For low shoes with 4-5 hole pair you should buy at least 75 cm long laces.
For most classic low shoes for men, with the so-called 5-hole lacing, the 75 cm long laces are just right. At least when it comes to slim to normal feet. Somewhat fuller feet often need the 90 length.
Especially with Derby lacings, a little more length is often required, as these lacings can be opened further. If you would use too short laces, it would mean that you would have to thread the ends of the laces through the holes every time you put on and take off your shoes.
By the way, Oxford distinguishes between shoes and Derby shoes.
Derby lacing = open lacing
Oxford lacing = closed lacing
With classic ladies' low shoes you can in principle use the same scale, but you should take one size smaller in each case.
So rather 60 cm - 75 cm instead of 75 cm - 90 cm for the five hole lacing.
Please always keep in mind that these words are suggestions based on a certain experience, but are not laws set in stone. Depending on the type of shoe and foot, there can be considerable deviations which can lead to mistakes in buying.
You can be quite sure of this if you measure the old shoelaces with a centimetre measure.
If you no longer have them, you can help yourself with a piece of string to find the right size.
1 hole pair = 2 lace holes per shoe