Tension is where most people go first when a stitch looks wrong, and it is where they should go last. Understanding what a balanced stitch looks like — and what the dial actually does — will save you a lot of unpicking.
What Tension Actually Means
A machine stitch is formed by two threads locking together: the upper thread from the spool, and the lower thread from the bobbin. Tension controls how hard each is pulled.
In a balanced stitch, the two threads meet and lock in the middle of the fabric layers. You see only upper thread on top and only bobbin thread underneath, and the stitch looks identical on both sides.
When the balance is off, that lock point moves up or down, and one thread gets pulled through to the other side.
Reading the Stitch
Test on a doubled scrap of the same fabric, with contrasting upper and bobbin thread. Contrasting colours make this immediately obvious and it is worth the two minutes.
- Bobbin thread showing on the top surface — upper tension is too tight. It is winning the tug of war and pulling the lower thread up. Reduce the upper tension number.
- Upper thread showing on the underside — upper tension is too loose. Increase the number.
- Loops or a tangle underneath — this is almost certainly not tension. See why your thread bunches underneath.
- Fabric puckering along the seam — often too much tension overall, but also commonly a blunt needle or too long a stitch for the fabric.
The Dial
The upper tension dial is numbered, typically 0 to 9, with 4 or 5 as the normal setting. Higher numbers mean tighter. That is the dial you should be using, and for the vast majority of sewing it is the only one you should ever touch.
Move it one number at a time and re-test. Small changes make a bigger difference than people expect.
Leave the Bobbin Screw Alone
There is a small screw on the bobbin case that adjusts lower tension. It is set at the factory and it very rarely drifts.
Adjusting it is a genuine trap: you lose your reference point, the screw is tiny and easy to drop inside the machine, and you can end up chasing a problem you created. If you have a machine with a removable bobbin case and you sew a lot with specialist threads, some people keep a second bobbin case adjusted for those — which is the sane version of this idea.
Things That Look Like Tension Faults and Are Not
Before adjusting anything:
- Rethread the machine with the presser foot up. This fixes most apparent tension problems outright.
- Change the needle. A blunt or bent needle produces skipped and uneven stitches that read as tension issues. See how often to change your needle.
- Check the needle suits the fabric. Sewing knit with a sharp needle causes skipped stitches no dial will cure — see needle sizes and types.
- Check the bobbin is wound evenly. A lumpy, loosely wound bobbin delivers thread inconsistently. Wind bobbins on the machine, not by hand.
- Clean the bobbin area. Lint under the tension spring is a classic cause — see cleaning a sewing machine.
When Tension Genuinely Needs Changing
There are legitimate reasons to move the dial:
- Heavy or thick fabric — often needs slightly higher tension.
- Very fine or sheer fabric — usually needs lower, or it puckers.
- Different thread weights top and bottom — almost always needs adjustment. See thread weight explained.
- Decorative or topstitching thread — usually needs looser upper tension.
- Knits and stretch stitches — often slightly looser, so the seam can extend without snapping the thread. See woven versus knit fabric.
A Sensible Habit
Note where your dial normally sits when everything is working. Then, whenever you change it for a specific fabric, you know exactly where to return to. Machines do not drift; people forget.
And test on a scrap of the actual project fabric, doubled, every time you start something new. It takes a minute and it catches needle, thread and tension problems before they are sewn into something you care about.
