Guides

Ordering the right amount of concrete takes three measurements and a few lines of arithmetic. This guide shows how to work out the volume of a rectangular slab, how much extra to allow, and how to turn the result into cubic yards or bags.

What to measure

• Length of the slab in feet.
• Width of the slab in feet.
• Thickness in inches. A 4 inch thickness is common for light-duty slabs, but use the thickness on your plan or from your contractor.

Measure the inside of the forms, because that is the space the concrete will fill.

The formula

• Thickness in feet = thickness in inches ÷ 12
• Cubic feet = length × width × thickness in feet
• Cubic yards = cubic feet ÷ 27, because a cubic yard is 3 ft × 3 ft × 3 ft = 27 cubic feet

Worked example

These figures are made up to show the arithmetic. A slab is 12 ft long, 10 ft wide and 4 in thick.

• Thickness in feet = 4 ÷ 12 = 0.333 ft
• Cubic feet = 12 × 10 × 0.333 = 40 cubic feet
• Cubic yards = 40 ÷ 27 = 1.48 cubic yards

Adding an allowance

Ground is rarely perfectly level, and some concrete is lost to spills. Many people add an extra percentage to the volume. You choose the number. With a 10% allowance in the example, 40 × 1.10 = 44 cubic feet, and 44 ÷ 27 = 1.63 cubic yards.

Turning the result into an order

• Ready-mix concrete is usually sold by the cubic yard. Ask your supplier about minimum loads, short-load fees and delivery charges before you order.
• For bagged concrete, the yield of one bag is printed on its label. Divide your cubic feet by the yield and round up to a whole bag.
• Round up rather than down. Running short in the middle of a pour is harder to fix than having a little left over.

Common mistakes

• Entering thickness in feet when the calculator expects inches, which makes the answer 12 times too large or small.
• Measuring the outside of the forms instead of the inside.
• Forgetting to multiply by the number of identical slabs.
• Skipping an allowance on uneven or soft ground.

Use the calculators

The Concrete Slab Calculator does these steps for a rectangular slab. For footings, walls or round columns, use the Concrete Calculator. You can browse every tool in Construction & Home Improvement Calculators.

Results are estimates. Check quantities with your supplier or a qualified professional before you order. See the Disclaimer.

Roof area is larger than the footprint of the house because a sloped roof covers more surface than the ground beneath it. The steeper the pitch, the bigger the difference. This guide shows the formula, a worked example, and how to turn the result into roofing squares and bundles.

What to measure

• The length and width of the roof footprint, in feet, including overhangs.
• The pitch, written as rise over run (for example 6/12 means 6 inches of rise for every 12 inches of horizontal run).

The formula

Pitch multiplier = square root of (1 + (rise ÷ 12)²). Roof area = footprint area × pitch multiplier.

Worked example

A 40 ft by 30 ft footprint has an area of 1,200 sq ft. At a 6/12 pitch, the multiplier is the square root of (1 + 0.5²) = 1.118. Roof area = 1,200 × 1.118 = about 1,342 sq ft.

Squares and bundles

Roofing is sold by the square, which is 100 sq ft. 1,342 sq ft is about 13.4 squares. Most asphalt shingles come three bundles to the square, so that is about 41 bundles before waste.

Adding an allowance for waste

Hips, valleys and cut edges create waste. A common allowance is 10 percent for a simple gable roof and more for complex roofs. At 10 percent, 13.4 squares becomes about 14.8 squares, or about 45 bundles.

Common mistakes

• Measuring the footprint without the overhangs.
• Using the pitch angle in degrees in a formula built for rise over run.
• Forgetting to add waste.
• Ordering by square footage instead of by squares or bundles.

Use the calculators

Try the roofing calculator, or browse all construction and home improvement calculators. Results are estimates; see the disclaimer.

Ordering pavers is a matter of area: divide the patio area by the area of one paver, then add an allowance for cuts and breakage. This guide walks through the steps with a real example.

What to measure

• The patio length and width, in feet.
• The size of one paver, in inches, from the product label.

The formula

Pavers needed = patio area ÷ area of one paver. Convert the paver size to square feet by multiplying its length by its width in inches and dividing by 144.

Worked example

A 12 ft by 10 ft patio has an area of 120 sq ft. A 6 in by 9 in paver covers 54 sq in, which is 0.375 sq ft. 120 ÷ 0.375 = 320 pavers.

Adding an allowance for cuts and breakage

Edges, curves and broken units call for extra pavers. A common allowance is 5 to 10 percent for straight layouts and more for curved or patterned ones. At 10 percent, 320 pavers becomes 352.

Common mistakes

• Using the nominal paver size instead of the size on the label.
• Ignoring joint spacing, which slightly changes coverage.
• Forgetting to buy extras for future repairs.
• Leaving out the base material, which is ordered separately.

Use the calculators

Try the paver calculator, or browse all construction and home improvement calculators. Results are estimates; see the disclaimer.