Modern sewing machines have stitch length controls marked in millimetres, mm.
Vintage machines give number of stitches per inch.

What is the relation between the two ?

Straight stitching

For straight stitching, if you set the stitch length to 3, the machine will sew about 8 stitches per inch.
If you set the stitch length to 2.5, the machine will sew about 10 stitches per inch.
As you shorten the stitch, the machine makes more stitches per inch, see table.
In this table for straight stitching, the number of stitches per inch is approximate, to the nearest whole number.

stitch length

Try sewing different straight stitch lengths and look at the effects.
2-2.5mm stitches are used for most sewing, but the other lengths have their uses.

Zig-zag stitching

zigzag length

Zig-zag stitch length is the length of one ‘zig’ – not the thread length but the length in the direction of stitching.

To get a closely stitched satin stitch needs testing.
Start by trying a stitch length of about 0.3mm.
Set the stitches too long and the zigzag nature of the stitching is obvious.
Too short, and you will get a big lump of thread which will jam in your machine.
Stitch some samples to find what gives a good result with your thread-fabric combination.

0.3mm length stitches make about 85 stitches per inch, or over 40 zig-zags. You can see why, now that machines can do zig-zag, it’s easiest to give stitch lengths in millimetres !

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