How is significant figures different to rounding?

Significant figures reflect the precision of a measuring device - in simple terms how many numbers it can measure down to. So an ordinary ruler for example can measure down to 1mm, and so numbers like 13.6 cm are possible to be recorded with it but 13.67 is not and neither is 13.60.

Rounding is often done for convenience. Take the value for pi for example which goes on forever. Using it with its infinite digits beyond the decimal is not very practical so we often round it 3.14. (Unless… see meme below ?)

When calculating answers you will often get a number with many decimals on your calculator. So the question becomes to how many significant figures should we round this? If you used practical data (the following is true in physics especially) then you always round to the lowest number of significant figures that was provided as this reflects the least precise measurement you could take. We do this because we recognise that the precision of our final answer is limited by the precision of our least precise measurement.

Hope that helps!

Now for the meme… a long standing jokes that engineers will just round irrational numbers to whatever is most practical!