Concrete Buying & Pour Planning Guide
Bags or ready-mix, what to confirm with your supplier, and how to plan the pour day.
Concrete Calculator
Calculate concrete for slabs, footings and posts. See the cubic yards you need, compare bag quantities, and decide how to buy.
Under 30 secondsShows every assumptionLast reviewed October 2026
What are you pouring?
Your slab
ft
ft
in
Measure the finished slab. Thickness is the depth of concrete, not including any gravel base.
Use the dimensions specified for your project. CalcBrite calculates quantity, not required structural thickness.
Your concrete estimate · Slab
You need about 1.48 cubic yards of concrete
No allowance added. Exact volume from your dimensions.
Cubic feet40.0 cu ftMeasured volume
Bags of 80 lb mix≈ 67 bagsAt 0.60 cu ft per bag
Ready-mix reference1.48 yd³Before any allowance or supplier rounding
Add an allowance?
An optional allowance can help account for uneven excavation, minor measurement differences and handling losses. Use the quantity appropriate for your project and confirm order increments with your supplier.
Bags or ready-mix?
Compare the quantity, handling effort and estimated cost before choosing how to buy.
Bagged concrete67 bags of 80 lb67 bags × 80 lb = 5,360 lb to buy, carry and mix by hand or in a mixer.
Yield per bag0.60 cu ft (typical label)
Total bagged weight5,360 lb (≈ 2.7 tons)
Why does bag yield vary?
Each product lists its own yield on the bag, and it depends on the mix. These are typical label yields for standard concrete mix: 40 lb ≈ 0.30 cu ft, 60 lb ≈ 0.45 cu ft, 80 lb ≈ 0.60 cu ft. Check your bag, or choose “From my bag” to use its exact yield.
Ready-mix delivery1.48 yd³Supplier order increment (optional)Delivered mixed, ready to place in one pour. Ask about minimum orders, short-load fees, how close the truck can get and how long you have to unload.
Required volume1.48 yd³
Rounded to incrementAsk your supplier
That’s a substantial number of bags to transport and mix. Consider checking ready-mix delivery availability.
Your concrete estimate
Concrete estimate — CalcBrite Slab: 12 × 10 ft, 4 in thick → 1.48 yd³ Calculated volume: 1.48 yd³ (40.0 cu ft) Allowance: none Bags: 67 × 80 lb (5,360 lb) Ready-mix: 1.48 yd³ Quantity only. Confirm mix, minimums and delivery with your supplier. calcbrite.com/concrete-calculator/No account needed. The share link keeps your dimensions and choices, never prices or anything personal.
How much concrete do I need?
Concrete is sold by volume3. Multiply length × width × thickness, all in feet, to get cubic feet, then divide by 27 for cubic yards8. A 12 × 10 ft slab 4 inches thick is 12 × 10 × 0.333 = 40 cubic feet, or about 1.48 cubic yards. For round holes, the area of the circle (π × radius²) times the depth gives the volume of one hole: four 12-inch holes 36 inches deep come to 9.4 cubic feet, about 0.35 cubic yards.
How many bags of concrete make a cubic yard?
It depends on the product. Typical label yields for standard concrete mix are about 0.30 cu ft for a 40 lb bag, 0.45 cu ft for 60 lb and 0.60 cu ft for 80 lb12, so one cubic yard (27 cu ft) is roughly 90, 60 or 45 bags. Specialty mixes yield differently: always check the bag.
Bags versus ready-mix: which should I choose?
Bags are easy to buy in small amounts and suit small pours, but every bag has to be carried, mixed and placed while the earlier batches are still workable. Ready-mix arrives mixed and can be placed in one pour, but suppliers may have minimum orders, short-load charges and access requirements5. Compare the bag count, the weight you’d handle, the help and equipment you have, and the prices you’re quoted.
How much extra concrete should I order?
The calculator shows the exact volume from your dimensions and adds nothing unless you choose an allowance. Small differences add up: the National Ready Mixed Concrete Association notes that a ⅛-inch difference in a 4-inch slab means a 3 percent shortage, and suggests ordering 4 to 10 percent more than the plan volume for waste, over-excavation and other contingencies43. The right amount depends on your site. Ask your supplier how they round orders and whether there’s a minimum load.
What should I confirm before ordering concrete?
Confirm the mix and strength your project specifies, the dimensions on your plans, whether the truck can reach the pour or you need a pump or wheelbarrows, how long you have to unload, and the supplier’s minimum order and fees5. CalcBrite calculates quantity only: thickness, reinforcement, footing depth and permits come from your plans, local code or a qualified professional.
Worked example: 12 × 10 ft slab, 4 in thick
- 10 ft × 12 ft = 120 sq ft
- 4 in ÷ 12 = 0.333 ft
- 120 × 0.333 = 40.0 cu ft
- 40.0 ÷ 27 = 1.48 yd³
- 40.0 ÷ 0.60 = 66.7 → 67 bags of 80 lb
- 67 × 80 lb = 5,360 lb to handle
Frequently asked questions
How do I calculate concrete for a slab?
