Endpoint Calculator — Find the Missing Endpoint of a Line Segment

Endpoint Definition in Geometry

In geometry, an endpoint is a point that lies at the end of a line segment. Every line segment has exactly two endpoints — unless it degenerates into a single point. We typically label them A and B, and the midpoint M sits exactly halfway between them.

The Endpoint Calculator solves the reverse midpoint problem: given one endpoint A and the midpoint M, it finds the missing endpoint B.

Endpoint Formula

The midpoint formula states:

M = ((xA + xB) / 2, (yA + yB) / 2)

Rearranging for the missing endpoint B:

xB = 2 × Mx − xA

yB = 2 × My − yA

How to Find the Endpoint

1. Enter the coordinates of the starting point A (x₁, y₁).
2. Enter the coordinates of the midpoint M (m_x, m_y).
3. Optionally select a unit system — metric (m, km, cm, mm) or US/Imperial (ft, in, mi, yd).
4. Click Calculate to instantly find endpoint B and the total segment length.

Example: Using the Missing Endpoint Calculator

Suppose starting point A = (2, 3) and midpoint M = (5, 8). Find endpoint B:

• B_x = 2 × 5 − 2 = 8
• B_y = 2 × 8 − 3 = 13
• Endpoint B = (8, 13)
• Distance |AB| = √((8−2)² + (13−3)²) = √(36 + 100) = √136 ≈ 11.66

Unit Systems

This calculator supports both the metric system (meters, kilometers, centimeters, millimeters) and the US/Imperial system (feet, inches, miles, yards). Select your preferred unit from the dropdown — the unit label is applied to the results for clarity.

FAQs

What is the endpoint of a line segment?

An endpoint is one of the two points that define a line segment. Together with the other endpoint, it fixes both the position and the length of the segment.

Can I use this calculator with negative coordinates?

Yes. The formula works for any real-valued coordinates, including negative numbers and decimals.

Is the endpoint formula different from the midpoint formula?

They are inverses of each other. The midpoint formula averages two endpoints to find the middle. The endpoint formula doubles the midpoint and subtracts the known endpoint to recover the missing one.

Does this calculator work in 3D?

This version works in 2D (x, y). For three-dimensional problems, apply the same formula to the z-coordinate separately.