@sigfpe in a reply to the tweet mentioned

Fₙ in ≈ Fₙ₊₂ cm

https://twitter.com/sigfpe/status/1158465136891269121

Fahrenheit to Celsius: (100F -30)/2= 35C

this was the size of my ruler: 12 inch ≈ 30 cm. therefore,

   2 inch ≈ 5 cm

A metre is about 40 inches, and an inch is about 2.5 cm. So, first into total inches, then take out as many 40s as you can, then multiply what's left.

5' 7" -> 5 * 12 + 7 inches -> 67 inches -> 1 metre 27 inches -> 1 metre 67 centimetres

Which isn't quite right, but is close-ish! And not really easy enough to call a shortcut either!

A yard is 0.9m.

1.1 yard per m or 1yd and 4in.

A foot is 30cm.

An inch is 2.5cm.

4in per 10cm.

2m is 6ft 8in =180+20=2m

5f 4in is 150+10=1.6m

1.6m is 3ft 4in + 24in=5ft 4in

Even converting to and back isn't bad... work in a machine shop for a while and it will be second nature. Also metric/standard tools do not cross the line on the floor, only you and the work piece on different pieces of equipment. 25um looks like a mil, but it's not!

For inches to cm in my head, I multiply by 10 and then divide by 2 twice. E.g. 7 inches -> 70 -> 35 -> ~17.5 cm.

Yeah it's not actually that bad, the base-12 system helps you out. You multiply the feet by 3, divide the inches by 4, add, multiply by 10. Optionally add 1.5% to really get that extra precision.

It's 30.5•(a+b/12) understood as about 1.015•10•(3a+b/4).

So I am 6’4”, that becomes 18+1=19, so I am about 190cm. Adding between 1-2% gives me that I am between 192cm and 194cm. I know I am on the lower end of 6’4” (maybe 6’ 3.75”) so I usually report 192cm.