Concrete Calculator
Cubic yards of concrete = length × width × thickness, all in feet, ÷ 27. A 10 × 10 ft slab 4 in thick is 33.3 ft³, or 1.23 yd³; with 10 % extra for waste, order 1.36 yd³ or 62 bags of 80 lb.
Concrete to order
1.36 yd³
36.67 ft³ · 1.04 m³ · includes 10 % extra
Estimate only. Check dimensions, loads and local code requirements with a qualified professional before you build.
- 80 lb bags (36.3 kg, 0.6 ft³ each)
- 62
- 60 lb bags (27.2 kg, 0.45 ft³ each)
- 82
- 40 lb bags (18.1 kg, 0.3 ft³ each)
- 123
- Volume before the extra
- 1.23 yd³ (33.33 ft³)
Ready-mix trucks usually sell in ¼ or ½ yd³ steps with a minimum load; bags are the usual choice under about 1 yd³.
How it works
Slab: volume (ft³) = length (ft) × width (ft) × thickness (in) ÷ 12
Footing: volume (ft³) = length (ft) × width (in) ÷ 12 × depth (in) ÷ 12
Round column or hole: volume (ft³) = π × (diameter (in) ÷ 24)² × depth (ft)
Steps: volume (ft³) = width × run × rise × n(n+1)/2 (all in ft)
Cubic yards = ft³ ÷ 27 · bags = ft³ × (1 + waste %) ÷ bag yield, rounded up
Assumptions
- Bag yields from the QUIKRETE Concrete Mix data sheet: 80 lb ≈ 0.60 ft³, 60 lb ≈ 0.45 ft³, 40 lb ≈ 0.30 ft³.
- Steps are solid concrete sitting on grade: each step is a full block down to the ground.
- The waste allowance (10 % by default) covers spillage, uneven subgrade and form bulging. It is added before bags are counted.
Worked examples
A 9 ft × 9 ft slab, 4 in thick
- 9 × 9 × 4 ÷ 12 = 27 ft³
- 27 ÷ 27 = 1.00 yd³
- 27 ÷ 0.60 = 45 bags of 80 lb, with no waste added
Result: 1 cubic yard, 45 bags of 80 lb (with 0 % waste). Sakrete gives the same figure: 45 bags of 80 lb make one cubic yard.
Checked by the automated test “concrete — 9 × 9 ft slab, 4 in thick = 27 ft³ = 1 yd³ = 45 bags of 80 lb”.
A 20 ft footing, 16 in wide and 8 in deep, plus 10 %
- 20 × (16 ÷ 12) × (8 ÷ 12) = 17.78 ft³ (0.66 yd³)
- 17.78 × 1.10 = 19.56 ft³
- 19.56 ÷ 0.60 = 32.6, so 33 bags of 80 lb
Result: 33 bags of 80 lb.
Checked by the automated test “concrete — footing 20 ft × 16 in × 8 in = 17.78 ft³ ; +10 % → 33 bags of 80 lb”.
This is an estimate — confirm with a licensed professional before you pour footings, structural slabs or steps: thickness, reinforcement and base depend on loads, soil and your local code.
Ordering: bags, yards and waste
- Round up, never down. The calculator rounds bags up to the next whole bag. With the default 10 % waste, the 9 × 9 ft slab above becomes 29.7 ft³, so 50 bags of 80 lb instead of 45.
- Sakrete’s pallet rule. Sakrete notes that one pallet of bags is about a cubic yard. It is a quick way to picture the job, but count your bags from the volume.
- Bagged mix is made for small pours. QUIKRETE Concrete Mix is meant for pours at least 2 in thick. Above a cubic yard you will be mixing 45 or more 80 lb bags by hand, so compare the cost of a ready-mix delivery.
Common mistakes
- Measuring thickness before the base is compacted. A slab planned at 4 in that ends up at 4½ in over a low spot uses 12.5 % more concrete. Check the depth at several points inside the forms.
- Counting one post hole for the whole fence. Use the “How many identical pours?” box so every hole is counted.
- Confusing concrete mix with cement. The yields here are for pre-blended mix (cement, sand and gravel). A bag of Portland cement alone does not make a bag’s worth of concrete.
- Forgetting the steps. Three 7 in steps, 4 ft wide and 11 in deep, need almost 13 ft³: more than a 4 × 9 ft slab 4 in thick (12 ft³). If the steps sit on gravel fill, measure only the concrete shell.
Frequently asked questions
How many bags of concrete are in a yard?
45 bags of 80 lb, 60 bags of 60 lb or 90 bags of 40 lb. A cubic yard is 27 ft³, and QUIKRETE lists the yield of its Concrete Mix at 0.60, 0.45 and 0.30 ft³ for those bag sizes. Add your waste allowance on top.
How many square feet does a yard of concrete cover?
81 ft² at 4 in thick, the 9 × 9 ft slab Sakrete uses as its reference. At 6 in thick the same yard covers only 54 ft², for example a 9 × 6 ft pad.
How many cubic feet is a 60 lb bag of concrete?
About 0.45 ft³ (12.7 L) for QUIKRETE Concrete Mix. An 80 lb bag gives about 0.60 ft³ and a 40 lb bag about 0.30 ft³. Other brands print their own yield on the bag.
How much concrete do I need for fence posts?
Choose "Round column or post hole", enter the hole diameter and depth, then the number of holes. Ten holes 10 in wide and 2 ft deep hold about 10.9 ft³: 19 bags of 80 lb or 25 bags of 60 lb before waste. The post itself takes up some of that space, so you may have a little left over.
Sources
Formula, examples and sources checked on by the Yardstick Math team. How we check our formulas.