Fun and games last night when I made a start on a commission, an adult sweater, and ended up with a back that would have worked for a short sleeve. Forgot to convert my swatch measurements to 10cm.
The formula is as follows: 10 * (Swatch measurement in cm / Swatch dimension). So if the swatch measured 14.3 cm wide, and it was a 40 st swatch, then 10cm would be 10 * (14.3 / 40), which is 27.97 sts.
It also works for rows. Don't forget that swatches are std : mid : bulky, 40 : 30 : 20 sts wide, and 60 : 40 : 30 rows high respectively.