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Trapezoid Area Calculator
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Quickly calculate the area of a trapezoid by entering the top base, bottom base, and height. Supports custom units and precision settings.
Overview
Understand what the tool solves, how it works, and the boundaries of its data.
A trapezoid’s area depends on the lengths of its two parallel sides and the perpendicular distance between them. Enter those measurements as a, b, and h. The slanted sides do not go into this area formula.
A = (a + b) × h ÷ 2
This is also average base length times height: ((a + b) ÷ 2) × h. The result is in square units matching the length unit you used.
For example, bases of 8 ft and 12 ft with a perpendicular height of 5 ft give (8 + 12) × 5 ÷ 2 = 50, or 50 square feet. The bases can be entered in either order because only their sum matters.
The height is the shortest distance between the parallel lines containing the bases: a segment at a right angle to both. In a right trapezoid, a side may also be perpendicular to the bases and equal the height. In a slanted trapezoid, the height is an interior or exterior perpendicular segment, not either angled leg. OpenStax presents the same formula and height relationship in its trapezoid area lesson.
A quick reasonableness check is to compare the answer with rectangles made from each base and the same height. With positive lengths, the trapezoid’s area falls between the shorter-base-times-height and longer-base-times-height values.
Guide
Follow the workflow and verify inputs and outputs with practical examples.
Choose the two sides that run parallel to each other. Their lengths go in the top-base and bottom-base fields; either side can go in either field.
Use a right-angle measurement between the parallel sides for the height. Do not substitute a sloping edge just because it is easier to measure.
Use one consistent unit for all values. The calculator does not provide a unit selector or convert feet, inches, meters, or other units.
The area updates from valid positive values and is displayed to four decimal places. Attach the corresponding square unit yourself: feet become square feet, inches become square inches, and so on.
Whole-number check: use 8, 12, and 5. The mean base is 10, so 10 × 5 = 50; the result appears as 50.0000.
Decimal check: use 3.6, 7.4, and 2.5. The mean base is 5.5, and 5.5 × 2.5 = 13.75, displayed as 13.7500. These examples use the same formula as the calculator.
Use cases
See how the tool fits into real work and everyday tasks.
When a diagram gives two parallel bases and a perpendicular height, enter the three known values and compare the result with your written work. Keep the unit in the final answer squared.
For a clearly trapezoidal panel or flat section on a drawing, calculate its nominal face area from the marked dimensions. Use the result as an area value, not as a material order quantity that accounts for waste.
If a flat garden bed or craft piece is well modeled by a trapezoid, calculate its area after measuring the parallel edges and perpendicular spacing in one unit.
Q&A
Find concise answers to common questions and confusing cases.
No. Height is perpendicular to the parallel bases. A sloping leg is a different measurement and cannot replace it in this formula.
No. The calculation adds the two base values, so switching their fields does not change the area.
All three fields need values that parse as numbers greater than zero. Check for an empty field, zero, or a negative value.
No. Convert all three lengths to the same unit before entering them; otherwise the arithmetic combines incompatible measurements.
Notes
Review scope, result limitations, and important precautions before use.
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