How many significant figures does the number 0.000504 have? *

Answer 1

This has 3 sig figs. The non-zero digits are significant and the zero between the non-zero digits is significant, but the others are "place holders". Rewriting this in scientific notation solves the quandary because we would write 5.04 x 10^-4. As we can see, only the numbers that are signification show in the expression.