Multiply length × width × thickness in feet (thickness in inches ÷ 12). Divide cubic feet by 27 for cubic yards. 12 × 10 ft at 4 in is 40 cu ft, about 1.48 yd³.8
How many 80-lb bags of concrete do I need?
Divide the cubic feet by the yield on the bag, typically about 0.60 cu ft for an 80 lb bag, and round up. 40 cu ft ÷ 0.60 ≈ 67 bags.12
How many cubic feet are in a cubic yard?
27. A cubic yard is a cube 3 ft on each side (3 × 3 × 3).8
Can I combine multiple pours?
Yes: add each pour and the calculator totals them. The total is for planning; separate pours may need different mixes, deliveries or pour days, so confirm with your supplier.
Does the calculator include waste?
No. It shows the measured volume. You can add an optional allowance of 5%, 10% or a custom amount. The National Ready Mixed Concrete Association suggests ordering 4 to 10 percent more than the volume from plan dimensions, for waste, over-excavation and other contingencies.43
Can I use the same calculator for footings and posts?
Yes. Choose Footing for continuous strip footings or Posts & Piers for round holes and tube forms.
How do I estimate ready-mix delivery costs?
Open “Compare my costs” and enter the price per cubic yard, delivery fee and any short-load fee your supplier quotes. CalcBrite doesn’t provide prices.5
How we calculate & sources
- Slab
- cu ft = length (ft) × width (ft) × thickness (in) ÷ 12
- Footing
- cu ft = total length (ft) × width (in) ÷ 12 × depth (in) ÷ 12
- Posts & piers
- cu ft = π × (diameter (in) ÷ 24)² × depth (in) ÷ 12 × holes
- Cubic yards
- yd³ = cu ft ÷ 27 · m³ = cu ft × 0.0283168
- Bags
- bags = ⌈ cu ft × (1 + allowance) ÷ yield per bag ⌉
- Ready-mix order
- order = ⌈ yd³ × (1 + allowance) ÷ increment ⌉ × increment (only if you set one)
Assumptions
- All math uses full precision; results are rounded only for display (yd³ and m³ to 2 decimals, cu ft to 1). 1 yd³ = 27 cu ft; 1 cu ft = 0.0283168 m³.8
- Typical label yields: 40 lb ≈ 0.30 cu ft, 60 lb ≈ 0.45 cu ft, 80 lb ≈ 0.60 cu ft.12 Your product may differ: use “From my bag”. Metric bag sizes (18, 27 and 36 kg) are the nearest kilogram equivalents of the same bags.
- No waste or allowance is added unless you choose one.
- Weights use the bag weight × whole bags.
- Prices are never prefilled; comparisons use only what you enter and exclude labor, rentals, taxes and site preparation.
- The 40-bag point where the page suggests checking ready-mix is a CalcBrite wording switch, not a published threshold.
- CalcBrite does not determine strength, reinforcement, thickness, footing depth, frost depth, soil suitability or permit requirements.
Sources
- QUIKRETE Concrete Mix No. 1101, product data sheet, The Quikrete Companies; revised October 2022 — “A 40 lb (18.1 kg) bag yields approximately 0.30 ft3 … A 60 lb (27.2 kg) bag yields approximately 0.45 ft3 … An 80 lb (36.2 kg) bag yields approximately 0.60 ft3” (manufacturer listing).
- SAKRETE High-Strength Concrete Mix, SAKRETE of North America (product page and technical data sheet) — “Yields 0.30 ft3 (40 lb bag); 0.45 ft3 (60 lb bag); 0.6 ft3 (80 lb bag)” (manufacturer listing).
- CIP 31: Ordering Ready Mixed Concrete, National Ready Mixed Concrete Association, Concrete in Practice (2000, 2016, 2023) — concrete is sold by volume in cubic yards (cubic meters); order 4% to 10% more than the volume from plan dimensions.
- CIP 8: Discrepancies in Yield, National Ready Mixed Concrete Association, Concrete in Practice (1979–2016) — “Include an allowance of 4 to 10 percent over plan dimensions for waste, over-excavation and other contingencies”; avoid short loads.
- Checklist for Ordering and Scheduling Ready Mixed Concrete, National Ready Mixed Concrete Association and American Society of Concrete Contractors — items to agree with the supplier include “additional charges for items like returned concrete, short loads”.
- Materials I.M. 340: Weight per Cubic Foot, Yield & Air Content (Gravimetric) of Concrete, Iowa Department of Transportation — “Normal weight concrete is typically in the range of 140 - 150 lbs./cu. Ft”.
- Unit weight of normal-weight concrete (145 + 0.001·f′c pcf), Federal Highway Administration, FHWA-HRT-05-057 Appendix C.
- Guide to the SI, Appendix B.8: cubic yard, cubic foot and short ton factors, NIST Special Publication 811.
- Last reviewed:
- October 2026
- Last updated:
- October 14, 2026
- Calculator version:
- 1.0