Ask ten buyers what GSM their running tee should be and you'll hear ten numbers. Most of them are guesses based on a garment they liked. The weight of a fabric is one of the first decisions in a tech pack, and it touches almost everything that comes after it: how the garment breathes, how it drapes, how well it hides what's underneath, how it survives fifty washes and what it costs you per piece.

This guide is the conversation our R&D team has with every new sportswear client. By the end you should be able to put a GSM range on your tech pack with a reason behind it.

What GSM actually measures

GSM stands for grams per square metre. It's the weight of one square metre of finished fabric. In our lab we cut a 100 cm² disc with a GSM round cutter, weigh it on a precision balance and multiply by 100. We take samples from the left, centre and right of the roll, because weight can drift across the width.

A higher number means more fiber in the same area. That usually means a denser, more opaque and more durable fabric that's also warmer and slower to dry. A lower number means a lighter, airier fabric that's more likely to show through or lose shape.

Match the weight to the garment

Start from how the garment will be used, not from a number. Drag the slider to see where each weight range lands and what we usually make with it.

Fiber and yarn count change the feel

Two fabrics at 180 GSM can feel completely different. A 180 GSM cotton jersey feels soft and substantial but holds sweat. A 180 GSM polyester interlock feels smooth and cool and dries in a fraction of the time.

Yarn count matters too. For cotton we use the English count (Ne): the higher the number, the finer the yarn. A 40s yarn makes a smoother, lighter face than a 20s yarn at the same weight. For synthetics we work in denier and filament count, where finer filaments give a softer hand and better wicking.

“Buyers ask us for a number. We ask them how the garment moves, sweats and washes. The number comes out of that.”

Knit structure matters as much as weight

The way loops are built affects stretch, recovery and surface texture. Here's how the five knits we use most compare.

What GSM does to cost

Knitted fabric is bought and sold by the kilogram. If you move a tee from 160 to 200 GSM, you're using roughly 25% more fabric by weight for the same pattern. On a 10,000-piece order that difference is real money.

Going lighter isn't free either. Very light fabrics often need finer, more expensive yarns to stay opaque and strong. The cheapest option is the weight that does the job, specified tightly enough that every roll matches it.

A quick tech pack checklist

• Target GSM with a tolerance, for example 180 GSM ±5%.
• Fiber composition, for example 88% polyester, 12% elastane.
• Knit structure and yarn count or denier.
• Finishes such as brushing, moisture-wicking or anti-pill, since some add weight.
• Whether GSM is measured before or after washing.
• A reference swatch or garment you want us to match.