The JavaScript Math.floor() method

Q: What is the difference between Math.floor() and Math.round()?

Math.floor() always rounds down; Math.round() rounds to the nearest integer (0.5 rounds up).

Q: What is the difference between Math.floor() and Math.trunc()?

They match on positive numbers. On negatives, floor() rounds down (more negative) while trunc() just removes the decimal (toward zero).

Method JavaScript All modern browsers Updated June 23, 2026

Quick answer

The Math.floor() method rounds a number down to the nearest integer. Math.floor(4.9) is 4, and Math.floor(-4.1) is -5 — it always goes toward negative infinity. It's the rounding partner of Math.random() for generating whole numbers.

Overview

Math.floor() always rounds down to the nearest whole number. 4.9 becomes 4; 4.1 also becomes 4. The decimal part is simply dropped for positive numbers. You'll see it most often paired with Math.random() to turn a random decimal into a random integer.

The subtlety is negatives. "Down" means toward negative infinity, not toward zero, so Math.floor(-4.1) is -5, not -4. If you instead want to chop off the decimal and keep heading toward zero, that's Math.trunc(). The two agree on positive numbers and differ on negatives.

It sits in a family of four: Math.floor() rounds down, Math.ceil() rounds up, Math.round() rounds to the nearest, and Math.trunc() just drops the decimals. Reach for floor when you specifically need the lower bound — page counts, grid positions, splitting items into rows.

Syntax

Math.floor(4.9)   // 4
Math.floor(4.1)   // 4
Math.floor(-4.1)  // -5  (toward negative infinity)

Parameters

The Math.floor() method accepts the following parameters.

Parameter	Description
`x`	The number to round down to the nearest integer.

Example

Live example

<pre id="out" style="font:15px ui-monospace,monospace"></pre>
<script>
  const total = 23, perPage = 10;

  const pages = Math.ceil(total / perPage);   // round up for page count
  const full  = Math.floor(total / perPage);  // full pages only

  document.getElementById('out').textContent =
    'pages needed: ' + pages + '\n' +
    'full pages:   ' + full; // pages needed: 3 / full pages: 2
</script>

Best practices

Use Math.floor() with Math.random() to produce random integers.
For negatives, remember it rounds toward negative infinity — use Math.trunc() to drop decimals toward zero instead.
Pick the right family member: floor (down), ceil (up), round (nearest).
It returns an integer but still a number type — no separate integer type is involved.

Frequently asked questions

What does Math.floor() do?

It rounds a number down to the nearest integer, toward negative infinity. Math.floor(4.9) is 4.

What is the difference between Math.floor() and Math.round()?

Math.floor() always rounds down; Math.round() rounds to the nearest integer (0.5 rounds up).

Why does Math.floor(-4.1) give -5?

Because "down" means toward negative infinity. To drop the decimal toward zero instead (giving -4), use Math.trunc().

What is the difference between Math.floor() and Math.trunc()?

They match on positive numbers. On negatives, floor() rounds down (more negative) while trunc() just removes the decimal (toward zero).

Math.ceil() Math.hypot()

References