Definition
Thread tension is the balance of pull on the upper and bobbin threads that makes them lock together midway between the fabric layers.
With balanced tension, the two threads interlock between the layers and the stitch looks the same on both sides. When it is off, one thread lies flat on the surface while the other is pulled through to its side in little dots or loops.
How to read the symptoms:
| What you see | Usual cause |
|---|---|
| Loops or a nest under the fabric | Upper thread not seated in the tension discs: rethread with the presser foot up |
| Bobbin thread showing as dots on top | Upper tension too tight |
| Upper thread showing as dots underneath | Upper tension too loose |
| Fabric puckering along the seam | Tension too tight for a light fabric, or stitch too short |
Before turning any dial, rethread the machine completely with the presser foot raised, check that the bobbin is inserted the right way round, and use the same thread top and bottom. Most tension problems disappear at this stage.
Then adjust only the upper tension, in small steps; many machines default to about 4 on a 0 to 9 dial. Leave the bobbin case screw alone unless your manual says otherwise, and test on a double layer of the fabric you are sewing.
Some techniques change tension on purpose: lowering it helps gathering and basting stitches pull out easily. See presser foot and sewing machine basics.