# math – How to round a number to significant figures in Python

## math – How to round a number to significant figures in Python

You can use negative numbers to round integers:

``````>>> round(1234, -3)
1000.0
``````

Thus if you need only most significant digit:

``````>>> from math import log10, floor
>>> def round_to_1(x):
...   return round(x, -int(floor(log10(abs(x)))))
...
>>> round_to_1(0.0232)
0.02
>>> round_to_1(1234243)
1000000.0
>>> round_to_1(13)
10.0
>>> round_to_1(4)
4.0
>>> round_to_1(19)
20.0
``````

Youll probably have to take care of turning float to integer if its bigger than 1.

%g in string formatting will format a float rounded to some number of significant figures. It will sometimes use e scientific notation, so convert the rounded string back to a float then through %s string formatting.

``````>>> %s % float(%.1g % 1234)
1000
>>> %s % float(%.1g % 0.12)
0.1
>>> %s % float(%.1g % 0.012)
0.01
>>> %s % float(%.1g % 0.062)
0.06
>>> %s % float(%.1g % 6253)
6000.0
>>> %s % float(%.1g % 1999)
2000.0
``````

#### math – How to round a number to significant figures in Python

If you want to have other than 1 significant decimal (otherwise the same as Evgeny):

``````>>> from math import log10, floor
>>> def round_sig(x, sig=2):
...   return round(x, sig-int(floor(log10(abs(x))))-1)
...
>>> round_sig(0.0232)
0.023
>>> round_sig(0.0232, 1)
0.02
>>> round_sig(1234243, 3)
1230000.0
``````