Construction calculators
Get accurate material estimates for your next project. No more guessing how much concrete to order.
Concrete Calculator
Calculate how much concrete you need for slabs, walls, columns, and stairs. Get cubic yards, 60lb/80lb bag counts, and cost estimates.
Use this toolHow to Figure Out How Much Concrete You Need
The calculation is straightforward: length × width × thickness gives you cubic feet. Divide by 27 to get cubic yards (ready-mix is sold by the yard). Then multiply by bag yield to get bag count. The mistake most people make is mixing units — measure everything in feet or convert inches to feet before you multiply.
Worked example — 10 ft × 10 ft slab, 4 inches thick:
- Convert 4 inches to feet: 4 / 12 = 0.333 ft
- Volume in cubic feet: 10 × 10 × 0.333 = 33.3 cubic feet
- Volume in cubic yards: 33.3 / 27 = 1.23 cubic yards
- 80 lb bags needed (each yields roughly 0.6 cu ft): 33.3 / 0.6 = 56 bags
- Add 10% for waste and uneven subgrade: 62 bags or 1.36 cubic yards
The concrete calculator does the whole thing in one pass — enter your slab dimensions, pick your bag size or ready-mix, and it gives you cubic yards, bag count, and estimated cost. It also handles walls, columns, stairs, and odd shapes so you are not doing the math by hand for every form.
Common Material Estimation Mistakes
Three errors show up on almost every DIY concrete project:
- Not adding waste. Ground is never perfectly level, forms shift, and some concrete stays in the mixer or bucket. Always add 10% to your calculated volume. Ordering exactly what the math says guarantees you come up short.
- Confusing bag sizes. A 60 lb bag yields about 0.45 cubic feet. An 80 lb bag yields about 0.6 cubic feet. That is a 33% difference per bag. Using the wrong bag size in your estimate can leave you 15-20 bags short on a medium pour. The calculator accounts for this automatically — you just pick the bag size you plan to buy.
- Ordering bags when you should order a truck. For pours over 2 cubic yards, ready-mix is almost always cheaper per yard and saves you hours of mixing. A yard of delivered concrete costs $120-$150. A yard's worth of 80 lb bags (45 bags at $6-$7 each) costs $270-$315 plus your labor. The calculator shows both options so you can compare the real cost of bagged vs delivered before you decide.
Frequently Asked Questions
How many bags of concrete do I need for a 10×10 slab?
A 10 ft × 10 ft slab at 4 inches thick needs roughly 56 bags of 80 lb concrete or 75 bags of 60 lb concrete, plus 10% for waste — so 62 bags of 80 lb or 83 bags of 60 lb. The concrete calculator adjusts for slab thickness, so if you pour a 6-inch slab instead of 4-inch, the numbers change to 84 bags of 80 lb.
How many cubic feet does an 80 lb bag of concrete cover?
One 80 lb bag of concrete mix yields roughly 0.6 cubic feet when mixed with water. At 4 inches thick, one bag covers about 1.8 square feet. For a 10×10 slab (100 sq ft), you need about 56 bags. The concrete calculator converts between cubic feet, bags, and cost automatically.
Should I use bagged concrete or order a ready-mix truck?
For pours under 1 cubic yard, bags are usually more practical because you can buy them at any home center and mix as needed. For 1-2 cubic yards, it depends on whether you have help mixing and placing. Above 2 cubic yards, ready-mix is cheaper, faster, and produces a stronger slab because the concrete is batch-mixed professionally. The calculator shows the cost difference before you decide.
How do I calculate concrete for a round column or footing?
Use the formula for cylinder volume: π × radius² × height. A 12-inch diameter column that is 8 ft tall has a radius of 0.5 ft. Volume: 3.14 × 0.5² × 8 = 6.28 cubic feet, or about 0.23 cubic yards — roughly 11 bags of 80 lb mix. The concrete calculator has a column mode so you do not have to run the cylinder formula every time.