JavaScript Number toPrecision() Method

What You’ll Learn

Number.prototype.toPrecision() formats a number to a chosen count of significant digits (same API as MDN Number.toPrecision). It may use plain or exponential form. This tutorial covers syntax, when expo appears, comparisons, five examples, and try-it labs.

Introduction

Significant digits measure how many meaningful figures you keep from the start of a number — not how many places sit after the decimal point. That is why (5.123456).toPrecision(2) is "5.1", while (1234.5).toPrecision(2) becomes "1.2e+3".

Use toPrecision() when you care about overall precision (science, sensors, rounded reports) more than a fixed UI decimal width.

This page is part of JavaScript Number Methods. Related formatters include toFixed(), toExponential(), and toString().

Understanding the toPrecision() Method

Call number.toPrecision() or number.toPrecision(precision) on a number primitive or Number object. You always get a string; the number itself does not change.

Omit precision and the result matches toString(). Pass a value from 1 to 100 to round to that many significant digits. Depending on the magnitude, the engine may print a plain string or switch to exponential notation.

📝 Syntax

General form of Number.prototype.toPrecision:

number.toPrecision()
number.toPrecision(precision)

Parameters

• precision (number, optional) — integer from 1 to 100. Count of significant digits. If omitted, behaves like toString().

Return value

Always a string. Special cases:

Exceptions

• RangeError — precision < 1 or > 100.
• TypeError — called on a non-Number this value (for example a plain object).

Common patterns

(5.123456).toPrecision();     // "5.123456"
(5.123456).toPrecision(5);    // "5.1235"
(5.123456).toPrecision(2);    // "5.1"
(1234.5).toPrecision(2);      // "1.2e+3"
(123.456).toPrecision(4);     // "123.5"

Examples Gallery

Examples follow MDN patterns. Use View Output or Try It Yourself for each case.

(5.123456).toPrecision();  // "5.123456"

5.123456

(5.123456).toPrecision(5);  // "5.1235"

5.1235

(5.123456).toPrecision(2);  // "5.1"

5.1

(1234.5).toPrecision(2);  // "1.2e+3"

1.2e+3

(123.456).toPrecision(4);  // "123.5"

123.5

🚀 Common Use Cases

• Science & engineering — report measurements with a fixed figure count.
• Sensor / telemetry UIs — keep noisy floats to a readable precision.
• Reports & exports — consistent significant-digit text across scales.
• Teaching precision — demonstrate significant figures live in the browser.
• Adaptive display — let the engine pick plain vs expo for the magnitude.
• Not fixed money columns — prefer toFixed(2) or Intl.NumberFormat for prices.

📝 Notes

• Does not change the number — returns a new string.
• precision must be 1–100 (unlike toFixed, which allows 0).
• Omitting precision matches toString() behavior.
• Exponential form appears when the exponent ≥ precision or < -6.
• Floating-point limits still apply — very high precision can show binary noise.
• For locale-aware formatting, use toLocaleString() / Intl.NumberFormat.

💡 Best Practices