Concrete Calculator

HomeMath turns every dimension into feet, multiplies them into cubic feet and divides by 27 for cubic yards. A slab or footing is length × width × thickness, a column is π × radius² × height, and steps are solid blocks on the ground, one run deep and one rise taller per step. The waste allowance is added to that volume, bags are the volume divided by the bag's yield and rounded up, and the ready-mix order is rounded up to the next quarter yard (quarter cubic meter in metric).

The published rules it uses: US customary units; metric conversion; premixed bag yields. Their sources, and how HomeMath tests every result, are on the about page.

Take a 14 × 18 ft patio, 4 inches thick. Work in feet throughout, then convert to yards at the end.

1. Area: 14 × 18 = 252 sq ft.
2. Thickness in feet: 4 ÷ 12 = 0.333 ft.
3. Volume: 252 × 0.333 = 84.00 cu ft.
4. Cubic yards: 84.00 cu ft ÷ 27 = 3.11 cu yd.
5. Waste: 3.11 cu yd × 1.10 = 3.42 cu yd, ordered as 3.50 yd.
6. Or bags: 92.40 cu ft ÷ 0.60 = 154.0, so 154 bags of 80 lb.

A shortcut for 4 in slabs: one cubic yard covers 81 sq ft, so divide the area by 81. 252 ÷ 81 = 3.11 yd, the same answer.

A continuous footing is a long, narrow slab: enter its length, its width and its depth as the thickness. Your plans or local code set the size by the load, the soil and the frost depth.

A round hole holds π × radius² × depth, so doubling the diameter quadruples the concrete. The post itself takes some room, which the calculator ignores and leaves as a small margin.

For a row of fence posts, the fence post concrete calculator takes the post out and counts the gravel; for tube forms with a footing under each, use the sonotube concrete calculator.

HomeMath treats steps as solid concrete on the ground, the way small entry steps are usually poured: each step is a block as wide as the stairs and one run deep, standing one rise taller than the step below. For 3 steps 4 ft wide with a 7 in rise and an 11 in run:

• Step 1: 4 × 0.917 × 0.583 ft = 2.14 cu ft
• Step 2: 4 × 0.917 × 1.167 ft = 4.28 cu ft
• Step 3: 4 × 0.917 × 1.750 ft = 6.42 cu ft

Total: 12.83 cu ft, or 0.48 cu yd before waste, which is 24 bags of 80 lb with 10% extra. A landing at the top is a slab of its own: add it with the slab tab.

For small pours you buy concrete in bags. The table gives the bags of each size for common small slabs 4 inches thick, with 10% waste and rounded up to whole bags.

To compare bag sizes, divide each bag's price by its yield: the lowest number is the cheapest concrete per cubic foot. Lighter bags are easier to carry and mix. Check the yield printed on your bag, since high-strength and fast-setting mixes can yield a little differently.

A bag's yield is the concrete it makes, printed on the bag. Quikrete's Concrete Mix data sheet lists five sizes; divide a yard's 27 cubic feet by the yield for the bags per yard, and round up. An 80 lb bag yields 0.60 cu ft, so a cubic yard takes 27 ÷ 0.60 = 45 bags.

The yield is also the coverage: at 4 inches thick an 80 lb bag covers 1.80 sq ft, so the bags per square foot depend on the thickness. For a pallet, multiply the bags on it by the yield and divide by 27: 42 bags of 80 lb hold 25.2 cu ft, 0.93 cu yd. The calculator counts the 40, 60 and 80 lb bags; for a 50 or 90 lb bag, divide its cubic feet the same way.

Multiply the area by the thickness in feet and divide by 27: 252 sq ft at 4 inches is 3.11 cu yd. For an area in square yards, multiply by 9 first, or type the length and width in yards: every length field takes yards. For cubic inches, divide by 46,656, the cubic inches in a cubic yard (36 × 36 × 36).

A cubic yard is 45 bags of 80 lb, or 3,600 lb to lift and mix before it sets. Bags suit post holes, footings and small pads; a slab of a yard or two is faster from a truck, which needs access to the site and often adds a short-load fee on a small order. The concrete cost calculator prices both at your own prices.

The concrete in a slab of each common size at 4, 5 and 6 inches thick, before waste. Add 10% for the amount to order.

A cubic yard is 27 cubic feet: divide cubic feet by 27, or multiply cubic yards by 27.

How far one cubic yard goes at each slab thickness:

How many yards a truck carries, and how far a yard of concrete goes, is in how many yards of concrete in a cement truck.

Switch the units to metric and every field takes meters and centimeters. The result gives the concrete in cubic meters and liters, the ready-mix order rounded up to the next quarter cubic meter, the bags, and the cost at your price per cubic meter. HomeMath converts by the exact definitions, 1 in = 2.54 cm, so a slab in meters gets the same answer as the same slab in feet.

1. Turn the thickness into meters: 10 cm ÷ 100 = 0.10 m.
2. Multiply length × width × thickness: 3 × 3 × 0.10 = 0.90 m³.
3. Add 10% waste: 0.99 m³, ordered as 1.00 m³.

A cubic meter is about 1.31 cubic yards, or 35.3 cubic feet; a cubic yard is about 0.76 m³.

Outside the US, premixed concrete is sold in kilogram bags with the yield in liters on the label, and it differs by brand. Type your bag's weight and yield in metric mode and the calculator counts your bags too: the concrete in liters ÷ the yield, rounded up to whole bags.

• Measure the thickness in several places. A 4 in slab that ends up half an inch thicker than planned takes 1/8 more concrete. Use the average, or the deepest spot if the base is rough.
• Measure inside the forms, since that is the space the concrete fills.
• Split odd shapes into rectangles and add them up; for a round slab, use the column tab with the slab's diameter and its thickness as the height.
• Keep the waste allowance unless the forms are rigid and the base is level and compacted.
• Check the bag's yield against the 0.30 / 0.45 / 0.60 cu ft the calculator uses.

For a shed or hot tub pad with its gravel base, use the concrete pad calculator; for block walls, the concrete block calculator.