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Masonry, drawn to scale

Isodomos

Concrete Slab Estimator and Block Calculator

Concrete to order (with waste)
1.63 cu yd

1.25 m³ · 44.0 cu ft including 10% waste

1.48 cu ydNet volume: 120.0 sq ft × 4 in = 40.0 cu ft (1.13 m³)
74 bags80 lb premix at 0.60 cu ft per bag
98 bags60 lb premix at 0.45 cu ft per bag

Sources: QUIKRETE Concrete Mix data sheet (bag yields) · QUIKRETE Mason Mix data sheet (13 blocks per 80 lb bag) · CMHA TEK 04-02A (block count and 5% waste) · NIST SP 811 Appendix B (27 cu ft per cu yd, 0.7645549 m³ per cu yd).

How It Works

Slab mode multiplies length by width by thickness to get cubic feet, then divides by 27 for cubic yards (there are 3 feet in a yard, so 27 cubic feet in a cubic yard) and multiplies by 0.02831685 for cubic metres. Both factors are NIST conversion factors. The metric toggle converts metres and centimetres with the exact 0.3048 m per foot and 2.54 cm per inch before the same math runs.

Waste is added on top of the net volume. Bag counts divide that gross volume by the yield QUIKRETE publishes on its Concrete Mix data sheet: 0.60 cu ft per 80 lb bag and 0.45 cu ft per 60 lb bag, rounded up to whole bags. Yields are approximate and do not allow for an uneven sub-base, which is what the waste percentage is for.

Wall mode counts standard 8 × 8 × 16 in nominal concrete block. The nominal face, joint included, is 8 × 16 in = 128 sq in, so a square foot of wall needs 144 ÷ 128 = 1.125 blocks. CMHA TEK 04-02A gives the same figure as 119 units per 100 sq ft with its 5% waste allowance. Mortar is counted at 13 standard blocks per 80 lb bag of QUIKRETE Mason Mix or Mortar Mix, the yield printed on both data sheets.

The calculator does not deduct openings. For a wall with a door or window, subtract the opening area from the wall area before you enter it, or leave it in as extra waste.

Worked Example

A 10 ft × 12 ft patio slab poured 4 in thick: 10 × 12 × (4 ÷ 12) = 40 cu ft, which is 40 ÷ 27 = 1.48 cu yd or 1.13 m³. With 10% waste the order is 44 cu ft = 1.63 cu yd (1.25 m³). In premixed bags that is 44 ÷ 0.60 = 73.3, so 74 bags of 80 lb, or 44 ÷ 0.45 = 97.8, so 98 bags of 60 lb.

A 40 ft × 8 ft block wall is 320 sq ft × 1.125 = 360 blocks. With 10% waste, order 396 blocks and 396 ÷ 13 = 30.5, so 31 bags of 80 lb mortar.

FAQ

Should I order ready-mix or bags?

The example slab needs 74 bags of 80 lb concrete, close to 3 tons of material to carry and mix. Ready-mix trucks are priced by the cubic yard with a short-load fee below their minimum, so compare the truck price for 1.63 cu yd against the bag count. Bags suit footings, small pads and repairs; slabs above roughly a cubic yard usually favour the truck.

Why do the cubic yards and the bag count both include waste?

Bag yields on the data sheet are already approximate, and the sub-base is never perfectly flat. The waste percentage is applied once, to the volume, and the bag count is taken from that gross volume so the two figures agree.

Does the block count change with block width?

No. A 4, 6, 8, 10 or 12 in wide block still has an 8 × 16 in nominal face, so the count per square foot stays 1.125. Width changes weight, grout volume and mortar bed area, but the tool’s mortar yield is the QUIKRETE figure for standard 8 × 8 × 16 units.

How much mortar does a block wall really use?

The data-sheet figure of 13 blocks per 80 lb bag is a conservative yield for full bedding. CMHA TEK 04-02A notes that mortar use varies with joint thickness, face-shell versus full bedding, and site practice, so check the actual rate after the first few courses.

What thickness should a slab be?

Common practice is 4 in for patios, walks and shed floors and 6 in for driveways or anywhere vehicles park; the QUIKRETE Concrete Mix data sheet lists the product for pours 2 in thick or more. Local code governs footings, so use the required depth from your permit drawings.

Sources