Newton's method calculator

Enter f(x), a starting guess and a tolerance. The derivative is worked out symbolically, then the iteration x ← x − f(x) / f′(x) runs and each step is listed.

The equation solved is f(x) = 0. For x^2 = 2 enter x^2 - 2.
A number or constant such as 1 or pi/2. Different guesses can find different roots.
Stops when |f(x)| or the size of the step is at most this. For example 1e-10.
Root (approximate)1.414213562
f(root)
4.511e-12
Iterations used
4
Start
x = 1, f(x) = -1
Step 1
x = 1.5, f(x) = 0.25
Step 2
x = 1.416666667, f(x) = 0.006944
Step 3
x = 1.414215686, f(x) = 0.000006007
Step 4
x = 1.414213562, f(x) = 4.511e-12

Runs in your browser: nothing you enter is sent anywhere.

How to use it

  1. Enter f(x) so that the equation is f(x) = 0.
  2. Give a starting guess near the root you want.
  3. Read the root and the step table. Each row shows x and f(x).

How it works

The tool differentiates f symbolically, then repeats x ← x − f(x) / f′(x). It stops when |f(x)| is at most the tolerance, or when a step moves x by no more than the tolerance (relative to 1 + |x|), or when the iteration limit is reached.

Worked example

Solve x² − 2 = 0 from x₀ = 1. Then f′(x) = 2x and:

  • x₁ = 1 − (−1) / 2 = 1.5
  • x₂ = 1.5 − 0.25 / 3 = 1.4166667
  • x₃ = 1.4142157, x₄ = 1.4142136 ≈ √2

Tips

  • Plot or evaluate f first to pick a guess near the root.
  • A guess where f′ is zero or tiny can throw the iteration far away.

Frequently asked questions

How does Newton’s method work?

From a guess x it follows the tangent line of f at x down to the x-axis. The point where the tangent crosses the axis is the next guess: x − f(x) / f′(x). Near a simple root the number of correct digits roughly doubles each step.

Why did it fail?

The method divides by f′(x), so a flat tangent stops it. It can also jump out of the domain (for example into negative numbers for ln or sqrt) or wander for a function with no root near the guess, such as x² + 1, which has no real root.

Which root does it find?

The one the iteration lands on, usually near the starting guess. A function with several roots needs different guesses.

Where does the derivative come from?

From the same symbolic differentiation as the derivative calculator, so you do not type f′(x).

More in Math

All tools in this category: Math ·Updated November 3, 2026

Privacy choices

Google Analytics shows us which tools are used and whether they work. It may set cookies. Your choice is kept in this browser.