I am buying new SDRAM memory. I need to convert 50 ns to Hertz so I know what to buy. (e.g. 533mH etc.)?

I wouldn't bother. Most RAM modules have speed designations like "PC 3200" or "PC2 6400". You need to find out which speed memory your motherboard supports. Then order that speed of SDRAM memory.

Your motherboard documentation will also tell you whether your DIMM slots are interleaved or not and whether you need 2 or 4 DIMM sticks.

Well to answer your question, the units of question are in cycles per second for Mhz, and seconds per cycle for 50 ns. Nano is 1.0 X 10-9, or 0.000000001 seconds. Divide 50 ns into 1, and you get 1/0.000000050 = 20 Mhz. I'm sure that this is not the answer you were looking for, but it is for the question you asked...

I agree with Tomas.. anyway. let me put this out to you using the units you specified:

50 ns to hertz which is unit/time, and you want to buy something in milihenries.

nanoseconds will never equal milihenries.