Zipper sizes are marked with a number – No.3, No.5, No.8 and, less often, No.4, No.7 or No.10 – and understanding it is the first step before you buy. The number is not the length; it is the approximate width of the closed teeth in millimetres. The higher the number, the stronger, wider and more visible the zipper. Below we explain how to read the gauge, what each size is used for and how to pick the right zipper for your project.
What the zipper number means
When a zipper is labelled "No.3" or "#3", the chain of teeth is roughly 3–4 mm wide when closed. A No.5 chain measures about 5–6 mm and a No.8 about 7–8 mm. Most manufacturers follow this convention, so a No.5 slider fits a No.5 chain of the same type (coil, metal or moulded plastic), but it will not work on a No.3 or No.8 chain.
Don't confuse the gauge with the length. Length is stated separately in centimetres – for example "No.3 coil zipper, 18 cm". The number determines strength and look; the length determines how big an opening the zipper closes.
Metal zipper closed-end brown / gold teeth T5 PMZ1008
Beige coil zippers #3 TAT TF334 – 10, 11, 13, 14, 15 cm non-separable
Metal zipper non-separable red / silver teeth T5 PMZ0005Zipper size chart
| Gauge | Closed teeth width | Total tape width | Typical use |
|---|---|---|---|
| No.3 | approx. 3–4 mm | approx. 24–26 mm | dresses, skirts, blouses, trousers, cushions, invisible zips |
| No.5 | approx. 5–6 mm | approx. 30–32 mm | jeans, jackets, hoodies, bags, backpacks |
| No.8 | approx. 7–8 mm | approx. 36–38 mm | winter coats, workwear, holdalls, tents, covers |
The figures are a guide and can vary by a millimetre or two between brands, but the relationship between the gauges stays the same.
When to choose No.3
No.3 is the finest and most widely used gauge for lightweight garments. No.3 coil zippers are soft and flexible and don't weigh down thin fabrics such as cotton, viscose, chiffon or jersey. That makes them the standard choice for:
- side zips in dresses and skirts;
- flies on women's and children's trousers;
- invisible (concealed) zippers – almost always No.3;
- cushion covers, pouches and small pockets.
At Bul-tex you'll find a wide range of closed-end No.3 coil zippers in dozens of colours and lengths from 10 cm to over 50 cm.
When to choose No.5
No.5 is the all-rounder – strong enough for daily wear without looking bulky. It is used on jeans (typically a No.5 metal zipper 15–20 cm long), light jackets, hoodies, bags and backpacks. Metal No.5 zippers are the classic choice for denim because they stand up to constant use and hot washes.
When to choose No.8 and larger
No.8 and No.10 are for places where the zipper takes real strain or needs to be operated with gloves on: winter and ski jackets, workwear, holdalls, tents and furniture covers. The slider is large and easy to grip, and the teeth resist splitting under load. On a simple dress, however, a No.8 would look heavy and distort the fabric.
Practical tips for choosing
- Match the gauge to the fabric – lightweight cloth needs No.3, heavy fabrics (denim, leather, canvas) need No.5 or more.
- Think about load – the more often it opens and the heavier the contents (for bags), the bigger the gauge.
- The slider must be the same gauge and type as the chain, or it won't close.
- Decide whether you need a closed-end or an open-end (separating) zipper – the gauge doesn't tell you that.
Frequently asked questions
Is the zipper number its length?
No. The number (No.3, No.5, No.8) shows the approximate width of the closed teeth in millimetres. Length is given separately in centimetres.
Can I put a No.5 slider on a No.3 zipper?
No. The slider has to match both the gauge and the chain type – coil, metal or plastic. Otherwise the teeth won't mesh or the slider will slip off.
What size zipper is used on jeans?
Most jeans use a No.5 metal zipper between 15 and 20 cm long, depending on the style and size. Children's jeans sometimes use a No.3 or No.4.
Browse all our zippers or go straight to the metal zippers for jeans and bags – you'll find the right gauge, colour and length for your project.
