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Interval Notation Calculator

Convert between interval notation, inequality notation, and set-builder notation. Calculate interval properties.

Reviewed by Christopher FloiedPublished Updated

This free online interval notation calculator provides instant results with no signup required. All calculations run directly in your browser — your data is never sent to a server. Enter your values below and see results update in real time as you type. Perfect for everyday calculations, homework, or professional use.

Left endpoint of the interval

Right endpoint of the interval

Results

Length of interval

1

Midpoint

0.5

Radius

0.5

How to Use This Calculator

1

Enter your input values

Fill in all required input fields for the Interval Notation Calculator. Most fields include unit selectors so you can work in your preferred unit system — metric or imperial, whichever matches your problem.

2

Review your inputs

Double-check that all values are correct and that you have selected the right units for each field. Incorrect units are the most common source of calculation errors and can produce results that are off by factors of 2, 10, or more.

3

Read the results

The Interval Notation Calculator instantly computes the output and displays results with units clearly labeled. All calculations happen in your browser — no loading time and no data sent to a server.

4

Explore parameter sensitivity

Try adjusting individual input values to see how the output changes. This is a quick and effective way to develop intuition about how different parameters influence the result and to identify which inputs have the largest effect.

When to Use This Calculator

  • Use the Interval Notation Calculator when you need a quick mathematical result without writing out all the steps manually, saving time on repetitive calculations.
  • Use it to verify hand calculations on tests or assignments and catch arithmetic mistakes.
  • Use it when teaching or explaining mathematical concepts to others, demonstrating how changing inputs affects the result.
  • Use it to explore the behavior of mathematical functions across a range of inputs.

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About Interval Notation Calculator

The Interval Notation Calculator works with intervals on the real number line, converting between different representations and computing key properties. Interval notation is a concise way to describe sets of real numbers, using brackets [ ] for included endpoints and parentheses ( ) for excluded endpoints. This notation is essential in calculus (domains, ranges, continuity intervals), analysis (open and closed sets), optimization (feasible regions), and probability (continuous distributions). The calculator computes the interval's length, midpoint, and radius, and helps translate between interval notation (like [2, 5)), inequality notation (like 2 ≤ x < 5), and set-builder notation (like {x | 2 ≤ x < 5}). Understanding intervals is foundational for working with functions, solving inequalities, and studying topology.

The Math Behind It

An interval is a connected subset of the real numbers. The four basic types are: closed [a, b] = {x : a ≤ x ≤ b}, open (a, b) = {x : a < x < b}, half-open [a, b) = {x : a ≤ x < b} and (a, b] = {x : a < x ≤ b}. Unbounded intervals use infinity: [a, ∞) = {x : x ≥ a}, (-∞, b] = {x : x ≤ b}, and (-∞, ∞) = all real numbers. Infinity is always paired with an open parenthesis because infinity is not a number and cannot be included. Key properties of an interval [a, b] include: length = b - a, midpoint m = (a + b)/2, and radius r = (b - a)/2. The interval can be expressed using absolute value: |x - m| ≤ r (closed) or |x - m| < r (open). Operations on intervals include: union (A ∪ B), intersection (A ∩ B), and complement (Aᶜ). Intervals are fundamental in the epsilon-delta definition of limits, in defining continuity and differentiability, and in the intermediate value theorem. In topology, open intervals form the basis for the standard topology on the real line, and understanding whether endpoints are included or excluded affects limit existence, integrability, and function behavior.

Formula Reference

Interval Notation

[a, b] or (a, b) or [a, b) or (a, b]

Variables: [ ] = included (closed), ( ) = excluded (open)

Absolute Value Form

|x - m| ≤ r ↔ [m-r, m+r]

Variables: m = midpoint, r = radius

Worked Examples

Example 1: Convert inequality to interval

Express -3 < x ≤ 7 in interval notation

Step 1:-3 is excluded (strict inequality <) → open parenthesis (
Step 2:7 is included (≤) → closed bracket ]
Step 3:Interval: (-3, 7]
Step 4:Length: 7 - (-3) = 10, Midpoint: (-3+7)/2 = 2

(-3, 7], length = 10, midpoint = 2

Example 2: Absolute value to interval

Express |x - 4| ≤ 3 as an interval

Step 1:|x - 4| ≤ 3 means -3 ≤ x - 4 ≤ 3
Step 2:Add 4: 1 ≤ x ≤ 7
Step 3:Interval: [1, 7]
Step 4:Midpoint: 4, Radius: 3

[1, 7]

Common Mistakes & Tips

  • !Using brackets [ ] with infinity: always write (-∞, a) or (b, ∞) with parentheses at infinity
  • !Confusing open and closed: < and > use parentheses ( ), while ≤ and ≥ use brackets [ ]
  • !Writing the bounds in wrong order: always lower bound first, upper bound second
  • !Forgetting that a single point {a} can be written as [a, a] (a degenerate interval)

Related Concepts

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Frequently Asked Questions

What is the difference between (a, b) as an interval and as a point?

Context determines the meaning. In interval notation, (a, b) represents all numbers between a and b. As an ordered pair, (a, b) represents a point in the coordinate plane. The context of the problem (number line vs. coordinate plane) clarifies which is intended.

Can intervals contain a single point?

Yes. The degenerate interval [a, a] = {a} contains exactly one point. The empty interval (a, a) = {} contains no points. Half-open intervals [a, a) and (a, a] are also empty.

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