Example 1
- Base a: 10
- Base b: 6
- Height: 4
32.00
a = 10, b = 6 and h = 4 gives 32.00.
Trapezoid area calculator
Type the two parallel bases and the height between them. The calculator takes half the sum times h and shows area in cm² or in².
A = ½(a+b)h. A rectangle is on the rectangle area calculator. A parallelogram is on the parallelogram area calculator.
Result
Base a (cm), Base b (cm) and Height h (cm). Area shows up here.
How it works
Trapezoid area is half the sum of two parallel bases times the height between them. The first card example, 10, 6 and 4, is 32.00, because the sum 16 times 4 is 64 and half of that is 32.
The neighboring 10 by 6 rectangle is 60 and does not split a sum. Height must be perpendicular to both bases, not along a leg.
Equal bases 5 and 5 at h = 5 give 25.00. Then the figure behaves like a parallelogram of base 5. That is allowed. A thin strip 7, 3 and 2 gives 10.00, also legal. Side order does not matter: 20 and 10 at h = 6 are the same 90.00 as 10 and 20.
The calculator does not run when a base or the height is missing. An empty b is not a hidden triangle ½ah.
Switching to inches relabels the result as in² and does not convert a typed 10. Slanted legs are not inputs: Pythagoras can give a leg, not this area.
A US school trapezoid has one pair of parallel sides. This calculator does not sort which base is longer. At 15, 5 and 4 you get 40.00. At 9, 6 and 8 you get 60.00. That is still ½(a+b)h, not a times b.
How to use
- Type base a, the longer or the shorter one, in cm or inches from the header.
- Type the second base b, parallel to a, in the same unit.
- Type height h perpendicular between the bases, not the length of a slanted leg.
- Click Calculate. The result uses two decimal places and cm² or in².
- The 10, 6 and 4 example is 32.00. That is not 60 from a 10 by 6 rectangle and not the sum 16 alone.
Formula
A = ½ × (a + b) × h
Not a rectangle a × b.
A of a trapezoid a = 10, b = 6, h = 4
Half the sum of the bases times height. a = 10, b = 6, and h = 4 give 32.00. Not a rectangle a × b.
- A
- Area ½(a+b)h. 10, 6, and 4 give 32.00. 8, 4, and 5 give 30.00. 12, 8, and 3 also 30.00.
- a
- One parallel base. 10 with b = 6 and h = 4 is 32. Two equal 5 and 5 at h = 5 give 25.
- b
- The other base. 6 with a = 10. 4 with a = 8 and h = 5 is 30.00.
- h
- Distance between the bases. 4 with 10 and 6. Not a slanted side.
Examples
Example 2
- Base a: 8
- Base b: 4
- Height: 5
30.00
a = 8, b = 4 and h = 5 gives 30.00.
Example 3
- Base a: 12
- Base b: 8
- Height: 3
30.00
a = 12, b = 8 and h = 3 gives 30.00.
Example 4
- Base a: 5
- Base b: 5
- Height: 5
25.00
equal bases 5 and h = 5 gives 25.00.
Example 5
- Base a: 15
- Base b: 5
- Height: 4
40.00
a = 15, b = 5 and h = 4 gives 40.00.
Example 6
- Base a: 20
- Base b: 10
- Height: 6
90.00
a = 20, b = 10 and h = 6 gives 90.00.
Example 7
- Base a: 7
- Base b: 3
- Height: 2
10.00
a = 7, b = 3 and h = 2 gives 10.00.
Example 8
- Base a: 9
- Base b: 6
- Height: 8
60.00
a = 9, b = 6 and h = 8 gives 60.00.
Frequently asked questions
Is this rectangle area a times b?
No. A rectangle multiplies two sides. A trapezoid adds a second base and halves the sum. 10 by 6 is 60 there, 32 here at h = 4.
Why do 10, 6 and 4 give 32, not 16?
First the bases sum to 16, then times height 4, then half. 16 times 4 is 64, half is 32.00. You do not divide the bases alone.
Can I type a slanted leg as h?
No. h is perpendicular to the bases. A slanted leg is longer and spoils 32.00.
What if I leave b empty?
Nothing is computed. Type the second base or load 10, 6 and 4. Empty b does not become a triangle ½ah.
Are 5 and 5 a mistake because that is a parallelogram?
No. Equal bases are allowed. 5 and 5 at h = 5 give 25.00. The sibling card takes one base and h.
Can I mix centimeters and inches?
Not in one run. All three lengths in cm or all in inches. The switch changes the label and does not scale the numbers.
Does swapping a and b change 90.00?
No. 20 and 10 are the same as 10 and 20. The calculator does not sort the longer base.
Why do 7, 3 and 2 give 10.00?
To show a thin trapezoid. Half of 10 times 2 is legal. The formula does not need a close to b.
Does Pythagoras replace ½(a+b)h?
No. Pythagoras gives a leg. Area still needs two bases and a perpendicular height.
Is 15, 5 and 4 equal to 40.00?
Yes. The sum 20 times 4 is 80, half is 40.00. Another set, 8, 4 and 5, gives 30.00.
Knowledge sources
The calculator computes the same formula as the definition below.
Page updated in 2026.