# How Many Significant Figures?

1. Oct 3, 2009

### fs93

What do physicists agree on the number of significant figures to use?

I've done some research and I found that the answer has the same number of significant figures as the least precise value used to calculate it.

ex: V=IR

[I=1.25474]
[R=5.2]

Hence, voltage is simply 6.5?

Thanks,

FS

2. Oct 3, 2009

### Bob_for_short

But if [I=1.25474],[R=5.20000], the voltage is 6.52468 V.

3. Oct 3, 2009

### fs93

Is that so? Aren't zeros after a decimal completely irrelevant? Especially considering that an ammeter will give you a reading of one decimal place.

4. Oct 3, 2009

### Bob_for_short

They are relevant if they are the amper-meter readings. In case of one decimal place they are irrelevant, one does not write them, and your post calculation is correct.

5. Oct 3, 2009

### fs93

Gotcha. Thanks Bob for Short :)

6. Oct 3, 2009

### DaveC426913

If I understand correctly (and it's been 2 decades or more), the zeros to the right can indeed count.

If I weigh a sample on a microgram scale and it comes out as 100.00g, surely that is accurate to five sig digs.

7. Oct 3, 2009

### Staff: Mentor

Yes, the general practice is to assume that "unnecessary" zeroes at the end are significant, and that if they were not significant, the writer would have omitted them.

This doesn't apply if the number is given as an integer without a decimal point. In that case, all the zeroes at the end are necessary, but not necessarily significant in terms of precision. If you're given the number 230000 without any additional information, it's impossible to say whether it has 2, 3, 4, 5, or 6 significant figures. However, if you're given 230000.0, then you can generally assume that it has 7 significant figures.