Given a number x and a number n, round number x to n significant figures and output the result.

Significant figures

The significant figures of a number are digits that carry meaning contributing to its measurement resolution. This includes all numbers except leading zeroes.

Bear in mind that leading zeroes after a decimal point are still insignificant figures.

When rounding a digit, you must round away from zero if the following digit is greater or equal than five.

All trailing zeroes after a decimal point are counted as significant.


The first number will be x, the number to be rounded. The second number will be n, the number of significant figures you should round x to.

x will be a number (your code should handle both integers and floating points) between -1,000,000,000 and 1,000, 000,000 inclusive. n will be a positive integer between 1 and 50 inclusive. n will never be greater than the nunber of digits in x.

The input will never be 0 or any form of 0, e.g. 0.000 or 000.


Inputs: 2.6754, 2
Output: 2.7

An output of 2.7000 would be invalid because the trailing zeroes after the decimal point are counted as significant figures.

Inputs: 0.00034551, 4
Output: 0.0003455

Inputs: 50237.1238, 3
Output: 50200

Note that this must not have a decimal point.

Inputs: 2374905, 1
Output: 2000000

Inputs: 543.0489, 4
Output: 543.0

Inputs: 15, 1
Output: 20

Inputs: 520.3, 3
Output: 520

If you wish, you can output 520. instead but not 520.0.

Inputs: -53.87, 2
Output: -54

Inputs: 0.0999, 2
Output: 0.10


Built-in functions and libraries which allow you to round a number to n significant figures are disallowed.


The shortest code in bytes wins.

  • 4
    \$\begingroup\$ For Inputs: 520.3, 3, isn't the decimal point in the answer 520. crucial? \$\endgroup\$ Sep 16, 2016 at 18:10
  • 5
    \$\begingroup\$ @GregMartin I believe that it is, as that's the only thing that makes it have 3 sig figs vs. 2 \$\endgroup\$
    – Suever
    Sep 16, 2016 at 18:12
  • 3
    \$\begingroup\$ @BetaDecay No it's not. The decimal point would be required for that. \$\endgroup\$
    – mbomb007
    Sep 16, 2016 at 18:16
  • 3
    \$\begingroup\$ "200 is considered to have only ONE significant figure" - chemistry.bd.psu.edu/jircitano/sigfigs.html \$\endgroup\$
    – mbomb007
    Sep 16, 2016 at 18:17
  • 4
    \$\begingroup\$ @DLosc That's why if that was the result you'd actually write it as 2.0 x 10^2, showing the 2 sigfigs. \$\endgroup\$
    – mbomb007
    Sep 16, 2016 at 19:13

3 Answers 3


PHP, 130 Bytes


PHP, 133 Bytes works with values <1 for the significant figures


PHP, 56 Bytes works but skip unneccessary Zeros


Someone has stolen or deleted the round function in PHP! To make the challenge more interesting. 127 Bytes

  • \$\begingroup\$ It skips the unnessary Zeros too. It is a version where I use not the native round function in PHP only for joke. It belongs to the 56 Byte Version which I would prefer \$\endgroup\$ Sep 19, 2016 at 19:59
  • \$\begingroup\$ Ok if not use library round .... \$\endgroup\$
    – user58988
    Dec 25, 2018 at 10:17

Python 3, 83 bytes

(similar to the PHP answer)

from math import *
def s(x,n):
 return y*round(x/y)

Test cases:

tests = [(2.6754,2), (0.00034551, 4), (50237.1238, 3),
        (2374905, 1), (543.0489, 4), (15, 1), (520.3, 3), (-53.87, 2)]

print ([s(x,n) for x,n in tests])


[2.7, 0.0003455, 50200, 2000000, 543.0, 20, 520, -54]

Apart from being slightly longer, another approach that I considered:

from math import *
def s(x,n):
 return "%.*f"%(n-z,10**z*round(x/10**z,n))

... produces an incorrect output for the input of (15, 1):

['2.7', '0.0003455', '50200', '2000000', '543.0', '10', '520', '-54']

... due to floating point imprecision in the round() function. It seems likely to me that I could find test cases that would break the "round to zero decimal places" method also if I looked hard enough.

Thus, it seems to me that my solution probably isn't 100% correct for all cases and wouldn't be unless it was computed in decimal. This issue may therefore affect solutions in any language that use FP arithmetic.

  • \$\begingroup\$ Save some bytes by putting the body of s on the same line, then using semicolons. def s(x,n):y=10**(ceil(log10(abs(x)))-n);return y*round(x/y) \$\endgroup\$
    – Cyoce
    Sep 19, 2016 at 19:13
  • \$\begingroup\$ Also, you can remove the space in import * to make it import* \$\endgroup\$
    – Cyoce
    Sep 19, 2016 at 19:18
  • \$\begingroup\$ your answer for me is not ok because the rules say :"Built-in functions and libraries which allow you to round a number to n significant figures are disallowed." And you use round function with n = 0 \$\endgroup\$
    – user58988
    Dec 25, 2018 at 10:14
  • \$\begingroup\$ @RosLuP: The round() function rounds to n decimal places, not to n significant figures, so it was allowed for this golf event. \$\endgroup\$
    – Simon
    Dec 25, 2018 at 21:30

Batch, 660 652 bytes

@echo off
set m=%1.
set s=
if %m:~,1%==- set s=-&set m=%m:~1%
if %m:~,1%==0 set m=%m:~1%&goto m
set d=%m:.=%
if %d:~,1%==0 set d=%d:~1%&goto d
for /l %%i in (1,1,%2) do call set d=%%d%%0
call set r=%%d:~%2,1%%
call set d=%%d:~,%2%%
if %r% leq 4 goto r
set r=
set r=%i:~-1%%r%
set d=%d:~,-1%
if %i% leq 9 set d=%d%%r%&goto r
if not "%d%"=="" goto i
set d=1%r:~1%
set m=1%m%
set m=%m:1.0=.%
if %m:~,2%==.0 set m=%m:.0=.%&set d=0%d%&goto r
set i=0
set p=.
if %m:~,1%==. echo %s%%i%%p%%d%&exit/b
if %i%==0 set i=
if "%d%"=="" set d=0&set p=
set i=%i%%d:~,1%
set d=%d:~1%
set m=%m:~1%
goto l

Explanation: Starts by suffixing a . to the parameter in case it doesn't have one already, then trims off the sign (which is saved) and any leading zeros. The resulting variable m is saved for later because it will tell us the desired magnitude of the result. Any .s are then removed, which could result in further leading zeros, so they are removed too. n zeros are suffixed to ensure that there are enough digits to round, then the nth and first n digits are extracted. If the nth digit is not 4 or less then we tediously add 1 to the string. If the string overflows then we increase the magnitude by prefixing a 1, but if it was originally less than 0.1 we do that by removing the 1 we just added and also a 0 after the decimal point. If the magnitude is still less than 1 then we copy the zeros after the decimal point to the result, however if it is 1 or more then we extract the integer portion of the answer, adding extra zeros if necessary to reach the decimal point (which is then deleted as it would show incorrect precision). Finally the sign, integer portion, decimal point and decimal portion are concatenated.


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