So you’ve got an event to attend and need to look both casual and formal at the same time. Wearing a suit jacket with jeans is one of the best ways to mix a casual and formal look, but it’s something that many people — style gurus in particular — frown upon. Why?
One reason is that suit jackets are usually designed to be worn exclusively with matching trousers. They’re cut to a specific length that makes them look best with matching trousers, and usually have a slightly longer length than a typical sports coat or blazer would. This can make a suit look too long to “fit” well with a pair of jeans.
If you’re going to wear a suit with jeans, choose a shorter jacket with more of an informal cut. A standard American or British style suit jacket with a cut down the center or two side vents is more suited to a pair of jean than an Italian suit with wider shoulders and no vents in the back.
Another thing to keep in mind is color. Very dark suits, especially black suits, look too formal to wear with a pair of jeans. Nothing looks like more of a mismatch than a pair of light blue jeans (extremely casual) worn with a black suit (something you’d normally see on a lawyer or banker). The whole look is too different to work well.
Instead, it’s better to wear a suit jacket that’s fairly informal — a blue or gray suit jacket usually the best. Gray and blue both go well with a pair of blue jeans as well — gray is obviously a soft contrast, while blue matches the color of the jeans entirely.
This article from GQ expands more on whether or not it’s okay to wear a suit jacket with jeans. For us, it’s something that you can get away with for some events and situations, but the color and cut of the jacket need to be on the casual side for it to look natural.