Thread weight labels can look like someone dropped numbers into a sewing basket and called it a system. In plain terms, 40/2 polyester sewing thread is a fine two-ply polyester thread suitable for many everyday sewing projects. The “40” refers to thread size and weight, and the “2” means two plies are twisted together. STI Ideal's Custom Thread Box uses 400-yard spools of 40/2 polyester sewing thread.
STI Ideal Threads has supplied sewing thread since 1999, with 932 colour shades and over 60 million spools sold. This guide explains what 40/2 means, how polyester thread behaves, and when this type of sewing thread makes sense.
What is thread weight?
Thread weight describes thread thickness. In many common sewing thread systems, a higher weight number usually means a finer thread, while a lower number usually means a thicker thread. That means a 40-weight thread is generally a medium-fine thread, not a heavy thread. It can be useful for everyday garment sewing, alterations, repairs, craft projects, quilting tasks, and small-batch making.
Thread weight affects:
- How visible the stitching looks
- How bulky the seam feels
- Which needle size works best
- How the thread moves through the machine
- Whether the thread suits light, medium, or heavy fabric
What does 40/2 mean?
In 40/2 thread, the first number indicates the thread size or weight, and the second number indicates ply. The “2” means the final thread is made from two strands twisted together. Two-ply construction helps create a balanced thread that can move smoothly through a sewing machine when paired with the right needle and settings.
In plain English: 40 is the thread size and weight, 2 is two plies, and polyester is the fibre. So 40/2 polyester sewing thread is a two-ply polyester thread with a practical everyday sewing weight.
Why polyester sewing thread?
Polyester thread is popular because it is smooth, versatile, and practical for many sewing projects. It can be a useful choice for garments, alterations, repairs, craft projects, and small studio work. Polyester sewing thread is often chosen for everyday sewing, garment seams, mending and alterations, craft projects, machine sewing, and broad colour matching.
STI Ideal sells 40/2 polyester sewing thread. It is not a cotton thread — if you are weighing the two fibres, see polyester vs cotton thread.
Is 40/2 polyester good for sewing machines?
Yes, 40/2 polyester sewing thread can work well for many sewing machine projects when used with the right fabric, needle, and tension. If stitching looks uneven, check these before blaming the thread:
- Is the machine threaded correctly?
- Is the bobbin wound evenly?
- Is the needle the right size?
- Is the needle old or bent?
- Is the thread catching on the spool?
- Is the tension suitable for the fabric?
Before blaming the thread, check the threading — it is the usual culprit.
What projects suit 40/2 polyester thread?
Use 40/2 polyester thread for garment construction, everyday repairs, alterations, lightweight to medium-weight fabrics, craft projects, quilting tasks where the thread should not feel bulky, small-batch production, and general sewing thread colour matching. For very heavy denim, upholstery, leather, or outdoor gear, check whether you need a heavier speciality thread.
How much is 400 yards of thread?
Each STI Ideal spool is 400 yards, approximately 366 metres. That is a practical size for makers who want enough thread for real projects without storing oversized cones. A 400-yard spool is large enough for many sewing tasks, easy to store, good for building a colour set, and less bulky than an industrial cone.
If your thread collection has become a drawer of mystery shades, a planned set of 400-yard spools is a calmer way to sew.
Quick reference table
| Term | Meaning | STI Ideal context |
|---|---|---|
| Thread weight | Thread thickness | 40 is a practical everyday size |
| Ply | Number of strands twisted together | 40/2 means two-ply |
| Polyester | Synthetic thread fibre | STI Ideal thread is polyester |
| 400 yards | Spool length | Approx. 366 metres per spool |
| Custom Thread Box | Custom colour set | 10 spools per box |
How to choose thread weight
Start with the project:
- For everyday garment sewing, choose a practical all-purpose weight.
- For lightweight fabric, avoid bulky thread.
- For visible topstitching, decide whether the thread should blend or stand out.
- For heavy denim or upholstery, check if a heavier thread is needed.
- For repairs, match colour and strength to the garment.
The goal is not to memorise every thread system. The goal is to choose a thread that behaves well with your fabric, needle, and machine.
Frequently asked questions
What does 40/2 thread mean?
40/2 means the thread is a size 40 thread made with two plies twisted together.
Is 40/2 polyester thread heavy duty?
No. It is better understood as a practical everyday sewing thread. Very heavy fabrics may need a heavier speciality thread.
Is polyester sewing thread good for beginners?
Polyester sewing thread can be a good beginner choice because it is versatile and works for many everyday projects.
How much thread is on each STI Ideal spool?
Each spool is 400 yards, approximately 366 metres.
What colours are available?
STI Ideal offers 932 colour shades. Customers can choose one shade or mix up to five colours in a Custom Thread Box.
Build your box
When you know which weight you need, you can order 400-yard thread spools in your own shades — 10 spools in one colour or a mix of up to five, from 932 colours of 40/2 polyester. Not sure where to start? Upload a fabric photo and let the colour matcher suggest the closest available shades, or browse the full range in our catalogue.