Endpoint Calculator — Find the Missing Endpoint from a Midpoint
Given the midpoint M of a line segment and one endpoint A, this calculator finds the other endpoint B by applying the midpoint formula in reverse. Enter all four coordinates and get the missing point instantly.
x-coordinate of the missing endpoint
- 1
Double the midpoint X
2 × 4 = 8 - 2
Endpoint X (B)
2 × 4 − 1 = 7Rearranging the midpoint formula: Mx = (Ax + Bx)/2 → Bx = 2·Mx − Ax.
How does this calculator work?
Given midpoint M and endpoint A, the missing endpoint B is found by B = (2·Mx − Ax, 2·My − Ay). This reverses the midpoint formula — each coordinate is solved independently. Enter the four known values and the result plots all three points on an XY diagram.
Formula
How this is calculated
The midpoint formula states that the midpoint M of segment AB has coordinates M = ((Ax + Bx)/2, (Ay + By)/2). Rearranging for the unknown endpoint B gives Bx = 2·Mx − Ax and By = 2·My − Ay. Each coordinate is solved independently, so the calculation is exact regardless of whether the coordinates are integers or decimals.
The calculator also reports the full segment length |AB| (using the distance formula √((Bx−Ax)² + (By−Ay)²)) and the half-length |MB|, which should equal |MA| as a consistency check. The XY plot shows all three points — A, M and B — on the segment so you can verify the geometry visually.
This works only in two dimensions. For 3-D problems (x, y, z), apply the same formula to each coordinate independently.
Frequently asked questions
If M = (mx, my) is the midpoint and A = (ax, ay) is one endpoint, the other endpoint B is (2·mx − ax, 2·my − ay). Each coordinate is simply twice the midpoint coordinate minus the known endpoint coordinate.
Yes. The formula works for any real coordinates — positive, negative, or zero. If the arithmetic produces a negative result, the endpoint simply lies in a different quadrant of the coordinate plane.
The midpoint calculator takes two endpoints and finds M. This calculator reverses the process: given M and one endpoint, it finds the other endpoint. Both use the same underlying midpoint formula, just rearranged.
TG we-Calculate Editorial Team. (2026). Endpoint Calculator — Find the Missing Endpoint from a Midpoint [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/endpoint-calculator
TG we-Calculate Editorial Team. "Endpoint Calculator — Find the Missing Endpoint from a Midpoint." TG we-Calculate. 2026. https://we-calculate.com/calculator/endpoint-calculator.
TG we-Calculate Editorial Team, "Endpoint Calculator — Find the Missing Endpoint from a Midpoint," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/endpoint-calculator
@misc{wecalculate_endpoint_calculator, title = {Endpoint Calculator — Find the Missing Endpoint from a Midpoint}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/endpoint-calculator}}, year = {2026}, note = {TG we-Calculate} }
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