How do you find the other endpoint?
Use B = 2M − A for each coordinate.
Endpoint Calculator
Recover the unknown endpoint when you know the midpoint and one end of a segment.
Enter midpoint and known endpoint
Provide M(xm, ym) and known endpoint A(x1, y1).
About this calculator
How to use Endpoint Calculator, plus the formulas and logic behind the results.
Tool description
The midpoint is the average of the endpoints. Solving for the unknown endpoint doubles the midpoint and subtracts the known point.
How to use
- Enter the midpoint coordinates.
- Enter the known endpoint.
- Select Calculate to find the other endpoint.
- Optionally verify that the midpoint of A and B recovers M.
Formulas & reference
xm = (x1 + x2)/2 → x2 = 2xm − x1ym = (y1 + y2)/2 → y2 = 2ym − y1
Check
- M is average of A and B
- B = 2M − A
- Example: M(3,4), A(1,2) → B(5,6)
- Same idea works in 3D with z
Calculation logic
Applies the midpoint inversion formula independently on each coordinate.
Works in 2D with any real coordinates.
Endpoint Calculator FAQ
Common questions about this calculator and how to use the results.
What if A equals M?
Then B also equals M (a degenerate segment of length zero).
Does this work with negatives?
Yes. The algebra is valid for all real numbers.
Is this the midpoint formula?
It is the midpoint formula solved for the unknown endpoint.
Can I use it on a number line?
Yes—apply the same idea with a single coordinate.
How do I verify?
Average A and B; you should recover M.