Trapezoid Side Calculator — Find a Missing Side
Solve for an unknown side of a trapezoid in two ways: find the missing leg when you know the perimeter and three sides, or find the missing base when you know the area, height, and one base.
Find which missing side?
d = P − a − b − c
- 1
Sum of the known sides (a + b + c)
10 + 6 + 5 = 21 - 2
Missing leg d = P − (a + b + c)
26 − 21 = 5
How does this calculator work?
Missing leg: d = P − a − b − c (perimeter minus the other three sides). Missing base: a = 2 × Area / h − b (rearrange the area formula). Pick the mode that matches what you know, enter the values, and the unknown side is computed instantly.
Formula
How this is calculated
A trapezoid has four sides: two parallel bases (a and b) and two non-parallel legs (c and d). If you know three of those sides and the total perimeter, the fourth side follows directly from P = a + b + c + d, rearranged as d = P − a − b − c. This is the "missing leg" mode.
Alternatively, if you know the area, the perpendicular height between the bases, and one of the bases, the other base comes from rearranging the area formula: Area = ½ × (a + b) × h → a = 2 × Area / h − b. This is the "missing base" mode. You might use it when you measured the area by another method (e.g., from a land survey) and need the length of one parallel side.
Both modes also report the midsegment = (a + b) / 2 as a bonus. In the missing-leg mode, if the two legs happen to be equal (isosceles trapezoid), the height is also derived using the Pythagorean theorem: h = √(c² − ((a − b)/2)²).
Frequently asked questions
Whenever you know the perimeter and three of the four sides. Subtract the three known sides from the perimeter: d = P − a − b − c. The result must be positive for the geometry to be valid.
When you know the area, the perpendicular height, and one parallel base. Rearranging the area formula gives a = 2 × Area / h − b. Commonly used in land-surveying and architecture.
If the computed missing side is zero or negative, the given inputs are geometrically impossible — for example, the perimeter is smaller than the sum of the three known sides, or the area and height imply a base shorter than zero.
Also known as
TG we-Calculate Editorial Team. (2026). Trapezoid Side Calculator — Find a Missing Side [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/trapezoid-side-calculator
TG we-Calculate Editorial Team. "Trapezoid Side Calculator — Find a Missing Side." TG we-Calculate. 2026. https://we-calculate.com/calculator/trapezoid-side-calculator.
TG we-Calculate Editorial Team, "Trapezoid Side Calculator — Find a Missing Side," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/trapezoid-side-calculator
@misc{wecalculate_trapezoid_side_calculator, title = {Trapezoid Side Calculator — Find a Missing Side}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/trapezoid-side-calculator}}, year = {2026}, note = {TG we-Calculate} }
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