Calculate Trapezoid Area Before You Pour
Use this Trapezoid Concrete Calculator to estimate concrete for trapezoidal trenches, tapered footings, channels, drainage sections, sloped forms, trapezoid-shaped slabs and repeated sections. Enter the two parallel dimensions, the perpendicular height or depth, and the concrete run or thickness to calculate square area, cubic feet, cubic yards, bags and optional material cost.
Choose trapezoid prism/trench, trapezoid slab, metric calculation or multiple identical sections. Results remain hidden until you press Calculate.
Estimate concrete for a trapezoidal cross section extending along a straight length.
This mode assumes the same trapezoid cross section continues along the full entered length. Changing widths or depths should be calculated as separate segments.
Calculate a trapezoid-shaped slab or pad from two parallel plan sides, perpendicular span and slab thickness.
The perpendicular span is the shortest distance between the two parallel sides—not the sloped side length.
Estimate a trapezoidal concrete prism using meters and cubic meters.
Keep every input in meters. The calculator converts the final metric volume to cubic yards for reference.
Estimate several identical trapezoidal prism sections in one combined concrete quantity.
Use this option only when every section has the same cross section and length. Different trapezoid sizes should be calculated separately.
Calculate area from two parallel bases and perpendicular height.
Extend the trapezoid section along a trench, beam or channel run.
Convert the final concrete volume into ready-mix ordering units.
Add local concrete pricing for a basic material-cost estimate.
A trapezoidal concrete section is common where the top and bottom widths are different. Examples can include tapered trenches, drainage channels, sloped footings, beams, retaining-wall bases and custom formwork. The key is to identify the two parallel sides of the trapezoid and the perpendicular distance between them.
Use Trapezoid Prism when the trapezoid is the cross section and the shape extends along a straight length. Use Trapezoid Slab when the trapezoid is the plan-view footprint of a slab. Use Metric Trapezoid for meters and cubic meters. Use Multiple Sections when several identical trapezoidal prisms are required.
Measure the top and bottom widths or the two parallel plan sides.
Use the perpendicular distance between the parallel sides.
Average the two parallel sides, then multiply by trapezoid height.
Multiply cross-section area by run length, or plan area by slab thickness.
Apply a practical ordering allowance after geometric volume is calculated.
The area of a trapezoid is the average of the two parallel sides multiplied by the perpendicular height. If the parallel sides are called A and B and the height is H, the formula is:
A and B must be the two parallel sides. H must be perpendicular to both.
If the trapezoid is a concrete cross section, multiply this area by the length of the section. If the trapezoid is a slab footprint, multiply the area by slab thickness.
Use consistent units before multiplying.
For example, a trench with a 12-inch top width, 24-inch bottom width and 12-inch depth has a cross-sectional area of 216 square inches, or 1.5 square feet. If that section extends 50 feet, the geometric volume is 75 cubic feet, equal to about 2.78 cubic yards before allowance.
Some concrete pads and slabs have a trapezoid-shaped footprint when viewed from above. In that case, calculate the plan area using the two parallel sides and perpendicular span, then multiply by the uniform slab thickness.
Convert slab thickness from inches to feet before calculating cubic feet.
For a slab with 20 ft and 30 ft parallel sides separated by 15 ft, the plan area is 375 ft². At 4 inches thick, the geometric concrete volume is 125 ft³, or about 4.63 yd³.
For rectangular slab calculations, use the Concrete Calculator. For thickness-focused comparisons, use the Concrete Slab Thickness Calculator.
Tapered concrete sections are often easier to estimate as a trapezoid than as several separate shapes. For a trench or footing with a wider bottom than top, the two widths become the trapezoid bases and the vertical depth becomes the trapezoid height. The section length then turns that area into volume.
If the width or depth changes along the run, divide the project into shorter sections and calculate each separately. A single trapezoid prism assumes one constant cross section for the full length.
This can be the top width of a trench, the shorter side of a slab footprint or either one of the two parallel edges.
This is the opposite parallel side. It can be longer or shorter than Side A.
Measure the shortest straight-line distance between the two parallel sides. Do not use a diagonal or sloping side as the trapezoid height.
For a cross-section calculator, use the distance the trapezoid extends. For a plan-area slab calculator, use the concrete thickness instead.
Identify the two parallel bases, perpendicular trapezoid height and the length or thickness that creates volume.
For smaller trapezoidal footings, channels or repairs, the calculator can estimate common bag quantities. It uses generic planning yields of approximately 0.30 ft³ for a 40 lb bag, 0.45 ft³ for a 60 lb bag and 0.60 ft³ for an 80 lb bag.
Actual bag yield can vary by product. Use the manufacturer's stated yield for final purchasing and round up to whole bags.
Enter a local ready-mix price per cubic yard to estimate the concrete-material portion of cost. The calculator multiplies the adjusted cubic-yard quantity by the price entered.
This does not automatically include excavation, forms, reinforcement, subbase, pumping, delivery, short-load fees, finishing, labor or curing.
Trapezoidal forms can vary because slopes, excavation and subgrade are not always perfectly uniform. The calculator includes 0%, 5%, 10% and 15% ordering allowances so you can compare geometric volume with a practical planning quantity.
Use field conditions, form tolerances and supplier ordering increments when selecting an allowance.
The two bases in a trapezoid formula must be parallel.
The trapezoid height must be perpendicular to the parallel sides.
Cross-sectional area alone is not concrete volume. Multiply by the run length.
Convert cross-section dimensions before multiplying by a length measured in feet.
Split changing geometry into multiple sections for a more accurate estimate.
For a trapezoid-shaped slab, calculate plan area first, then multiply once by slab thickness.
The calculator uses the standard trapezoid area formula and rectangular-prism extension. Its arithmetic is accurate for the dimensions entered, but actual concrete use can differ where excavation, side slopes, forms or depths vary from the simplified geometry.
For concrete technical resources, visit the American Concrete Institute and the National Ready Mixed Concrete Association. For other concrete shapes, use the Concrete Calculator.
You can return to the Trapezoid Concrete Calculator above to compare parallel-side dimensions, depths, lengths and ordering allowances.
Common questions about trapezoid area, tapered trenches, trapezoid slabs, cubic yards, concrete bags and material cost.