Do I need to round off to significant figures when converting units?

Question

For example, I'm going to convert 87 kg to lbs using dimensional analysis. There are 2.20462 lbs in a kg.

1. How am I supposed to know how many decimal places of lbs should I use?

2. Assuming I used 2.205 lbs to convert, and I come up with the answer of 191.835 kg, am I supposed to round off the answer to the significant figure as well? How will I know how many significant figures to round off to?

Answer 1

Since the conversion requires multiplication, as in 87 kg x 2.20462 lg/kg, your answer should have no more signifiant figures than the least number of sig. figs. in the numbers being multiplied. 87 has 2 s.f. and 2.20462 has 6 s.f., so your answer should be limited to 2 significant figures. That would be 191.80194 rounded to

190 lbs (2 significant figures)

Answer to #2 is same as above. You round off to the number of sig. figs. indicated by the least number of sig. figs. in the numbers being multiplied. Your answer of 191.835 would still round to 190 (2 sig. figs) because the value of 87 kg has only 2 sig. figs.