Simpson's Rule Calculator

Author: Henrick Yau

- 28 September 2026

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Calculate definite integrals numerically using Simpson's Rule. This calculator approximates the integral of a function over a specified interval by fitting parabolic arcs through equidistant points.

Integration Parameters

Function to Integrate:

Use x as the variable. Math functions: sin, cos, tan, log, sqrt, etc.

Lower Bound (a):

Upper Bound (b):

Number of Intervals (n):

Must be an even number for Simpson's Rule

Visualization Options

Show function plot

Show approximation areas

Understanding Simpson's Rule

Simpson's Rule is a numerical approximation method for definite integrals. It approximates the function using quadratic polynomials, which provides better accuracy than simpler methods like the Trapezoidal Rule.

The Formula

\begin{align} \int_{a}^{b} f(x) \, dx \approx \frac{h}{3} \left[ f(x_0) + 4f(x_1) + 2f(x_2) + 4f(x_3) + \ldots + 2f(x_{n-2}) + 4f(x_{n-1}) + f(x_n) \right] \end{align}

Where:

h = (b - a) / n is the width of each subinterval
n is the number of subintervals (must be even)
x_i = a + i·h are the evenly spaced points

Error Term

Simpson's Rule has an error term of O(h⁵), which means the error is proportional to h⁴. Specifically:

\text{Error} \approx -\frac{b-a}{180}h^4 f^{(4)}(\xi)

Where f⁽⁴⁾(ξ) is the fourth derivative of f at some point ξ between a and b.

Comparison with Other Methods

Midpoint Rule: Error is O(h³), less accurate than Simpson's Rule
Trapezoidal Rule: Error is O(h³), less accurate than Simpson's Rule
Simpson's 3/8 Rule: Similar accuracy to Simpson's Rule
Gaussian Quadrature: More accurate for certain function classes

Disclaimer: This calculator provides numerical approximations that may contain errors, especially for functions with singularities or rapid oscillations. Always verify important results with alternative methods or analytical solutions when possible.

What Simpson's Rule does for definite integrals

The Simpson's Rule Calculator is an interactive tool that estimates the value of a definite integral. Instead of solving complex integrals by hand, this calculator applies a reliable numerical method to approximate the area under a curve, known as Simpson's Rule. It is especially useful for functions that are difficult or impossible to integrate analytically.

\[ \int_{a}^{b} f(x) \, dx \approx \frac{h}{3} \left[ f(x_0) + 4f(x_1) + 2f(x_2) + \dots + 2f(x_{n-2}) + 4f(x_{n-1}) + f(x_n) \right] \]

This method divides the interval into an even number of parts and fits parabolas through points on the graph of the function. It delivers better accuracy than the trapezoidal or midpoint rule.

Why Use It?

Whether you are a student, teacher, engineer, or curious learner, the Simpson’s Rule Calculator helps you:

Estimate definite integrals quickly
Visualise how the area under the curve is approximated
Understand the impact of changing interval numbers
Perform error analysis and view convergence behaviour

It also complements other tools like the Integral Calculator for solving definite or indefinite integrals and the Antiderivative Calculator for finding antiderivatives. If you are working with multivariable functions, check out the Partial Derivative Calculator to compute partials or analyse multivariable differentiation.

Entering your function and interval

Follow these simple steps to get an accurate approximation of your definite integral:

Enter the function you want to integrate in the input box (use x as the variable).
Set the lower and upper bounds for the integration interval.
Choose the number of intervals (must be an even number).
Optionally, enable function plotting and approximation visuals.
Click "Calculate Integral" to view the result, plot, and breakdown.

You can reset the calculator at any time using the "Reset" button.

When to approximate area under a curve

Use the Simpson’s Rule Calculator to:

Approximate the area under curves when the exact integral is hard to compute
Compare numerical results with exact solutions from an integral solver
Analyse convergence by increasing intervals
Gain insights into error behaviour across different interval counts

It is especially handy for checking work or supplementing results from tools like the Second Derivative Calculator or the Directional Derivative Calculator in multivariable analysis.

Questions about function input and accuracy

Q: What kind of functions can I input?
Any function using x as the variable. Common expressions include polynomials, trigonometric functions, exponentials, and logs. For example: x^2 + sin(x).

Q: Why must the number of intervals be even?
Simpson’s Rule relies on fitting parabolas across pairs of intervals. An odd number of intervals would break this pairing.

Q: How accurate is this method?
Simpson's Rule is highly accurate for smooth functions and improves with more intervals. The calculator also shows error and convergence information.

Q: What if my function is undefined at some point?
Avoid functions with singularities or discontinuities within the interval. These can cause inaccurate results or evaluation errors.

How this tool supports calculus study

This calculator is a helpful companion for studying calculus and solving real-world problems involving integration. It is part of a broader suite of mathematical tools like the Derivative Calculator, Inverse Derivative Calculator, and Limit Calculator that simplify learning and applying advanced maths concepts.