In the screenshot below:
-- upward arrows are support levels
-- downward arrows are resistance levels
Note how price occasionally reverses at these points, and also how support can become resistance, and vice versa.
It's not easily possible to "calculate" these, due their their abstract, subjective nature (if that's what you're asking). But here's a possible (albeit simplistic) starting point:
1. Find significant highs and lows. A significant high could be a candle high that has at least X lower candle highs on both its left, and its right. Vice versa for significant lows.
2. Define a tolerance level of Y pips. Where two or more significant highs and/or lows occur within Y pips of each other, that defines an S/R zone. Or you might want to stipulate a minimum of 3 or even 4 significant highs/lows, and possibly increase the value of Y (and/or decrease the value of X).
3. After Z candles have passed, a significant high or low becomes obsolete, and is removed from consideration.
Fibo retracements and pivot points might also be considered as S/R levels: see here.
Trendlines, and/or round numbers (i.e. X.XX00, X.XX50) can also provide imaginary S/R levels.
