Concrete Calculator
Ordering too little concrete means a cold joint and a second delivery fee; ordering too much is money poured into the ground. This concrete calculator turns your project dimensions into an exact volume — in cubic feet and cubic yards — for the three most common pours: slabs (patios, driveways, shed floors), footings (continuous trenches under walls), and round columns (deck piers, fence posts, Sonotube forms).
It also converts that volume into premixed bag counts for 40, 60, and 80 lb bags, adds the industry-standard 10% waste allowance for spillage and uneven subgrade, and estimates total material cost if you enter your local price per cubic yard. As a rule of thumb: if your project is under about 1 cubic yard, bags from the home center usually win; above that, a ready-mix truck (typically $125–$175 per yard in most US markets) is cheaper and far less work. Enter your dimensions below — every field has a unit toggle so you can mix feet and inches exactly the way your tape measure reads.
| Premix bag size | Exact need | With 10% waste |
|---|---|---|
| 40 lb bag (yields 0.30 cu ft) | ||
| 60 lb bag (yields 0.45 cu ft) | ||
| 80 lb bag (yields 0.60 cu ft) |
Bag yields are manufacturer-typical values (40 lb ≈ 0.30, 60 lb ≈ 0.45, 80 lb ≈ 0.60 cu ft). Always round up — running short mid-pour is far worse than one extra bag.
How to Use This Concrete Calculator
- Pick your shape: Slab for flat pours (patios, driveways, garage floors), Footing for continuous trenches, Round Column for cylindrical piers and post holes.
- Enter dimensions exactly as measured — every field has a feet/inches toggle, so a 4-inch slab thickness needs no mental conversion.
- Read your volume: the big number is cubic yards (how ready-mix is sold); cubic feet is below it (how bags are measured).
- Choose bags or truck: the bag table shows exact counts for 40/60/80 lb bags; over ~1 yard, price out ready-mix delivery instead.
- Order with waste: use the “+10% waste” figures for your actual order — subgrade is never perfectly level.
- Optionally add your local price per yard to see a total material cost estimate.
Formulas and Assumptions
- Slab / Footing: volume = length × width × thickness (or depth), all in feet
- Round Column: volume = π × (diameter ÷ 2)² × height
- Unit conversions: 1 cubic yard = 27 cubic feet; inches are converted to feet before multiplying
- Bag yields (industry-typical): 40 lb ≈ 0.30 cu ft, 60 lb ≈ 0.45 cu ft, 80 lb ≈ 0.60 cu ft — bag counts round up to the next whole bag
- Waste allowance: +10% covers spillage, over-excavation, and uneven subgrade; cost is estimated on the waste-adjusted yardage
- Slab thickness guide: 4 in for patios/walkways, 4–6 in for driveways, 6 in+ for heavy vehicles; footings typically 8–12 in deep, twice wall width
FAQ
How many 80 lb bags of concrete make a yard?
One cubic yard is 27 cubic feet; at 0.60 cu ft per 80 lb bag, that is 45 bags exactly — plan on 50 with waste. For 60 lb bags it is 60 bags, and for 40 lb bags, 90.
How much concrete do I need for a 10×10 slab?
At the standard 4-inch thickness, a 10×10 slab needs 33.33 cubic feet, or 1.23 cubic yards — about 56 eighty-pound bags. Order 1.36 yards with waste allowance.
Should I use bagged concrete or ready-mix?
Bags make sense up to roughly 1 cubic yard (small slabs, a few post holes). Beyond that, mixing 45+ bags by hand costs more in labor than a short-load delivery fee, and truck-poured concrete is more consistent.
How thick should a concrete slab be?
Four inches is standard for patios and walkways. Driveways for passenger cars use 4–6 inches; RVs or heavy trucks call for 6 inches or more plus reinforcement.
What does concrete cost per yard in 2026?
Ready-mix typically runs $125–$175 per cubic yard in most US markets, plus short-load fees under about 5 yards. Bagged mix works out to roughly $250–$350 per yard equivalent — convenient, but a premium.