Simpson's Rule Calculator for a Function

Approximate an integral (given by a function) using the Simpson's rule step by step

An online calculator for approximating a definite integral using Simpson's (parabolic) 1/3 rule, with steps shown.

Related calculators: Simpson's Rule Calculator for a Table, Simpson's 3/8 Rule Calculator for a Function

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Simpson's Rule: An Easy Approach to Approximating Integrals


Integrals are important in math, engineering, and science, but solving them can be tough. Luckily, there's a method called Simpson's Rule that helps us estimate integrals. In this article, we'll learn about Simpson's Rule and how it can be used to approximate integrals. This practical approach will make it easier for students to learn and practice integral approximation.

Understanding Simpson's Rule

Simpson's Rule is named after the mathematician Thomas Simpson and is based on the principle of approximating the area under a curve by dividing it into smaller sections and fitting parabolic curves to each section. The rule is an extension of the trapezoidal rule, but it achieves a higher degree of accuracy by using quadratic polynomial approximations. Simpson's Rule is a way to find a good estimate for integrals. Instead of using straight lines like other methods, it uses curved lines called quadratic polynomials. These curves give us more accurate results.

Simpson’s Rule Formula and Calculation Process

Simpson's Rule is derived from the idea that a curve can be approximated using a quadratic polynomial over small intervals. The formula for approximating the definite integral using Simpson's Rule is as follows:

$$\int_a^b f(x) dx \approx \frac{h}{3}\left(f(a)+4f(a+h)+2f(a+2h)+4f(a+3h)+\ldots+2f(a+(n-2)h)+4f(a+(n-1)h)+f(b)\right),$$

where $$$h$$$ represents the step size, $$$n$$$ is the number of intervals, $$$a$$$ is the lower limit, $$$b$$$ is the upper limit, and $$$f(x)$$$ is the function being integrated. The above formula is also known as Simpson’s 1/3 rule.

Since the Simpson’s 1/3 rule uses quadratic polynomials to approximate a function, there is another formula that uses cubic polynomial which is known as Simpson’s 3/8 Rule. It is written as:

$$\int_a^b f(x) dx \approx \frac{3h}{8}\left(f(a)+3f(a+h)+3f(a+2h)+2f(a+3h)+3f(a+4h)+3f(a+5h)+2f(a+6h)+\ldots+3f(a+(n-2)h)+3f(a+(n-1)h)+f(b)\right)$$

The calculation process involves dividing the interval $$$[a,b]$$$ into an even number of subintervals, applying the Simpson's Rule formula to each subinterval, and summing up the results. The more subintervals used, the more accurate the approximation becomes. Online calculators can help to approximate an integral without doing any manual calculations.

Simpson’s Rule Example 1

Let's say we want to find the area under the curve of the function $$$f(x)=x^2$$$ from $$$x=0$$$ to $$$x=2$$$ using Simpson's Rule. We divide the interval $$$[0,2]$$$ into $$$4$$$ equal subintervals, so each subinterval has a width of $$$h=\frac{2-0}{4}= 0.5$$$.

Now we evaluate the function at each endpoint and within the subintervals:


Using the Simpson's Rule formula, we calculate the approximation:

$$\int_0^2 x^2 dx\approx\frac{0.5}{3}\left(0+4(0.25)+2(1)+4(2.25)+4\right)\approx 2.6667$$

So, the area under the curve $$$f(x)=x^2$$$ from $$$x=0$$$ to $$$x=2$$$ is approximately $$$2.6667$$$.

Simpson’s Rule Example 2

We want to approximate the value of the integral $$$\int_0^2\left(x^3+2x\right)dx$$$ using Simpson's Rule.

In this case, we'll divide the interval $$$[0,2]$$$ into four equal subintervals, so $$$n=4$$$. Each subinterval will have a width of $$$h=\frac{2-0}{4}= 0.5$$$.

We need to calculate the values of the function $$$f(x)=x^3+2x$$$ at the endpoints and within the subintervals. Let's compute those values:


Using the values obtained in above, we can now apply the Simpson's Rule formula to approximate the integral:


So, using Simpson's Rule, the value of the integral $$$\int_0^2\left(x^3+2x\right)dx$$$ is approximately $$$5.53125$$$.

Applications of Simpson's Rule

Simpson's Rule finds its application in various fields such as physics, engineering, economics, and more. It enables students to estimate areas under curves and solve complex problems where analytical integration is difficult or impossible.

Some practical applications include calculating the volumes of irregular shapes, approximating definite integrals in physics equations, and estimating the values of integrals in statistics.

Advantages and Limitations

Simpson's Rule offers several advantages over other numerical integration techniques:

  • Increased accuracy

    Simpson's Rule utilizes quadratic approximations, providing a more accurate estimation of the integral compared to simpler methods like the trapezoidal rule.

  • Relatively simple implementation

    While the concept behind Simpson's Rule involves quadratic interpolation, its application is straightforward and can be easily programmed in computer algorithms or calculators.

  • Versatility

    Simpson's Rule can be applied to both smooth and oscillatory functions, making it a versatile tool for a wide range of applications.

However, it is important to note that Simpson's Rule also has its limitations:

  • Inefficient for highly oscillatory functions

    When dealing with functions that exhibit rapid oscillations, Simpson's Rule may require a large number of subintervals to achieve acceptable accuracy, resulting in increased computational time.

  • Unsuitable for improper integrals

    Simpson's Rule is designed for definite integrals over finite intervals and is not directly applicable to improper integrals.


Simpson's Rule is a practical and effective method for approximating definite integrals. Its accuracy, ease of implementation, and versatility make it a valuable tool for students learning about numerical integration.

By understanding the concept, formula, and applications of Simpson's Rule, students can enhance their problem-solving skills in mathematics, engineering, and various scientific disciplines.

With practice and experience, they can utilize this numerical technique to efficiently approximate integrals and gain valuable insights into complex mathematical problems.