The only way to be sure what your current 1 rep max for a particular exercise is, is to test it. There are calculators and tables around, but they are not particularly reliable. The problem is that they rely on an assumed ratio of your 1 rep max to (say) your five rep max. However, because people vary in the proportion of fast twitch/slow twitch muscle fibres they have (and even in the same person, different muscles will vary in proportion)the ratio assumed is just someone's guess at what the average might be.
Some people can do 20 reps with 80% of their 1 rep max. Others can only do 5. As well as fast twitch/slow twitch ratio, the level of muscle fiber recruitment you are capable of will also influence what your 1 rep max/5 rep max ratio is.
You can use tables or calculators as a (very rough) guide, but IMHO they are not really much use. Really, the only way is to test yourself. Of course, once you have established your 1 rep max once, you could find out your 5 rep max at the same time. If it is (say) 90% of your 1 rep max then you could assume that, for that exercise, the ratio is going to stay fairly close to 90% for quite a while. This will save you having to test your 1 rep max all the time. However, you couldn't assume that the same ratio applies to other exercises. You would expect the ratio for calf raises to be different from that for bench press, for example.