I would think after a certain point, you would just remember the formula and be able to use it on any calculator.
The simple formula for pounds and inches:
(703 * weight in pounds) / (height in inches) ^ 2
That's 703 multiplied by your weight in pounds, divided by the square of your height in inches.
An example of a lady who weighs 250 lbs and is 5'6" would thus have a BMI of about 40.3, as shown below:
(703 * 250) / (66) ^ 2 =
(703 * 250) / (66 * 66) =
(175,750) / (4,356) =
~ 40.35
There is an even simpler formula that uses meters and kilograms, but I can't recall it off hand. However, as an American I admit I'm not used to thinking in metric.
Once you remember this, you won't need to visit a BMI web site, download yet another smart phone program, etc.
Other ratios worth remembering if you can:
1 kg = ~2.205 lbs
75 kg = 165.3 lbs
100 kg = 220.5 lbs
125 kg = 275.6 lbs
1 inch = 2.54 cm
1 foot = 30.48 cm
1 "stone" = 14 lbs (primarily the British/UK residents may use the term "stone"
18 stone = 252 lbs