Concrete Calculator

How the concrete calculator works

Concrete is ordered by volume, so every answer starts with one question: how many cubic feet (or cubic metres) does the form hold? The calculator works out that volume for the shape you choose, adds the extra you ask for, and then converts it two ways: into the cubic yards (or m³) a ready-mix plant sells, and into the number of premixed bags you would need instead.

Bag counts use the yields printed on manufacturer data sheets, not guesses. QUIKRETE lists a 40 lb bag of its Concrete Mix at about 0.30 ft³, a 60 lb bag at 0.45 ft³ and an 80 lb bag at 0.60 ft³; its Fast-Setting mix lists 0.375 ft³ for a 50 lb bag, and 11.6 L and 14 L for the 25 kg and 30 kg bags sold in Canada. Other brands are similar but not identical, so if your bag prints a different yield, divide by that number instead.

The formulas

Slab or footing:  volume = length × width × thickness
Round column:     volume = π × (diameter ÷ 2)² × height
Straight steps:   volume = width × run × rise × n(n + 1) ÷ 2
Cubic yards = cubic feet ÷ 27      1 yd³ = 0.7646 m³
Bags = volume with extra ÷ bag yield  (always round up)

The one place people slip is mixing units. Thickness is usually measured in inches but length in feet, so divide the inches by 12 first: a 4 in slab is 0.333 ft thick. In metric, convert millimetres to metres by dividing by 1,000, so 100 mm becomes 0.1 m. The step formula treats a flight as a stack of blocks: the top step is one rise tall, the next is two rises tall, and so on, which is where n(n + 1) ÷ 2 comes from. It assumes the steps sit on solid ground with no hollow underneath and no landing; add a landing as a separate slab.

Worked examples

1. A 10 × 10 ft patio, 4 in thick

10 × 10 × 0.333 = 33.3 ft³, which is 33.3 ÷ 27 = 1.23 yd³. With 10% extra the order becomes 36.7 ft³, or 1.36 yd³. In bags that is 36.7 ÷ 0.60 = 61.1, so 62 bags of 80 lb, or 82 bags of 60 lb. At this size it is worth getting a ready-mix quote too: 62 bags is roughly 2.5 tons of material to lift and mix by hand.

2. A 3 × 2.5 m shed base, 100 mm thick (metric)

3 × 2.5 × 0.1 = 0.75 m³; with 10% extra, 0.825 m³ (825 litres). Using the 11.6 L yield of a 25 kg bag, 825 ÷ 11.6 = 71.1, so 72 bags of 25 kg. If your local bags are a different size or brand, use the yield printed on the bag.

3. A 12 in round column, 4 ft tall

π × 0.5² × 4 = 3.14 ft³ (0.116 yd³). With 10% extra that is 3.46 ft³, so 6 bags of 80 lb. Tube forms are a common way to pour these; the calculator doesn’t subtract any rebar or anchor bolts, which take up very little volume.

4. Three straight steps, 36 in wide

With a 7 in rise and 11 in run: 3 ft × 0.917 ft × 0.583 ft × (3 × 4 ÷ 2) = 9.6 ft³, or 10.6 ft³ with 10% extra. That works out to 18 bags of 80 lb or 0.39 yd³.

Bag yield reference

Bags per yd³ and per m³ are rounded up from the yields above. Yields are approximate and vary slightly with water content.

Tips before you order

• Measure the form, not the plan. Measure the inside of the formwork in a few places. A base that dips by half an inch over a whole slab adds about 12% to a 4 in pour, which is why an allowance for extra matters.
• Running short is worse than leftovers. A pour that stops halfway leaves a cold joint. Order the extra, and have a plan for the leftovers, such as a small form for a stepping stone.
• Bagged or ready-mix? Bags suit small jobs and places a truck cannot reach. For bigger pours, compare a ready-mix quote, including any short-load fee, with the price of the bags plus mixer hire and labour.
• Control joints. QUIKRETE’s slab instructions call for 1 in deep control joints every 6–8 ft (1.8–2.4 m). Plan them before you pour.
• Protect your skin and eyes. Manufacturer safety warnings state that wet cement mixes can cause severe skin burns and eye damage. Wear impervious gloves, eye protection and a dust mask when mixing.