Concrete Calculator

The formula

Coverage and yield assumptions

Bag yields follow published figures for standard mixes such as Quikrete: an 80 lb bag ≈ 0.60 cu ft, a 60 lb bag ≈ 0.45 cu ft and a 40 lb bag ≈ 0.30 cu ft. One cubic yard is 27 cu ft, so it takes about 45 bags of 80 lb mix per yard. Ready-mix plants sell by the cubic yard, often in quarter-yard steps with a minimum order or short-load fee.

How thick should the slab be?

Four inches is standard for patios, walkways and shed floors. Driveways for passenger cars are commonly 4–5 in and 6 in for heavier vehicles or poor soils. Thickened edges and footings are deeper; follow your plans and local code. The Portland Cement Association recommends a well-compacted, uniform subgrade, which matters more for long-term cracking than extra thickness.

Base, forms and reinforcement

Most slabs sit on 4 in or more of compacted gravel. Set forms with 2× lumber staked every few feet and check them for level or the slope you want (about 1/8 to 1/4 in per foot away from buildings). Wire mesh, rebar or fiber reinforcement controls crack width; it does not prevent cracks. Cut or tool control joints at spacing about 2–3 times the slab thickness in feet (for a 4 in slab, roughly every 8–12 ft) to a depth of one quarter of the thickness.

Bags or ready-mix?

Mixing 40–60 bags by hand or with a rented mixer is a long, hard day for two people. Above about one cubic yard, a ready-mix truck or a short-load supplier usually saves money and gives better, more consistent concrete. For a truck, measure the actual depth of your forms at several points, since an extra half inch over a large area adds up quickly.

Curing

• Keep fresh concrete moist for at least 7 days with curing compound, wet burlap or plastic.
• Avoid pouring when freezing is expected without cold-weather protection.
• Light foot traffic after 24–48 hours; cars after about 7 days for normal mixes.

Common mistakes to avoid

• Measuring nominal thickness when the forms are deeper in places.
• Pouring on soft or unevenly compacted soil.
• Adding extra water to make mixing easier, which weakens the concrete.
• Skipping control joints, leading to random cracking.
• Letting fresh concrete dry out in sun and wind instead of curing it.

Waste factor

Add 5–10% for spillage, uneven subgrade and form deflection. Use 10% for DIY bag pours and uneven ground, and 5% for carefully screeded subgrade with a truck order. Ordering short is far more costly than leaving a few bags over.

Imperial and metric

Switch the toggle above the inputs to enter ft ↔ m, in ↔ cm. The page converts metric entries to imperial before calculating, using NIST factors (1 ft = 0.3048 m, 1 in = 2.54 cm), and shows key results in both systems.

Worked examples

Patio 10 × 10 ft, 4 in

Inputs: length 10 ft; width 10 ft; thickness 4 in; how many identical pours 1; waste factor 10%.

Result: You need 1.36 cubic yards of concrete (1.04 m³).

• Units to buy (rounded up): 62 80 lb bags
• Volume: 36.7 cu ft
• 80 lb bags (0.60 cu ft each): 62
• 60 lb bags (0.45 cu ft each): 82
• 40 lb bags (0.30 cu ft each): 123

Driveway apron 20 × 12 ft, 5 in

Inputs: length 20 ft; width 12 ft; thickness 5 in; how many identical pours 1; waste factor 5%; cost per unit $6.5.

Result: You need 3.89 cubic yards of concrete (2.97 m³).

• Units to buy (rounded up): 175 80 lb bags
• Volume: 105 cu ft
• 80 lb bags (0.60 cu ft each): 175
• 60 lb bags (0.45 cu ft each): 234
• 40 lb bags (0.30 cu ft each): 350
• Estimated material cost: $1,137.50

Six footings 2 × 2 ft, 12 in

Inputs: length 2 ft; width 2 ft; thickness 12 in; how many identical pours 6; waste factor 10%.

Result: You need 0.98 cubic yards of concrete (0.75 m³).

• Units to buy (rounded up): 45 80 lb bags
• Volume: 26.4 cu ft
• 80 lb bags (0.60 cu ft each): 45
• 60 lb bags (0.45 cu ft each): 59
• 40 lb bags (0.30 cu ft each): 89

Quick reference for common sizes

The table repeats the first worked example at smaller and larger sizes, keeping every other setting the same, so you can see how the concrete calculator result scales.

Limitations

The calculator assumes rectangular shapes with uniform thickness. For round columns and post holes use the post hole calculator. Structural footings, foundations and suspended slabs must be designed to code.