Why 0.1 + 0.2 ≠ 0.3 in JavaScript? Understanding Number Precision and Fixes

Most JavaScript developers have encountered floating‑point precision errors, for example console.log(0.1+0.2===0.3) // false. In JavaScript all numbers, both integers and decimals, are represented by the Number type, which follows the IEEE‑754 64‑bit double‑precision format.

Number storage standard

JavaScript Number uses the IEEE‑754 64‑bit double‑precision floating‑point representation. The 64 bits are divided into three fields:

sign: 1 bit

exponent: 11 bits

fraction (mantissa): 52 bits

The exponent range is 0‑2047. When the exponent is 0 or 2047, special meanings apply depending on whether the fraction is zero (see the table). For normal numbers the exponent is stored with a bias of 1023, and the leading mantissa bit is implicit 1. The value of a normal number is:

For subnormal numbers the leading bits are 0 instead of 1 and the bias is 1022, giving the value:

Key Number properties

Understanding the storage format clarifies several important Number properties:

Number.MAX_VALUE – the largest representable number.

Number.MIN_VALUE – the smallest positive subnormal number.

Number.EPSILON – the difference between 1 and the smallest number greater than 1.

Number.MAX_SAFE_INTEGER – the largest integer that can be represented exactly (2^53‑1). Example:

Math.pow(2,54)===Math.pow(2,54)+1 // true

Number.MIN_SAFE_INTEGER – the negative of Number.MAX_SAFE_INTEGER (‑9007199254740991).

Why 0.1 + 0.2 ≠ 0.3

0.1 in binary is 0.0001100110011… (repeating). After rounding to 52 fraction bits the stored value becomes slightly larger. The same occurs for 0.2 and 0.3. Adding the stored binary values of 0.1 and 0.2 yields a binary fraction that rounds to 0.30000000000000004, not exactly 0.3. The following code shows the binary strings and their lengths.

var a = 0.1;
console.log(a.toString(2)); // 0.0001100110011001100110011001100110011001100110011001101
var b = 0.2;
console.log(b.toString(2)); // 0.001100110011001100110011001100110011001100110011001101
var c = 0.3;
console.log(c.toString(2)); // 0.010011001100110011001100110011001100110011001100110011
var d = 0.1 + 0.2;
console.log(d.toString(2)); // 0.0100110011001100110011001100110011001100110011001101
console.log(d.toString(2).length); // 54

The resulting binary sum is rounded to 52 fraction bits, producing the final value 0.01001100…110100, which differs from the binary representation of 0.3.

Floating‑point precision solutions

Depending on the use case, you can mitigate precision loss in several ways:

For display only, use Number.prototype.toFixed together with parseFloat.

function formatNum(num, fixed = 10) {
  return parseFloat(num.toFixed(fixed));
}
var a = 0.1 + 0.2;
console.log(formatNum(a)); // 0.3

For arithmetic, convert operands to integers, perform integer operations, then scale back. Example addition function:

function add(num1, num2) {
  var decimalLen1 = (num1.toString().split('.')[1] || '').length;
  var decimalLen2 = (num2.toString().split('.')[1] || '').length;
  var baseNum = Math.pow(10, Math.max(decimalLen1, decimalLen2));
  return (num1 * baseNum + num2 * baseNum) / baseNum;
}
console.log(add(0.1, 0.2)); // 0.3

Reference material:

https://en.wikipedia.org/wiki/IEEE_754

https://en.wikipedia.org/wiki/Double-precision_floating-point_format

https://en.wikipedia.org/wiki/Normal_number_(computing)

https://en.wikipedia.org/wiki/Denormal_number