Jeans that fit in the morning and bag at the knee and seat by lunchtime are not badly made. They are behaving exactly as the fabric is designed to.

Understanding why makes the problem manageable.

Why it happens

Two different mechanisms, and they need different answers.

Cotton relaxes. Pure cotton denim has no recovery — no ability to return to shape. It moulds to you as you wear it, which is why raw denim enthusiasts prize it, and why a rigid pair grows roughly half a size across a week.

Elastane fatigues. Denim with 1 to 3% elastane recovers well when new. Heat degrades elastane, so every hot wash and every tumble dry permanently reduces how much it springs back. A pair that used to recover overnight and now does not has usually been washed too hot too often.

The knee and seat go first because those are the points that flex most.

What to do about it

Wash cold, always. 30 degrees or below. This is the single highest-impact change, and it matters most for anything with elastane. Heat is what kills recovery.

Never tumble dry. The dryer is worse than the wash, because it combines heat with agitation. Hang them.

Wash less often. Denim genuinely does not need washing every wear. Every three to ten wears is normal, depending on how you live. Airing between wears does most of the work.

Wash inside out. Protects the surface colour and reduces abrasion on the face of the fabric.

Hang by the waistband or fold. Hanging by the hem lets the weight of the wet fabric pull the leg longer.

Getting the shape back

For a pair that has relaxed during wear rather than through damage, the fix is short and warm rather than long and hot.

  • Cold wash and hang dry restores most of the shape in a pair with elastane still in good condition.
  • A short warm tumble — ten minutes, then hang while slightly damp — will tighten a pair up. Use this sparingly, because it costs you a little elastane every time.
  • Steam from a shower while the jeans are on a hanger firms the fabric slightly with no heat damage.

None of this reverses fatigued elastane. Once recovery is genuinely gone, it is gone.

Buying for it

The composition tells you what you are getting.

  • 100% cotton — will mould and grow, will not spring back, but holds its shape for years and ages well. Size for a snug fit knowing it will relax.
  • 98–99% cotton with 1–2% elastane — the practical middle. Comfortable, holds shape reasonably, degrades slowly if washed cold.
  • Below about 95% cotton, with more elastane and often polyester — very comfortable on day one, and the fastest to bag out and stay bagged.

If a pair feels remarkably comfortable in the changing room, that comfort is usually elastane, and elastane is the part with a limited lifespan.

Reading a composition label covers the rest of the label, including what else that percentage list predicts.

Sizing around it

The practical rule: buy rigid cotton snug, buy stretch denim true.

Rigid cotton will give you about half a size within a week. Stretch denim will feel loose within hours if you size up, because it is already doing the accommodating for you.

How should jeans fit covers where to judge the fit, which is not where most people look.