How many significant figures are in 0.001125?

Answer 1

The first significant figure is the first non-zero digit - start counting from that point onwards, and stop counting when you reach just zeros remaining in the number:

0.001125 has four sig fig.

By expressing the number in standard form (scientific form to Americans) this can be easily seen:

0.001125 = 1.125 x 10-3

There are four digits in the mantissa (the number (1.125 in this case) before the "x 10-3").

When rounding to a number of significant figures, it can result in a zero, eg rounding 1.195 to 3 sig fig would round to 1.20; normally this trailing zero would be left out (the result written as 1.2), but to emphasise the fact that it is to 3 sig fig, it may be left in - if there are trailing zeros after a decimal point (where they would normally be left out) they can be counted as significant figures.

However, 1195 rounded to 3 sig fig is 1200, but given 1200, it is impossible to tell if the number is rounded to 2, 3 or even 4 sig fig, so the best that can be given in such a case is to say it is to two sig fig - the trailing zeros are before the decimal point and are not counted.