8:5, 5:3

Roughly speaking, it's about 8 km for every 5 miles, or 5 km for every 3 miles.

So if someone says he walked 25 miles, you can use the 8:5 ratio—

25 = 5 × 5, so 8 × 5 = 40 km.

If I walk from Yangpyeong Station to Yeoju's intercity bus terminal, that's 33 km.

5 km × 6 = 30 km, so 

3 mi. × 6 = 18 mi.

If I add another 3 km to the above, how many miles am I adding? Well, from the 8:5 ratio above, I can see that 4 km = 2.5 miles, so 3 km is a little less than that. Let's round the figure and say it's about 2 miles. Therefore:

18 + 2 = 20

So 33 km is about 20 miles.

Google says 33 km is 20.5052 miles. I'm off by a half mile (or about a kilometer).

If we use the 5:3 ratio, we get very similar numbers.

5 km = 3 miles

10 km = 6 miles

20 km = 12 miles

30 km = 18 miles

Converting from round-numbered kilometers is easy.

And with the 8:5 ratio, converting from round-numbered miles is easier:

5 mi. = 8 km

10 mi. = 16 km

15 mi. = 24 km

20 mi = 32 km (as we see above, it's closer to 33 km)

Here's why 8:5 and 5:3 are so similar. If I divide 8 by 5, I get exactly 1.6. If I divide 5 by 3, I get 1.667—almost 1.7. These ratios work for very rough calculations, but if your cell phone's calculator has unit conversion, as mine does, just use that for more exact numbers. Failing that, if you have access to Google, just type in "km to mi" or "mi to km" to convert distances. (You can also do this for temperatures: Google "f to c" or "c to f," for example. Google will even convert between units that are not easily convertible, e.g., about how many grams are in 2.25 cups of all-purpose flour? That's taking a volume measurement and coming out with a weight measurement. I Googled "2.25 cups all-purpose flour in g" for the answer.)

Am I preaching to the converted?