While touring in Europe it's convenient to be able to convert between km and miles in your head. A rough & ready method involves memorising three numbers – 3, 5 and 8 – and the fact that 1 km in miles is approximately equal to the first of these numbers divided by the second, or alternatively the second divided by the third. That is:
1 km ≅ 3/5 or 5/8 miles
(Tip: To help memorise 3, 5 and 8, note that 3 + 5 = 8)
When converting a km figure to miles (or km/h to mph), look for ways to use the divisors 5 or 8. For every 5 km accounted for, substitute 3 miles; and for every 8 km accounted for, substitute 5 miles. When converting the other way, look for ways to use the divisors 3 or 5, then substitute 5 and 8 km respectively.
This might sound cumbersome at first but after a little practise you will soon become adept. And the practise can be done in your head while you're on the road. A few days into your trip and you'll be impressing your friends and family with your uncanny mental faculty.
According to Google, 1 kilometre = 0.621 miles. So:
Hence, in cases where you have a choice between the two ratios (like when converting 40 km to miles) 5/8 should be preferred (i.e. 5 * 5 miles is a more accurate than 8 * 3 miles). And if you have stooped to using a calculator (hopefully you're not driving!), simply doing the whole conversion using 5/8 will give pretty good accuracy.
If you regularly need to convert other units you can apply the same method after memorising the applicable approximate conversion ratios. Examples:
|miles ↔ nautical miles|
|1 mile ≅ 6/7 or 7/8 nm||(-1.4%, +0.7% error)|
|mph ↔ m/s|
|1 mph ≅ 4/9 or 5/11 m/s||(-0.6%, +1.7% error)|
|ft ↔ m|
|1 ft ≅ 3/10 or 4/13 m||(-1.6%, +0.95% error)|
Some of these figures aren't so easy to deal with and only miles ↔ nm exhibits the same '3 easily remembered digits' characteristic as km ↔ miles.
For ft ↔ m you can generally get by just memorising 3/10. It is, of course, very easy when converting feet to metres: just divide by 10 and multiple by 3 (e.g. 6' = 1.8 ). But how easy it is to convert metres to feet depends on how easy you find it to divide the particular value by 3; e.g.: 100 m ≅ 333 ft; 150 m ≅ 500 ft; 800 m ≅ ...hmmm
Side note: This kind of mental arithmetic would be easier in most cases if we adopted the dozenal system (base 12) rather than decimal (base 10).