a half-size on Keen.
Lone Peak 8 is a zero-drop, foot-shaped trail last — wide at the toes by design. Newport H2 is also wide, but it is a bumper sandal last with a different instep and drop. Matching 27.2 cm gets length close; it does not mean Lone Peak’s toe splay equals Newport’s bumper room. People who size Altra TTS because of the foot-shaped toebox should not assume Keen needs the same number plus a trail offset.
If you sit between Altra 10 and 10.5, this chart says Keen may land at 10 or 11, not 10.5. That is a centimeter stair-step, not a fit verdict. Start at the Keen US next to your Altra cm, then use each model guide for drop and instep rather than chasing another half.
Even when charts match, lasts differ. Check these guides:
Altra: Lone Peak 8
Keen: Newport H2
Also compare Altra to: Salomon
Check prices: Altra Keen (paid link)
Clothes sizing uses a separate compare path.