Cotton vs polyester: how to compare footprint per kilo

The question appears in every textile project: which fibre has the lower footprint? The honest answer starts with “it depends”, and it is worth understanding on what.

Why is there no single figure?

Because a fibre’s footprint is not a fixed property. It changes with:

• Origin: where and how the cotton is grown or the polymer is made.
• Energy: which electricity mix and fuels power each stage.
• Fibre type: virgin or recycled.
• Process: spinning, weaving, dyeing and finishing.

How to compare rigorously

1. Same boundary. Compare cradle to gate with cradle to gate, not with the full life cycle.
2. Same functional unit. A kilo of fibre is not a kilo of garment; the garment may weigh and last differently.
3. Same kind of data. If one figure comes from a real fabric and the other is an average, they are not comparable.
4. Same standard. A calculation following ISO 14067 fixes what is included.

What role does the physical factor play?

A physical factor starts from what the product is: material, weight, process (including its energy) and transport. It lets you see the effect of changing fibre or supplier, something a spend-based calculation does not capture; we explain it in physical vs spend-based factors.

What to do with this

Calculate the specific style with your supplier’s data and compare alternatives with the same boundary. If you want to start from a complete garment, see the footprint of a t-shirt.

Four traps when comparing fibres

1. Comparing a per-kilo-of-fibre figure with a per-garment figure. They do not measure the same thing.
2. Mixing real data with an average. The latter will always look more “normal” and is not comparable.
3. Forgetting fibre blends. A 60/40 fabric is not simply the average of two figures: the process changes.
4. Assuming recycled is always better. It depends on the recycling process and the energy used; that is why you have to calculate it.

How do you decide between alternatives?

1. Define the garmentWeight, use and expected life of the style.
2. Set boundary and standardSame boundary and same standard for every option.
3. Calculate each optionWith each supplier's data, using physical factors.
4. Decide with the breakdownLook at where the difference lies, not just the total.

What data should you ask the supplier for?

• Exact composition and percentage of recycled fibre, if any.
• Country and plant of each stage (spinning, weaving, dyeing).
• Type of energy for each stage, if known.
• Weight per metre or per unit of the fabric.
• Any declaration or certificate they already have, to use as primary data.

What can you do this week?

If you are weighing a fibre switch, first write down the question you want to answer: reduce the style’s footprint? answer a customer that has set a target? prepare a tender? The answer determines the boundary and unit you should use. Then ask your current and alternative suppliers for the same information, in the same format, so the comparison is fair.

Avoid deciding from a table of per-kilo figures taken from a generic report. Those tables help you get oriented, but not choose: they do not include your process, your supplier or your destination. Calculating two or three specific options takes very little time and avoids a decision based on an average.

How does Xain do it?

Xain compares alternatives with the same method: physical life-cycle factors on a process tree, cradle-to-gate boundary and ISO 14067. Where the supplier provides primary data, it replaces the generic data. That way you can see the effect of changing fibre or supplier on your style, not on an average. If you want to compare two fabrics, tell us the case.