What needle size do I need for my fabric?

Match two things: the size, and the point.

Size is the number on the packet — 70/10, 80/12, 90/14, 110/18. The larger number is European, the smaller American; they describe the same needle.

• 60/8 – 70/10 — silk, chiffon, fine cotton lawn
• 80/12 — quilting cotton, poplin, shirting, most dressmaking
• 90/14 — heavier cotton, linen, light upholstery
• 100/16 – 110/18 — denim, canvas, multiple layers

Point type matters as much as size, and more often goes wrong.

• Universal — wovens. The default, and the wrong choice for knits.
• Jersey (ballpoint) — knits. A rounded tip pushes the loops aside instead of piercing them, which is what stops skipped stitches.
• Jeans — a reinforced blade and sharp point that penetrates dense weave without deflecting.
• Microtex — a very sharp point for tightly woven microfibre and silk.
• Leather — a cutting point. Never use on woven fabric, which it will slice.

When in doubt, go finer. A needle slightly too small leaves a smaller hole and is more forgiving on the fabric than one slightly too large, which perforates visibly on anything fine.