Concrete Calculator

How to use the concrete calculator

1. Keep Imperial selected (feet and inches).
2. Choose the shape: Slab for patios, pads, sidewalks and footings with a rectangular section, or Round column for post holes, piers and form tubes.
3. Enter the dimensions. Slabs take length and width in feet and thickness in inches; columns take diameter in inches and height in feet.
4. Enter how many identical pieces you're pouring, such as 6 deck footings.
5. Keep 10% waste or adjust it. Uneven ground and hand-dug holes eat concrete.
6. Choose the method. Premix bags shows the number of bags for the size you pick. Mix your own shows bags of cement, cubic yards of sand and gravel and gallons of water; the 1:2:3 ratio can be changed under Advanced options.

How the calculator works

• Slab volume = length × width × thickness
• Column volume = π × (diameter ÷ 2)² × height
• Total = volume × number of pieces × (1 + waste %)

One cubic yard is 27 cubic feet. Ready-mix plants sell by the yard; bags are sold by weight.

Premix bags

Bag yield is estimated as bag weight divided by a fresh density of about 2,135 kg/m³ (133 lb/ft³). That gives roughly 0.30 ft³ for a 40-lb bag, 0.45 ft³ for 60 lb and 0.60 ft³ for 80 lb, the same yields Quikrete prints for its Concrete Mix. At 0.60 ft³, a full cubic yard takes 45 bags of 80 lb, which matches the figure Sakrete gives. High-yield or specialty mixes yield differently, so check the bag.

Mixing your own

For a 1:2:3 mix by volume (cement : sand : gravel), the calculator assumes the dry materials take up 1.54 times the finished volume, cement weighs 1,440 kg/m³ loose, and you add water at half the cement's weight. For one cubic yard that comes to about 623 lb of cement (7 bags of 94 lb), 0.51 yd³ of sand, 0.77 yd³ of gravel and 37 gallons of water. Wet sand bulks up and needs less added water, so add water gradually until the mix is workable.

Concrete examples

10 × 10 ft patio, 4 inches thick

With 10% waste you need 1.36 yd³ (36.7 ft³). That is 62 bags of 80 lb, or 82 bags of 60 lb.

Six deck footings, 10 in diameter, 3.5 ft deep

The six holes plus 10% waste come to 0.47 yd³ (12.6 ft³): 21 bags of 80 lb or 28 bags of 60 lb.

12 × 20 ft parking pad, 4 inches

With 10% waste this is 3.26 yd³ (88 ft³), or 147 bags of 80 lb. Mixed on site at 1:2:3 it needs 22 bags of 94-lb cement, 1.67 yd³ of sand, 2.51 yd³ of gravel and about 122 gallons of water. At this size, a ready-mix truck usually costs less and gives a more uniform pour.

Using the result

Round up, not down: running short in the middle of a pour leaves a cold joint. Unopened bags can usually be returned, so buy a few extra. As a rule of thumb, above about one cubic yard, ordering ready-mix is often cheaper and far less work than mixing by hand.

This is a quantity estimate, not a structural design. Footings, foundations, retaining walls and anything carrying a load must follow your local building code and the drawings of a licensed engineer or architect, including the required strength (psi), reinforcement and frost depth. Check with your building department whether the work needs a permit.