Concrete Calculator
Calculate how much concrete you need in cubic yards, cubic feet and 60 lb or 80 lb bags. Includes a waste allowance and cost estimate.
Concrete needed
1.3 cu yd
- Volume
- 35 cu ft
- Metric volume
- 0.99 m³
- 60 lb bags
- 78
- Estimated material cost
- $507.00
Assumes 60 lb bags yield 0.45 cu ft each and a 5% waste allowance.
Concrete is ordered in cubic yards, but most projects are measured in feet and inches. This calculator converts your dimensions, adds a waste allowance, and shows the equivalent number of 60 lb or 80 lb bags if you plan to mix by hand.
It works for slabs, pads, footings and short walls. Bagged concrete is practical up to about one cubic yard; above that, a ready-mix delivery is usually cheaper and far less work.
How the concrete calculator works
Volume in cubic feet = length × width × thickness, with every dimension converted to feet. Divide by 27 to get cubic yards. Bags needed = volume ÷ yield per bag, where a 60 lb bag yields about 0.45 cu ft and an 80 lb bag yields about 0.60 cu ft. The waste allowance is applied before the bag count.
Worked example
A 10 × 10 ft slab at 4 in thick is 10 × 10 × 0.333 = 33.3 cu ft, or 1.23 cu yd. With 5% waste that becomes about 1.30 cu yd — roughly 78 bags of 60 lb or 59 bags of 80 lb.
How to measure a slab
- Measure the inside of the forms at the widest point. For an irregular shape, break it into rectangles and add the volumes.
- Measure thickness in inches and convert to feet by dividing by 12. A 4-inch slab is 0.333 ft thick.
- Use 4 inches for patios, walkways and shed pads; 5–6 inches for driveways and vehicle loads.
- Add 5–10% for spillage, uneven subgrade and over-excavation. The calculator applies the allowance before the bag count.
How much area does a cubic yard cover?
| Thickness | Coverage per cubic yard |
|---|---|
| 4 in | 81 sq ft |
| 5 in | 65 sq ft |
| 6 in | 54 sq ft |
| 8 in | 40.5 sq ft |
Common concrete mistakes
- Ordering the exact calculated volume with no waste allowance.
- Mixing 60–80 bags by hand for a slab that should be delivered ready-mix.
- Adding water on site to make the mix flow; it weakens the concrete.
- Pouring on soft, uncompacted subgrade or over frozen ground.
- Finishing too early or too late, which leads to dusting or craze cracks.
Frequently asked questions
How many bags of concrete do I need for a 10 × 10 slab?
A 4-inch slab needs about 1.23 cubic yards. With a 5% waste allowance, plan on roughly 78 bags of 60 lb or 59 bags of 80 lb. At that size, ready-mix concrete is usually cheaper and much faster than mixing dozens of bags.
How thick should a concrete slab be?
Four inches is typical for patios, walkways and shed pads. Driveways and slabs that carry vehicles are usually 5–6 inches thick, with reinforcement and a compacted base. Check local building codes and soil conditions before pouring.
Should I add rebar or wire mesh?
A 4-inch slab usually gets welded wire mesh or #3 rebar on a 12–18 inch grid, positioned near the middle of the slab. Driveways, structural slabs and expansive soils need engineered reinforcement rather than a generic rule of thumb.
How much area does one cubic yard of concrete cover?
One cubic yard is 27 cubic feet. At 4 inches thick it covers about 81 square feet, at 5 inches about 65 square feet, and at 6 inches about 54 square feet.
Is it cheaper to mix bags or order ready-mix?
Bagged concrete makes sense up to roughly one cubic yard, or about 60–80 bags. Above that, ready-mix is cheaper per yard and gives you a consistent mix. Also factor in the time and equipment needed to mix and place that many bags.
How long does concrete take to cure?
Concrete sets in 24–48 hours but reaches design strength around 28 days. Keep it moist and protected from traffic for at least 7 days, and avoid heavy loads until the 28-day mark.
Assumptions and accuracy
- Dimensions are converted to feet before the calculation; 1 ft = 12 in = 0.3048 m.
- Waste allowances are estimates. Add more for irregular shapes, many cuts and uneven subgrade.
- Bag yields, coverage rates and material densities vary by product. Check the label or ask your supplier.
- Local building codes and site conditions always take precedence over a general-purpose calculator.
- Read our full methodology