CalcaTools

Endpoint Calculator

Finds the missing endpoint of a segment from its midpoint and known endpoint, plus the segment's length and slope.

Last updated: June 2026 · Free · No sign-up required

Enter the midpoint M and the known endpoint A; the tool solves for the other endpoint B.

Results

Enter values above and click Calculate to see your result instantly.

Quick reference

Midpoint MEndpoint AMissing endpoint B
(3, 4)(1, 2)(5, 6)
(0, 0)(2, −3)(−2, 3)
(−1, 5)(4, 5)(−6, 5)
(2.5, 0)(1, −2)(4, 2)

functions Shows the working, not just the answer

For students and teachers, the final number is only half the value. Every math tool here exposes the formula it applied, the intermediate steps, and the rounding rule, so you can follow along, check your homework, or use the answer in a proof or report with confidence.

calculate Accurate to the spec

Calculations use 64-bit floating point with sensible rounding for the domain (currency to 2 decimals, percentages to 4 decimals, algebra to 6 significant figures). Where exact rational arithmetic matters — fractions, factorials, simplification — we use a dedicated BigNumber path so 1/3 + 1/6 returns ½, not 0.49999.

school Free for classroom use

Educators are welcome to link to any math calculator on CalcaTools from a class site, Google Classroom, or worksheet. The pages are mobile-friendly, free, ad-supported (so we can keep them free) and have no sign-up wall — students just click and use them in class or at home.

tips_and_updates Pair with the spoke articles

Below the calculator we link a small set of plain-English explainer pages — "What is a percentage?", "Why does PEMDAS matter?", and so on. They cover the underlying concept in 4–6 short paragraphs. Read those before the calculator if the topic is new, or after if you want the extra context.

Interpretation guide

GivenSolve with
Midpoint + one endpointThis tool: B = 2M − A
Two endpointsMidpoint formula: M = (A + B)/2
Endpoint + length + slopeParametrize along the direction vector
Slope undefinedSegment is vertical (x's are equal)

Formula & methodology

Formula: B = (2xₘ − x₁, 2yₘ − y₁) — double the midpoint, subtract the known endpoint

  1. Enter the midpoint coordinates (xₘ, yₘ).
  2. Enter the endpoint you know, A = (x₁, y₁).
  3. Because the midpoint is the average of the endpoints, doubling it and subtracting A isolates the other endpoint.
  4. Read off B, the full segment length and the slope of the line through the points.

Worked example: midpoint (3, 4), known endpoint (1, 2) → B = (2·3 − 1, 2·4 − 2) = (5, 6); the segment AB is 5.657 units long with slope 1.

Frequently asked questions

How do you find the missing endpoint of a segment?
Double each midpoint coordinate and subtract the known endpoint: B = (2xₘ − x₁, 2yₘ − y₁). With midpoint (3, 4) and endpoint (1, 2), the other endpoint is (5, 6).
Why does the endpoint formula work?
The midpoint is the average of the two endpoints: M = (A + B)/2. Multiplying both sides by 2 and subtracting A solves for B exactly — no guessing involved.
Does it work with negative or decimal coordinates?
Yes. Midpoint (0, 0) with endpoint (2, −3) gives (−2, 3); decimals pass straight through the same arithmetic.
What if my segment is vertical?
Both endpoints share the same x, so the slope is undefined — the calculator reports 'undefined (vertical)' instead of dividing by zero. The endpoint formula itself still works normally.
Can I find an endpoint in 3D?
The same identity holds per coordinate: z₂ = 2zₘ − z₁. Run the calculator on x and y, then apply the same doubling step to z by hand.