The shirt is too loose in the middle, causing it to puff out around the waist (often called “blousing”).
Reduce the midsection width of the shirt.
If you think you need darts
Darts are a great way to resolve the issue of excess fabric around the lower back of the shirt. If you find that the midsection in your shirt size is as slim as you prefer in the front of the shirt, but there is still some excess fabric you’d like to remove from the curve in your lower back, try reducing the midsection width another 1/2″ and applying darts to your shirt size. Read more about darts here.
Better assessing the problem
- Make sure you have tried the shirt on tucked into the pants you plan to wear it with.
- Make sure you have tried the shirt on with/without an undershirt as you plan to wear it.
If you’re looking for a very tailored midsection that will have almost no extra fabric blousing around the waist:
- Be very careful about reducing the midsection width such that: 2*midsection width < waist body measurement+3.5.
- Keep in mind that a very tight midsection width with no blousing around the waist may not look good untucked with casual pants/shorts.
- Remember that everybody’s stomach expands when sitting down, so be sure to take that into consideration.
- Depending on the shape of your waist and hips, the “default” bottom width that is calculated may end up being too tight around the hips, so consider switching it to manual and keeping it where it was before the new midsection width was used or only going a bit smaller.
- Be careful about reducing the midsection width such that it is less than 4.5 inches below the chest width. Unless you are a body builder with a significant drop between your chest and midsection width. For most guys, this extreme shape can cause awkward draping of fabric around the torso. If you’re confident the midsection needs to be smaller, consider reducing the chest width as well.
If the midsection is already pretty tight
Is the blousing of fabric around the lower waist because the shirt is not staying tucked in all the way? The extra fabric at the lower back is more in the shape of horizontal folds. If this is the case, consider:
- Could the shirt be longer? Increasing the shirt length (to enable a deeper tuck) can prevent it from riding up at the back.
- Is the bottom width of the shirt tight around the hips? If the bottom width is fairly tight around the hips/butt, it can cause it to ride up as you move around. Widening the bottom width to not be too tight around the hips can reduce this tendency. Conversely, you don’t want the bottom width to be so loose aroung the hips that this in itself is causing the blousing.