Stitch length and tension are not separate settings — they change the same thing, which is how much thread ends up in each stitch.
A longer stitch pulls more thread through the tension discs for every cycle of the needle. At an unchanged dial setting the stitch looks and feels slightly looser, and on a long stitch a small imbalance is much easier to see.
A shorter stitch takes less thread and reads as tighter. Very short stitches on fine fabric are a common cause of puckering that people then try to fix with the dial, when the real fault is that the fabric has been perforated into a gathered line.
What this means in practice. If you lengthen the stitch for denim or canvas, expect to check the tension again. If you shorten it for silk, expect to loosen the top tension rather than tighten it.
The extreme case is satin stitch. At 0.3 to 0.5 mm the stitches sit against each other and the top tension normally has to come down, so the bobbin thread is drawn to the underside and does not show along the edges of the band.
The other extreme is basting. At 5 mm the stitch is meant to be loose and removable, and judging tension by it is pointless.
Read them together. When a seam looks wrong, ask what length it was sewn at before touching the tension. On heavy fabric a longer stitch fixes more problems than a dial ever will.
Questions people actually ask
What length should I use as a default?
Around 2.5 mm for general woven construction, 3 to 3.5 mm for heavy fabric, and 2 mm or slightly less for fine fabric.
My seam puckers at the right tension.
Try lengthening the stitch slightly. Puckering is as often a length problem as a tension problem.
Last reviewed 2026-08-11