Scroll Down for instructions and definitions
To use the calculator, you need to know the lengths of all 4 trapezoid sides.
Lines BC and AD are parallel and are called bases.
Lines AB and DC are the non-parallel sides and are called legs.
Lines AC and DB are called diagonals.
The line perpendicular to lines AD & BC is
called the height or altitude.
The line parallel to lines AD & BC, is at the midpoints of lines AB and DC
and is called the median.
The length of the median = (Line AD + Line BC) ÷ 2
* * * * * * * * * E x a m p l e * * * * * * * * *
A trapezoid has bases that are 30 and 55 centimeters in length and the non-parallel sides (or legs) are 15 and 20 centimeters.
(height)2 = (a+b-c+d) • (-a+b+c+d) • (a-b-c+d) • (a+b-c-d) ÷ (4 • (a -c)2)
What is the area of the trapezoid?
Going by the diagram, we shall label the 4 sides as:
a = 55 b = 15 c = 30 d = 20
Before we can use the area formula, we first have to determine the height of the trapezoid.
(height)2 = (55+15-30+20) • (-55+15+30+20) • (55-15-30+20) • (55+15-30-20) ÷ (4 • (55 -30)2)
(height)2 = (60) • (10) • (30) • (20) ÷ (4 • (25)2)
(height)2 = 360,000 ÷ 2,500
(height)2 = 144
height = 12 cm
Now to use the area formula:
trapezoid area = ((sum of the bases) ÷ 2) • height
trapezoid area = ((55 + 30) ÷ 2) • 12
trapezoid area = 510 cm²
To see how to calculate trapezoid area without using formulas, click here.
* * * * * * * * * Trapezoids * * * * * * * * *
ALL TRAPEZOIDS have the following
1) ONE pair of opposite sides are parallel.
(BC and AD)
2) The sum of the angles attached to the same leg = 180°
∠ 'A' plus ∠ 'B' = 180°
∠ 'C' plus ∠ 'D' = 180°
2 special cases of trapezoids are worth mentioning.
The isosceles trapezoid has
both legs of equal length, each pair of angles along each base are equal AND both diagonals
The right trapezoid has
two right angles.
The default setting is for 5 significant figures but you can change that
by inputting another number in the box above.Return To Geometry Index
Answers are displayed in scientific notation and for easier readability, numbers between
.001 and 1,000 will be displayed in standard format (with the same number of
The answers should display properly but there are a few browsers that will show
no output whatsoever. If so, enter a zero
in the box above. This eliminates all formatting but it is better than seeing no
output at all.
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