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 | 232 mm | US 3.5 | 53% | +4.0 mm |
| US 4 | 235 mm | US 3.5 | 88% | +1.0 mm |
| US 4.5 | 239 mm | US 4 | 100% | 0.0 mm |
| US 5 | 242 mm | US 4.5 | 88% | +1.0 mm |
| US 5.5 | 245 mm | US 5 | 88% | +1.0 mm |
| US 6 | 249 mm | US 5.5 | 100% | 0.0 mm |
| US 6.5 | 252 mm | US 6 | 88% | +1.0 mm |
| US 7 | 259 mm | US 6.5 | 65% | -3.0 mm |
| US 7.5 | 262 mm | US 7 | 88% | +1.0 mm |
| US 8 | 265 mm | US 7.5 | 88% | +1.0 mm |
| US 8.5 | 269 mm | US 8 | 100% | 0.0 mm |
| US 9 | 272 mm | US 8.5 | 88% | +1.0 mm |
| US 9.5 | 275 mm | US 9 | 88% | +1.0 mm |
| US 10 | 279 mm | US 9.5 | 100% | 0.0 mm |
| US 10.5 | 282 mm | US 10 | 88% | +1.0 mm |
| US 11 | 285 mm | US 10.5 | 88% | +1.0 mm |
| US 11.5 | 289 mm | US 11 | 100% | 0.0 mm |
| US 12 | 292 mm | US 11.5 | 88% | +1.0 mm |
| US 12.5 | 295 mm | US 12 | 88% | +1.0 mm |
| US 13 | 299 mm | US 12.5 | 100% | 0.0 mm |
| US 14 | 305 mm | US 13 | 76% | -2.0 mm |
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Frequently Asked Questions
How does the Estella size compare to the Happy?
The Happy provides about 4mm more internal length than the Estella. We suggest ordering 1/2 size down in Happy. The Estella has an internal fit offset of -8mm compared to -4mm for the Happy.
Do these shoes fit wide feet comfortably?
The Estella has a standard medium width profile, while the Happy is standard medium.