For example, when I need to talk with my American friends about body weight:

$\mathrm{KG}\times2+10\%$

($100\times2=200$, $200+10\%=220$... Pretty accurate and easy to do mentally, both ways)

However, when I need to talk with my British friends about body weight, how do I approximate from KG to Stones?

Additionally, if I have a similar formula for Stones-lbs, that will be useful too!

I mean, I know a Stone is 14 lbs, but how to approximate 247lbs in Stones without pulling out a calculator?

For your reference, a Stone is about 6.35 KG

I am thinking that a "good, practical" approximation follows these rules:

- 5% accuracy is kinda bad... Imagine weighing 100kg and wanting to lose 5kg - the error alone is making discussion about it difficult! I am thinking up to 2% error
- Maximum of 3 steps
- Only addition, subtraction, multiplication and division allowed (% also allowed)
- Multiplication only by "easy" factors: 2, 3, 4, 5, 10
- Division only by "easy divisors": 2, 10
- Only "easy" percentages like: 1%, 10%...