Well, as surprising as it might sound there seem to be many units in the kitchen which are not so easy to convert. One example: How many milliliters does a table spoon have? Google says 14.7868 (american table spoon). I have read from 6 to 25... I mean the problem is that the people who writes those recipes use different definitions of the same unit. And that is just one example. How should you convert units which are so different between different authors?

1 US customary table spoon (~14.8ml) is conventionally taken as 15ml, a teaspoon as 5ml, a cup as 240ml. That's roughly what you get when you buy a set of measuring spoons/cups as well; some may be a tad smaller (following the US measurements), and some are aligned with these metric values — the difference is mostly negligible.

These are quite standardised unless you are using something like Australian tablespoons¹ (but then any American following that recipe would run into problems as well). The US customary cup and spoon measures were introduced exactly to solve the problem of measurement ambiguity in recipes! Nineteenth century cookbooks can be hard to follow because of the significant variation in the interpretation of smidgens, dashes, pinches, and (then) spoons and cups.

1: Are these really a thing? Any Australian care to chime in?

I'm Australian, a 'standard' tablespoon is 20ml, but recipes sometimes use 25. Not sure about the rest of the world. It probably doesn't actually matter too much for most recipes; the ones that need that level of accuracy tend to specify weights.

I created http://www.recipastely.com/ to simplify recipe conversion between us and iso units a few years back. It's not perfect but it seems to work fine for me.

For unit conversion to metric, I made a browser plugin: https://github.com/falk-hueffner/metric-cooking It can also convert from volume to mass, e.g. annotate "3/4 cup plus 2 tablespoons packed light-brown sugar" with "[190 g]". It's a bit hacky (basically a giant regexp) but works nicely for me.