It took me a second to parse what you were saying. Essentially since it’s 2.2LBS per kg the first divide gets you the number on both sides of the decimal. Very cool

I have similar way of calculating square meters from square feet. "Subtract 10% divide by 10". So for example 900 square feet: 900 - 90 = 810/10 = 81. Real answer is 83.6, close enough for me.

Any similar shortcut for feet inches to cm? I find this conversion to be slower to compute than miles to km or lbs to kg.

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.

@sigfpe in a reply to the tweet mentioned

Fₙ in ≈ Fₙ₊₂ cm

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

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!

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.

I do something similar but, because I'm faster at multiplying, I do "its double plus its half". E.g. 7 inches = 14 + 3.5 = 17.5 cm

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!

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

   2 inch ≈ 5 cm

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

Just remember 100F was Fahrenheit's wife's body temperature when she had a low grade fever. Normal body temperature is 36.8C, and with a low fever it's between 37 and 38C. It's so easy! haha.

Not quite. It is 37.78.

In temperature couple of degrees means a lot.

For outside temperatures it's close enough though. As an American who travels to Central America quite a bit being about to do: (C*2) + 30 to get F is close enough.

So n * (1-0.5) * (1-0.1)... This could also work for miles to km conversion in a similar way: n * (1+0.5) * (1+0.1). Not as accurate as golden ratio but still acceptable. :D

You saved my life :)