Concrete Calculator
This concrete calculator estimates how much concrete any small-to-medium project needs — rectangular slabs, round pads, strip footings, and square or round columns. Enter your dimensions, pick a shape, and get cubic yards with a waste allowance plus the number of 80, 60 and 40 lb pre-mix bags required. Use it to order ready-mix or to size a bagged-concrete purchase before you start mixing.
Only the dimensions that match this shape are used.
Used for: rectangular slabs and strip footings.
Used for: rectangular slabs and strip footings.
Used for: slabs and strip footings. Typical slab: 4 in.
Used for: round slabs and round columns.
Used for: square columns / piers.
Used for: square and round columns / piers.
Covers spillage, uneven subgrade and form bulge.
Concrete needed
1.36yd³
- Volume (cubic feet)
- 36.67ft³
- 80-lb bagsEach 80-lb bag yields ≈ 0.60 ft³.
- 62
- 60-lb bagsEach 60-lb bag yields ≈ 0.45 ft³.
- 82
- 40-lb bagsEach 40-lb bag yields ≈ 0.30 ft³.
- 123
- Estimated total weight≈ 4,000 lb per cubic yard.
- 5,432lb
How it was calculated
- Gross volume: 10 ft × 10 ft × 0.33 ft = 33.33 ft³.
- With 10% waste: 33.33 × 1.10 = 36.67 ft³ (1.36 yd³).
- 80-lb bags: 36.67 ÷ 0.60 = 61.1 → 62 bags (rounded up).
How it works
Volume is length × width × thickness for rectangles (slabs and strip footings), π × (diameter ÷ 2)² × height for round shapes, and side² × height for square columns — all computed in cubic feet.
Cubic feet are converted to cubic yards by dividing by 27, because one cubic yard equals 27 cubic feet (3 ft × 3 ft × 3 ft).
The waste allowance multiplies the gross volume — 10% is the standard recommendation for formed pours, 15% for hand-mixed or uneven subgrades.
Bag counts divide the waste-adjusted volume by each bag's yield (0.60 ft³ for 80-lb, 0.45 ft³ for 60-lb, 0.30 ft³ for 40-lb) and round up, since you cannot buy a fraction of a bag.
Worked examples
10 × 10 ft patio slab at 4 inches
Volume = 10 × 10 × (4 ÷ 12) = 33.33 ft³. With 10% waste: 36.67 ft³ = 1.36 yd³. That needs 62 bags of 80-lb mix (36.67 ÷ 0.60 = 61.1, rounded up) — or 1.36 yards if you order ready-mix.
Six 12-inch round deck piers, 3 ft deep
One pier = π × 0.5² × 3 = 2.36 ft³, so six piers = 14.14 ft³. With 10% waste: 15.56 ft³. At 0.60 ft³ per 80-lb bag, that is 26 bags (15.56 ÷ 0.60 = 25.9, rounded up).
How to measure
- Measure forms, not the finished drawing: tape the inside of the formwork for the true pour dimensions.
- Measure thickness at several spots — subgrade is rarely perfectly flat, and the deepest spot governs.
- For round piers, measure the diameter of the hole or sonotube, not the post that sits on it.
- Round fractional inches down to the nearest half inch; extra depth only makes the estimate safer.
- Re-measure after forms are set and braced, since staking can shift boards by an inch or more.
Assumptions & limitations
Planning estimate
- Bag yields (80-lb ≈ 0.60 ft³, 60-lb ≈ 0.45 ft³, 40-lb ≈ 0.30 ft³) are manufacturer rules of thumb for standard pre-mix; high-strength or fast-setting mixes can differ slightly.
- Concrete is assumed to weigh ≈ 4,000 lb per cubic yard — a rule of thumb for estimating haul and handling weight, not a precise density.
- The calculator assumes vertical sides and a flat bottom; bell-bottom piers, sloped footings or irregular excavations need extra volume added manually.
- Results are a planning estimate only — not a substitute for code compliance or professional design. Footing sizes and slab thickness for structures must meet local building codes.
- Waste percentage does not account for concrete lost to pump lines or washout on large ready-mix orders.
Frequently asked questions
Multiply length × width × thickness (all in feet), then divide by 27. A 10 × 12 ft slab at 4 inches is 10 × 12 × 0.333 = 40 ft³, or 1.48 yd³ — order about 1.6 yd³ with a 10% waste allowance.
About 45 bags. One cubic yard is 27 ft³ and an 80-lb bag yields roughly 0.60 ft³, so 27 ÷ 0.60 = 45. For 60-lb bags it is 60 bags, and for 40-lb bags it is 90 bags per yard.
Bags make sense under roughly 1 cubic yard (about 45 eighty-pound bags). Above that, ready-mix is usually cheaper per yard and far less labor. The crossover varies by local pricing, so price both.
Use 10% for typical formed pours on prepared subgrade. Bump to 15% for hand-dug footings, uneven ground, or hand mixing, where spillage and over-excavation are common. Small precision jobs like a single pier can use 5%.
No — and it does not need to. Steel reinforcement displaces a negligible amount of concrete (well under 1% of the volume), so it is ignored in standard estimating practice.