What's difference between significant figures and decimal places

I do know how to do them, but i seem to get the same answers like 8.6 to one decimal place is 9 isnt it? and to 1 sf its also 9 so what actually is the difference what do you have to remember so you dont get mixed up?

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Reply 1

Jelephant

Well to one decimal place it is 8.6 :P

The number of decimal places is the amount of digits after the decimal point. The number of significant figures is the amount of digits in total, not including any zeros before the decimal point i.e. 0.009 has three significant figures ("009")

Reply 2

MeepMeep09

Decimal places you only count the digits after the "." before you round

Significant figures you count from the start of the number unless its a 0. number in which case you start from the first digit other than zero before you round

So take 1706.9438

to 3 decimal places - 1706.9438 would be 1706.944
to 3 significant figures - 1706.9438 would be 1710

Reply 3

Alex-R

To one decimal place means that there is one decimal place in your answer so
9.12
Becomes
9.1

Significant figures is how many numbers are there, regardless if they are decimals
1.23
To 2 significant figures is 1.2
0 is not counted as a significant figure unless its after a significant figure so
0.0124
to 2 significant figures is not 0.0 but 0.012

Reply 4

Alex-R

Jelephant

0.009 has three significant figures ("009")

Really? Isn't that 3 decimal places and 1 significant figure.

Reply 5

130ss

sig figs is number before decimal.. so 100 is sig fig
100.053- decimal places is after the DOT.

Reply 6

MeepMeep09

Alex-R

Really? Isn't that 3 decimal places and 1 significant figure.

That's what I would say too...

Reply 7

paddyman4

134.567

To 2 decimal places is: 134.56
To 2 significant figures is: 130

Reply 8

paddyman4

Jelephant

The number of significant figures is the amount of digits in total, not including any zeros before the decimal point i.e. 0.009 has three significant figures ("009")

No that has 1 significant figure. You start counting when you hit the first non zero. So 0.009057 to 3 sig fig is 0.00906.

Reply 9

dbmag9

paddyman4

134.567

To 2 decimal places is: 134.56
To 2 significant figures is: 130

This, but to clarify; 0.000134567

to 2 decimal places is: 0.00
to 2 significant figures is: 0.00013

Reply 10

Icy_Mikki

Jelephant

not including any zeros before the decimal point i.e. 0.009 has three significant figures ("009")

That's wrong- 0.009 is one significant figure.

"Round to 5 decimal places" means you should round whatever number you've been given so that there are (a maximum of) 5 digits after the decimal point.

"Round to 5 significant figures" means you should, essentially, imagine the decimal point was to the left of the first non-zero digit, and then round to 5 decimal places.

Reply 11

username234311

paddyman4

134.567

To 2 decimal places is: 134.56
To 2 significant figures is: 130

I get it now, but surely to 2 decimal places 134.567 is 134.57 ???

Reply 12

Jelephant

Ah yeah, see what you mean guys, thinking to fast for my own good :P

Reply 13

paddyman4

krussel

I get it now, but surely to 2 decimal places 134.567 is 134.57 ???

yeah my bad

im learnin eh

Generally in an exam, or anything for that matter, round to something like 3dp rather than 3sfs. Also, just to make it clear, for sfs, you only count from the first digit unless it's a 0. And after that first digit you count all the digits including 0.

for example.
0.0005275045490452
to 8 sfs us
0.00052750455. (notice how you include the 0 in the middle of that number to be the 5th significant figure)

Reply 16

username219321

decimal places is after the dot, significant figures is figures after (and including) the first none-zero figure

Reply 17

xoe.xox

yeah it is because the three zeros arent worth anything so the first sig fig is nine...

Reply 18

Notnek

Original post by krussel

Reply 19

DFranklin

Original post by notnek

Something to add to what other people have said:

When rounding using sig figs or decimal places, always check that your answer makes sense i.e. check that the number has actually been rounded. If your answer is completely different to the original number then you've probably made a mistake.

E.g. Round 54 to one significant figure. Answer : 5
Round 0.0034 to two significant figures. Answer 0.00However, note that
0.0034 to 2 decimal places is 0.00. So:

Completely different number when using sig figs means you have almost certainly made a mistake.
Completely different number when using decimal places may actually be correct. (Other than the decision to use decimal places, which was probably a bad choice).

This is particularly relevant when considering a previous comment saying "Generally ... round to something like 3dp rather than 3sfs. " which I would 100% disagree with.

Although since the thread is 6 years old I suspect we are talking to ourselves here...