True-to-Size Millimeter Sizing Comparison
Spring Step Estella vs Spring Step Happy
Detailed size conversion chart, cavity measurements in millimeters, and crowd-sourced fit advice for choosing your perfect size.
Interactive Size Converter
Select what size you wear in the Estella to see your calculated size in the Happy.
Complete Size Conversion Matrix
Direct millimeter-calibrated mapping comparing all available sizes between both shoes.
| Estella (US) | Internal Cavity | Happy Equivalent | Match Accuracy | Fit Difference |
|---|---|---|---|---|
| US 3.5 | 238 mm | US 3.5 | 93% | +0.6 mm |
| US 4 | 241 mm | US 4 | 93% | +0.6 mm |
| US 4.5 | 245 mm | US 4.5 | 93% | +0.6 mm |
| US 5 | 248 mm | US 5 | 93% | +0.6 mm |
| US 5.5 | 251 mm | US 5.5 | 93% | +0.6 mm |
| US 6 | 255 mm | US 6 | 93% | +0.6 mm |
| US 6.5 | 258 mm | US 6.5 | 93% | +0.6 mm |
| US 7 | 265 mm | US 7 | 93% | +0.6 mm |
| US 7.5 | 268 mm | US 7.5 | 93% | +0.6 mm |
| US 8 | 271 mm | US 8 | 93% | +0.6 mm |
| US 8.5 | 275 mm | US 8.5 | 93% | +0.6 mm |
| US 9 | 278 mm | US 9 | 93% | +0.6 mm |
| US 9.5 | 281 mm | US 9.5 | 93% | +0.6 mm |
| US 10 | 285 mm | US 10 | 93% | +0.6 mm |
| US 10.5 | 288 mm | US 10.5 | 93% | +0.6 mm |
| US 11 | 291 mm | US 11 | 93% | +0.6 mm |
| US 11.5 | 295 mm | US 11.5 | 93% | +0.6 mm |
| US 12 | 298 mm | US 12 | 93% | +0.6 mm |
| US 12.5 | 301 mm | US 12.5 | 93% | +0.6 mm |
| US 13 | 305 mm | US 13 | 93% | +0.6 mm |
| US 14 | 311 mm | US 14 | 93% | +0.6 mm |
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Frequently Asked Questions
How does the Estella size compare to the Happy?
Both shoes share a very similar length profile. We recommend ordering your standard size in both. The Estella has an internal fit offset of -1.6mm compared to -1mm for the Happy.
Do these shoes fit wide feet comfortably?
The Estella has a snug/narrow width profile, while the Happy is snug/narrow.