I have two weights. One is 50 grams and one is 100 grams. I don't know how exact the given weight is, i.e. I don't know if the 50 grams is 50.00 grams or 50.0 grams. I use these numbers (50 and 100) in a calculation involving several other numbers that are not weights but other things. I get an answer that looks something like 9,381562307564 and I have to round it. How do I round it? I normally take all the numbers that I used in the calculation and look at each number to see how many significant figures it has and then I round the answer to the smallest number of significant figures I found among all the numbers I used in the calculation. BUT what I'm thinking about is this: let's say we know the weights are exactly 50.00 grams and 100.00 grams. Wouldn't that mean that 50.00 has 4 significant figures and 100.00 has 5 significant figures? Should I do what I usually do and of these say that 4 is the lowest and use that if it is the lowest of all the numbers? OR is some other method better? Because isn't the whole point of using significant numbers that the answer reflects the accuracy (or precision, don't know which...) of the numbers I used when I did the calculation? And 50.00 grams and 100.00 grams are both accurate to within 0.01 grams, right? So the accuracy of these two weights should be/is the same even though they have a different amount of digits, right? I'm confused... Another thing: when you have the measurement/number (whatever you call it, English is my second language) 100 grams, all you know for certain is that it has at least 1 significant figure, right? But in this case it probably has more than 1 significant figure, right? So I should not use 1 because it's probably accurate to within at least 1 gram and not only to within 100 grams. Is the significant number method not always the best method? Is there a decimal places method or something? I only learned about this briefly in school and now I have to use it in physics.