JavaScript Math.round(), floor(), ceil(), trunc() Compared

The Math object has four ways to turn a decimal into a whole number. They behave differently, especially with negative numbers.

Method	Rule
`Math.round(x)`	Nearest integer (.5 goes up)
`Math.floor(x)`	Down, toward negative infinity
`Math.ceil(x)`	Up, toward positive infinity
`Math.trunc(x)`	Cut off the decimals

script.js

console.log(Math.round(4.5));  // → 5
console.log(Math.round(4.4));  // → 4
console.log(Math.floor(4.9));  // → 4
console.log(Math.ceil(4.1));   // → 5
console.log(Math.trunc(4.9));  // → 4

Negative numbers

This is where they differ.

script.js

console.log(Math.round(-4.5));  // → -4
console.log(Math.floor(-4.1));  // → -5
console.log(Math.ceil(-4.9));   // → -4
console.log(Math.trunc(-4.9));  // → -4

floor always goes down (more negative), while trunc simply drops the decimals. Math.round(-4.5) is -4, because .5 rounds toward positive infinity.

Rounding to decimals

Math.round rounds to whole numbers only. To keep two decimals, multiply, round, divide, or use toFixed() for display.

script.js

const value = 3.14159;

console.log(Math.round(value * 100) / 100); // → 3.14
console.log(value.toFixed(2));              // → 3.14   (a string)

Typical uses

script.js

// How many pages for 23 items at 10 per page? (always round up)
console.log(Math.ceil(23 / 10)); // → 3

// Which row does item index 7 belong to, with 3 per row?
console.log(Math.floor(7 / 3));  // → 2

// Round a percentage
const done = 7, total = 9;
console.log(Math.round((done / total) * 100) + "%"); // → 78%

lightbulb

Pick by intent

Number of pages needed: ceil. Whole units that fit: floor. Nearest value: round. Drop decimals regardless of sign: trunc.