A correctly tensioned lockstitch looks the same from both sides — the two threads lock in the middle of the fabric rather than on either surface. A visible difference tells you exactly what is wrong.
Bobbin colour showing on top → top tension too tight. Loosen it in half-numbers.
Top colour showing underneath → top tension too loose, or the thread is not seated in the tension discs. Re-thread with the presser foot up before touching the dial.
Both look right but the seam puckers → tension is balanced but too tight overall, or the stitch is too short for the fabric.
Test with contrasting threads. Top in one colour, bobbin in another, on a doubled scrap of the actual fabric. On matching thread this is invisible until the garment is finished, which is why so many people never notice.
One thing that is not a fault. The top side of a zigzag or a decorative stitch legitimately looks different from the underside — the geometry is not symmetrical. Judge balance on a straight stitch.
And re-test when you change fabric weight. A tension right for denim is too tight for lawn.
Questions people actually ask
Which side should I judge from?
Both, always. Judging from one side is how people spend an evening adjusting the wrong dial.
My machine has no tension dial.
Some computerised machines set tension automatically. If the stitch is unbalanced on one of those, the fault is threading, needle or bobbin — not a setting.
Last reviewed 2026-08-11