# Endpoint Calculator

URL: https://speedycalculator.com/calculators/math/endpoint/
Last updated: 2026-10-30

> The endpoint calculator finds the missing endpoint B of a segment when you know the other endpoint A and the midpoint M.

## Inputs

- Starting point A – x
- Starting point A – y
- Midpoint M – x
- Midpoint M – y

## Results

- Endpoint B – x₂
- Endpoint B – y₂
- Length of AB

## Formula

```
x₂ = 2x − x₁,  y₂ = 2y − y₁
```

## Explanation

If the midpoint is halfway, the missing end is as far beyond the midpoint as the known end is before it. This is the midpoint formula solved backwards.

### How to use the Endpoint Calculator

- **Starting point A – x**
- **Starting point A – y**
- **Midpoint M – x**
- **Midpoint M – y**

Results update as you type, and everything is calculated in your browser.

### How it works

Each coordinate of the midpoint is the average of the endpoints, so the missing coordinate is twice the midpoint minus the known coordinate.

### Inputs explained

FieldWhat to enterStarting point A – xA numberStarting point A – yA numberMidpoint M – xA numberMidpoint M – yA number

### What each result means

- **Endpoint B – x₂** = 2×[Midpoint M – x] - [Starting point A – x]
- **Endpoint B – y₂** = 2×[Midpoint M – y] - [Starting point A – y]
- **Length of AB** = 2×sqrt(([Midpoint M – x] - [Starting point A – x])^2 + ([Midpoint M – y] - [Starting point A – y])^2)

### Example values

How the main result changes when **Starting point A – x** changes and everything else stays at the example values above:

Starting point A – xEndpoint B – x₂191.58.5282.57.53746

## Worked example

Inputs — Starting point A – x: 2; Starting point A – y: 3; Midpoint M – x: 5; Midpoint M – y: -1.

Results — **Endpoint B – x₂ = 8**; Endpoint B – y₂ = -5; Length of AB = 10.

## Key terms

- **Midpoint**: The point halfway along a segment.

## FAQ

**How do I find an endpoint from a midpoint?**
Double the midpoint and subtract the known endpoint, coordinate by coordinate.

**Does it work in 3D?**
Yes: z₂ = 2z − z₁.

**What is the length of the segment?**
Twice the distance from A to M.

**Can the endpoint have negative coordinates?**
Of course.


## Sources

- [OpenStax – College Algebra 2e](https://openstax.org/details/books/college-algebra-2e)
- [OpenStax – Precalculus 2e](https://openstax.org/details/books/precalculus-2e)