dba_afish said:
I think just having decent benchmarks helps the most. most of the time you're not going to need to do actual conversions so just having a vague sense is better.

32°c ≈ 90°f (at/above most would consider "hot" weather)
8°c ≈ 45°f (at/below most would consider "cold" weather)
1cm ≈ ⅓in, 1m ≈ 1yd, 2km ≈ 1mi
100kph ≈ 60mph

C to F is actually really easy! F = C x 1.8 + 32