A denim jacket is the easiest layer to own and the easiest to get slightly wrong. The wash is rarely the problem. The cut is: how long the jacket is, and where its hem lands against the waistband of whatever you wear underneath. Get that one relationship right and the rest of the outfit mostly sorts itself out.
Denim jackets come in three broad lengths. The numbers below are rough, measured from the back of the collar to the hem, and they vary from brand to brand, so check the size chart of any jacket you are considering.
Look at a jacket hem and a waistband as two horizontal lines. When the lines coincide, or when the hem sits clearly past the waistband, the outfit looks planned. When the hem lands a few centimetres below the waistband, the two lines compete. That small gap is the usual reason a jacket "feels off" even though the colours and fit are fine.
That gives a simple rule of thumb: put the hem on the waistband, or at least a hand's width past it. Use the picture below to see it. Choose a cut, change the rise of the jeans, then drag the red handle to move the hem and watch the verdict change. The measurements are approximate and the figure is a generic outline, not anyone's proportions.
Length gets the attention, but the shoulder seam matters as much. On a classic or cropped cut the seam sits at the edge of your shoulder. On an oversized cut it sits lower on the upper arm, on purpose. If a jacket that is meant to be fitted has a seam that droops, you have bought the wrong size, not a different style.
The sleeve should end somewhere between the wrist bone and the base of the thumb. Roll the cuff once if it is long. A roll of one or two turns also shows a little of the lining or the inner colour, which gives an oversized jacket some structure.
Each layer adds a little bulk, and bulk changes where the hem sits and how the jacket hangs. A cropped jacket wants a slim layer under it, such as a fine tee or a thin knit, because anything thick makes it ride up. An oversized jacket can take a heavier knit or a shirt without complaint. A classic cut sits in the middle and copes with both.
If you wear a longer top, make sure it ends above the hem or well below it. A top that peeks out a few centimetres below the jacket creates the same "in between" problem as a jacket hem that is close to the waistband.
Stiff, heavy denim holds its shape, so a cropped cut stays boxy and a classic cut keeps a clear outline. Soft denim, or a blend with a little lyocell or hemp, drapes more and moves with you. That suits an oversized cut, where a stiff fabric can feel like wearing a board. A lighter wash looks larger on the eye and a darker one looks tidier, so if you are unsure, a mid-blue is the most forgiving starting point.
You may already own the right jacket. Try it against the rule above: stand in front of a mirror, find where the hem lands against your usual jeans, and check if a different pair of jeans or a different rise brings the two lines together. A change of jeans can fix more than a new jacket. If you do want another one, second-hand denim jackets are easy to find in every size, and new ones across the high street tend to cost around €40–140.