The normal formula to convert a European size to a US size is to subtract 10 from the European size to get the approx US chest size, and that works pretty well for this style.
Use the chart I put together below to choose a size that will be big enough for both your chest and belly. The jackets are cut more casually and so the taper from chest to waist isn’t nearly as aggressive as a sport/race jacket, so the belly size won’t be an issue for most, but I put my estimate of max belly in the chart so you can make sure the size you choose based on your chest measurement will be big enough for your belly as well.
| Jacket Size | Max Chest | Max Belly/Waist |
|---|---|---|
| 46 | 36 | 34 |
| 48 | 38 | 36 |
| 50 | 40 | 38 |
| 52 | 42 | 40 |
| 54 | 44 | 42 |
| 56 | 46 | 44 |
| 58 | 48 | 46 |