What is the Trapezoid Side Calculator?
A trapezoid (or trapezium) is a four-sided polygon. Its perimeter is simply the sum of all four side lengths. If you know the perimeter and three of the four sides, you can easily find the missing fourth side by subtraction. This calculator does that instantly for you.
How to Use It
Enter the total perimeter (P) of the trapezoid, then enter the three side lengths you already know (a, b, and c). The calculator subtracts the sum of the known sides from the perimeter and returns the length of the unknown side (d). Use the same unit for every value — centimeters, meters, inches, or any consistent length unit.
The Formula Explained
The perimeter of any trapezoid is \(P = a + b + c + d\). Rearranging to solve for the unknown side gives $$d = P - (a + b + c)$$ Because perimeter is just a total of the parts, finding one missing part only requires subtracting the parts you already have.
Worked Example
Suppose a trapezoid has a perimeter of 30 and three known sides of 8, 6, and 10. The sum of the known sides is $$8 + 6 + 10 = 24$$ The unknown side is therefore $$d = 30 - 24 = 6$$ The calculator returns 6 as the missing side length.
FAQ
Can the result be negative? If the result is negative or zero, your inputs are inconsistent — the three known sides already exceed (or equal) the perimeter, which is geometrically impossible for a real trapezoid.
Does it matter which side is unknown? No. The math is the same regardless of which side (a, b, c, or d) is missing — just enter the three sides you know.
Do I need angles or height? No. Perimeter depends only on side lengths, so angles and height are not required for this calculation.
