Javascript roundoff number to nearest 0.5

Question

Can someone give me an idea how can i round off a number to the nearest 0.5.
I have to scale elements in a web page according to screen resolution and for that i can only assign font size in pts to 1, 1.5 or 2 and onwards etc.

If i round off it rounds either to 1 decimal place or none. How can i accomplish this job?

score 254 · Accepted Answer · edited Jun 25 '15 at 15:45

254

Write your own function that multiplies by 2, rounds, then divides by 2, e.g.

function roundHalf(num) {
    return Math.round(num*2)/2;
}

edited Jun 25 '15 at 15:45

saeraphin

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answered May 26 '11 at 11:56

newtron

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Just used this to clean up a reduce function on monetary values that was coming back with like 9 decimal points... (num*100)/100 worked perfectly. – Dustin Kreidler Feb 12 '19 at 16:53
If you want to end up with either 13.0 or 13.5, I combined your answer with the one below: function roundHalf(num) { return (Math.round(num*2)/2).toFixed(1); } – Dan D Jun 29 '19 at 18:04
1

rounding the num*2 is not working for all case..try any decimal like 15.27 => using your formula will give => 15 where in fact it should have returned 15.5. **** I think using toFixed will be better (num*2).toFixed()/2 – sfdx bomb Dec 02 '19 at 16:12
4

@sfdxbomb Have you checked this? In my browser's console `roundHalf(15.27)` returns 15.5 – malarres Aug 28 '20 at 11:04

score 115 · Answer 2 · edited Jan 04 '16 at 12:33

115

Here's a more generic solution that may be useful to you:

function round(value, step) {
    step || (step = 1.0);
    var inv = 1.0 / step;
    return Math.round(value * inv) / inv;
}

round(2.74, 0.1) = 2.7

round(2.74, 0.25) = 2.75

round(2.74, 0.5) = 2.5

round(2.74, 1.0) = 3.0

edited Jan 04 '16 at 12:33

Tunaki

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answered Jan 04 '16 at 12:21

Michael Deal

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2

What does `inv` mean? What's the `inv` variable represents? – Deilan Oct 22 '18 at 12:36
3

@Deilan I'd guess `inverse`. – Alex K Jan 30 '19 at 17:03

score 13 · Answer 3 · edited Sep 15 '20 at 21:34

13

Just a stripped down version of all the above answers:

Math.round(valueToRound / 0.5) * 0.5;

Generic:

Math.round(valueToRound / step) * step;

edited Sep 15 '20 at 21:34

cela

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answered Sep 15 '20 at 19:00

Paul Lee

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score 5 · Answer 4 · answered Sep 04 '19 at 21:20

To extend the top answer by newtron for rounding on more than only 0.5

function roundByNum(num, rounder) {
    var multiplier = 1/(rounder||0.5);
    return Math.round(num*multiplier)/multiplier;
}

console.log(roundByNum(74.67)); //expected output 74.5
console.log(roundByNum(74.67, 0.25)); //expected output 74.75
console.log(roundByNum(74.67, 4)); //expected output 76

score 3 · Answer 5 · edited Jan 23 '17 at 09:38

3

Math.round(-0.5) returns 0, but it should be -1 according to the math rules.

More info: Math.round() and Number.prototype.toFixed()

function round(number) {
    var value = (number * 2).toFixed() / 2;
    return value;
}

edited Jan 23 '17 at 09:38

URL87

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answered Nov 25 '14 at 15:59

Julia Savinkova

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@Yuri To expand on what you're saying, `round` rounds to the next integer greater than the given value, which in terms of negative numbers would be towards the positive integer spectrum. -2.5 would go to -2. Is that correct? – Danny Bullis Nov 23 '15 at 22:44
Yup just verified. `Math.ceil(-1.75) == -1` and `Math.floor(-1.75) == -2`. So for anyone getting tripped up by this, just think of it as `ceil` returns a **greater than** number, `floor` returns a **less than** number. – Danny Bullis Nov 23 '15 at 22:46
Rounding -0.5 does equal 0. – Dan Oct 28 '22 at 20:23

score 2 · Answer 6 · answered May 24 '22 at 07:05

2

The highest voted answer above fails for:

roundHalf(0.6) => returns 0.5
roundHalf(15.27) => returns 15.5

The fixed one is as follows:

   const roundHalf = (num) => {
      return Math.floor(Math.ceil(num * 2) / 2)
    }

answered May 24 '22 at 07:05

Omer Gurarslan

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11
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0.6 should return 0.5 and 15.27 should return 15.5. The question is "nearest" not floor – Dominic Aug 26 '23 at 22:50

score 1 · Answer 7 · answered Jul 04 '18 at 15:44

    function roundToTheHalfDollar(inputValue){
      var percentile = Math.round((Math.round(inputValue*Math.pow(10,2))/Math.pow(10,2)-parseFloat(Math.trunc(inputValue)))*100)
      var outputValue = (0.5 * (percentile >= 25 ? 1 : 0)) + (0.5 * (percentile >= 75 ? 1 : 0))
      return Math.trunc(inputValue) + outputValue
    }

I wrote this before seeing Tunaki's better response ;)

score 1 · Answer 8 · answered May 26 '11 at 12:04

1

var f = 2.6;
var v = Math.floor(f) + ( Math.round( (f - Math.floor(f)) ) ? 0.5 : 0.0 );

answered May 26 '11 at 12:04

Blazes

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if f = 1.9, this will result in v = 1, which is incorrect. – bogatyrjov Apr 01 '14 at 08:09

score 1 · Answer 9 · answered Mar 31 '22 at 15:57

These answers weren't useful for me, I wanted to always round to a half (so that drawing with svg or canvas is sharp).

This rounds to the closest .5 (with a bias to go higher if in the middle)

function sharpen(num) {
  const rem = num % 1
  if (rem < 0.5) {
    return Math.ceil(num / 0.5) * 0.5 + 0.5
  } else {
    return Math.floor(num / 0.5) * 0.5
  }
}

console.log(sharpen(1)) // 1.5
console.log(sharpen(1.9)) // 1.5
console.log(sharpen(2)) // 2.5
console.log(sharpen(2.5)) // 2.5
console.log(sharpen(2.6)) // 2.5

score 0 · Answer 10 · answered Jan 14 '21 at 13:27

As a bit more flexible variation of the good answer above.

function roundNumber(value, step = 1.0, type = 'round') {
  step || (step = 1.0);
  const inv = 1.0 / step;
  const mathFunc = 'ceil' === type ? Math.ceil : ('floor' === type ? Math.floor : Math.round);

  return mathFunc(value * inv) / inv;
}

Javascript roundoff number to nearest 0.5

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