Concrete calculator

Concrete is ordered by volume, but a pour needs more than a number of yards. This calculator covers the whole job.

Shapes: slab (patio, sidewalk, driveway), continuous footing, pier pad, square column, round column or form tube, and solid concrete steps with a top landing.

Three ways to get the concrete:

• Ready-mix: cubic yards rounded up to the next ¼ yd³ for ordering.
• Premix bags: 40, 60 or 80 lb bags at about 0.30, 0.45 and 0.60 cu ft per bag.
• Mix your own: a 1:2:3 cement, sand and gravel mix by volume, with 94 lb bags of portland cement.

Plus: for slabs, a rebar grid (bar count, total length, intersections to tie) or welded wire mesh sheets; formwork area, a sketch with dimensions, and a materials list with editable prices and a total. Every assumption is listed on the page. Export to PDF or Excel for free, no sign-up. It runs in your browser, and you can switch to metric.

Multiply in feet to get cubic feet, then divide by 27 for cubic yards.

Shape	Volume	Example
Slab	length × width × thickness	10 × 12 ft × 4 in = 40 cu ft
Continuous footing	total length × width × depth	100 ft × 20 in × 10 in = 138.9 cu ft (5.14 yd³)
Pier pads	length × width × depth × count	6 × (24 × 24 × 12 in) = 24 cu ft
Square column	side × side × height × count	4 × (12 × 12 in × 8 ft) = 32 cu ft
Round column or tube	π × (diameter / 2)² × height × count	8 × (12 in dia × 4 ft) = 25.1 cu ft
Solid steps	width × tread × riser × n(n+1)/2, plus landing	see below

Steps: the first step sits on one riser of concrete, the second on two, and so on, which is where n(n+1)/2 comes from. Three steps, 4 ft wide, 11 in treads, 7 in risers: 4 × 0.917 × 0.583 × 6 = 12.8 cu ft. Add a 3 ft deep landing at the top (3 risers high) and the total is 33.8 cu ft, 1.25 yd³, with about 23.9 sq ft of forms.

Example: a 10 ft × 12 ft slab, 4 in thick.

1. Volume: 10 × 12 × 4/12 = 40 cu ft = 40 / 27 = 1.48 yd³
2. Ready-mix order: rounded up to ¼ yd³ = 1.5 yd³; with 10% waste, 1.63 → 1.75 yd³
3. Premix bags: 40 / 0.60 = 66.7 → 67 bags of 80 lb (74 with 10% waste), or 89 bags of 60 lb, or 134 bags of 40 lb
4. Forms: perimeter 44 ft × 4 in = 14.7 sq ft of form boards

Mixing your own at 1:2:3. Dry ingredients shrink when water is added, so dry volume ≈ wet volume × 1.54. For 1 yd³: 27 × 1.54 = 41.58 cu ft dry, split 1:2:3:

• cement: 41.58 / 6 = 6.93 → 7 bags of 94 lb (1 cu ft each)
• sand: 13.9 cu ft
• gravel: 20.8 cu ft

For the 40 cu ft slab (before waste) that is 11 bags of cement, 20.5 cu ft of sand and 30.8 cu ft of gravel. At that size, ready-mix is usually far less work.

Rebar grid. Enter bar spacing, bar size and stock length. The calculator keeps about 2 in of edge cover, places bars both ways, adds lap length where a run is longer than one bar, and counts the intersections you will tie.

Example: a 12 × 10 ft slab, bars 18 in on center, 2 in cover, 20 ft stock. You get 7 bars 11 ft 8 in long and 8 bars 9 ft 8 in long: 159 ft in total, 8 bars by length and 56 intersections.

The bar count assumes you reuse offcuts. Check your cut list: here each 11 ft 8 in piece leaves 8 ft 4 in, too short for a 9 ft 8 in bar, so a real cut list needs 11 bars.

Welded wire mesh is counted by sheet area with an overlap allowance. Formwork is the area of the sides in contact with concrete; for footings poured against earth you can leave it out.

Reinforcement size, spacing and placement for structural work come from your plans or engineer; the calculator counts material, it does not design.

• Convert inches. A 4 in slab is 0.333 ft thick, not 4.
• Order by the yard. 40 cu ft is about 1.5 yards; suppliers do not sell cubic feet.
• Order a little extra. Running short mid-pour leaves a cold joint. Ask about short-load fees on small orders.
• Measure the real forms. Uneven subgrade makes a slab thicker in spots.
• Use the inside diameter of round form tubes. Ten 10 in post holes, 3 ft deep, take 16.4 cu ft: 28 bags of 80 lb, or 30 with 10% waste.
• Cure it. Keep new concrete damp and protected from sun and frost for the first days.

Planning the steps first? The stair calculator gives riser and tread sizes. For an odd-shaped slab, find the area with the square footage calculator.