what’s the difference with English system of measurements and SI?

SI units are metric (the SI bit comes from Système international d’unités). There are standards of the weights and measures that are kept at the Bureau International des Poids et Mesures.

Here is their website:

http://www.bipm.org/en/home/

The ‘English’ measures are the Imperial measurements and we don’t in fact use them in Britain anymore, we use the metric system now (maybe because we no longer have an empire?).

Imperial measures were subject to change. A foot was just that, the length of a man’s foot. The Egyptians used cubits which was the length of a man’s forearm.

They did become standardised eventually but just aren’t practical as they’re not based on the decimal system, 12 inches is a foot etc.

There’s some more info here:

http://www.cftech.com/BrainBank/OTHERREFERENCE/WEIGHTSandMEASURES/MetricHistory.html

The saying ‘rule of thumb’ comes from when men were allowed to beat their wives with sticks no thicker than their thumb, thankfully that unit of measurement didn’t survive…… The Imperial system was somewhat misogynistic!

metric is based on water standard is based on various other things that are not consistent.