🏗️ Concrete Calculator PRO
Volume, rebar, formwork, material breakdown & cost — all in one.
Dimension Diagram — updates as you type
Slab plan view. Length × width gives the area; thickness gives the depth of the pour.
1 Project Type
2 Shape
3 Dimensions
4 Rebar (Optional)
5 Options
Labour Cost
Results
Volume
Bags Needed
Material Breakdown (approximate)
Rebar
Formwork
Cure Time
⚠️ Pre-Export Checklist
- Footings measured on-site
- Sub-grade compacted and level
- Reinforcement placed per structural drawings
- Formwork secured and oiled
- Building permit obtained
- Weather forecast checked (no rain for 24h)
- Pump truck / wheelbarrows arranged
Professional Concrete Estimation Guide
Show formulas, worked examples & pro tips ▼
Formulas
Rectangle = L × W × T
L-Shape = (L₁ × W₁) − (L₂ × W₂) × T
Trapezoid= ((Top + Bottom) ÷ 2) × Length × T
Circle= π × r² × Depth × Quantity
Stairs= Number of steps × (Riser × Tread × Width)
Material Mix (1:2:3 ratio):
Cement = Volume ÷ (1+2+3) × 1.5 (bags per cu ft)
Sand = Volume × (2 ÷ 6) × 1.5 tons per cu yd
Gravel = Volume × (3 ÷ 6) × 1.5 tons per cu yd
Rebar Weight= Bars × Length × Weight per ft (varies by bar size)
Worked Example — 10×10 ft Slab, 4" thick
- Volume: 10 × 10 × (4÷12) = 33.3 cu ft = 1.23 cu yd
- Bags (80 lb): 33.3 ÷ 0.60 = 56 bags → with 10% waste = 62 bags
- Cement: ~62 bags × 94 lb = 2.9 tons cement
- Sand: ~1.1 cu yd = 1.6 tons
- Gravel: ~1.6 cu yd = 2.4 tons
- Water: ~33.3 × 6 gal/cu yd = 25 gallons
- Rebar (#4 @ 12" O.C. both ways): 11 bars × 10 ft = 110 ft = 73.5 lb
Common Mistakes to Avoid
- Forgetting to convert inches to feet (4 in = 0.333 ft, not 4)
- Not adding waste — always 5-15% depending on site conditions
- Using wrong bag yield — 80 lb = 0.6 cu ft, 60 lb = 0.45, 40 lb = 0.30
- Ignoring rebar overlap — add 40× bar diameter for each splice
- Not checking concrete pump accessibility for large pours
- Pouring in direct sunlight without curing compound
Pro Tips
- For pours >1 cu yd, order ready-mix trucks instead of bags
- Use a concrete vibrator to eliminate air pockets
- In hot weather (>90°F), add ice to mix water to slow setting
- Spray curing compound immediately after finishing
- Control joints should be spaced at 24-30× slab thickness
- Order 10% extra — uneven subgrade always uses more concrete
Drag to rotate — Scroll to zoom
Related Calculators
Frequently used together on the same project.
Client & Quote (prints on PDF & Excel)
Adjustments (applies to exports)
Contingency is added as a separate line on the exported cost summary. Waste % is printed as an advisory reminder.
Project notes (prints on PDF & Excel)
Result compare (A/B)
| Item | Pinned | Current | Change |
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Saved rates auto-apply from your Price Library. Advanced details stay in your browser — nothing is uploaded except inside your own PDF/Excel exports.
Whether you are pouring a patio slab, setting fence footings, or building a foundation for a new garage, getting the concrete volume right is the difference between a smooth pour and a costly disaster. Our free online concrete calculator takes the guesswork out of the job by computing the exact volume of concrete you need in cubic feet, cubic yards, or cubic meters, and then converts that figure into the number of pre-mixed bags required. We have seen homeowners order twenty percent too much concrete because they estimated by gut feel, and we have also seen jobs halt mid-pour because the short-order fee ate into the profit margin. This tool eliminates both problems in seconds.
Simply enter the dimensions of your project — length, width, and thickness for a slab, or diameter and depth for a circular footing — and the calculator does the rest. It factors in standard bag sizes of 40, 60, and 80 pounds, so whether you are mixing by hand in a wheelbarrow or loading a mixer, you get the right count. For commercial projects where ready-mix trucks are involved, the calculator also provides the volume in cubic yards, which is the standard unit contractors use when ordering from a batch plant. The waste factor field lets you add five to ten percent extra for spillage, uneven subgrade, and form irregularities — a step that many DIYers skip and later regret. The cost estimator then multiplies the bag count or volume by your local price per bag or per cubic yard, giving you a ballpark budget before you even call the supplier. We built this calculator based on real-world construction experience, not textbook theory, so every output is something you can take to the job site with confidence.
📖 How to Use This Concrete Calculator
Step 1: Choose Your Shape
Select Rectangle for slabs, walls, and footings, or Circle for round footings, columns, and sonotubes. The shape determines which dimension fields appear.
Step 2: Enter Dimensions
Type the length, width, and thickness for rectangles, or diameter and depth for circles. Use consistent units — feet for Imperial, meters for metric. For a standard 4-inch patio slab that is 12 ft by 16 ft, enter 12, 16, and 0.33.
Step 3: Select Bag Size
Choose 40 lb, 60 lb, or 80 lb bags. A standard 80 lb bag yields about 0.6 cubic feet of mixed concrete. The calculator uses the yield per bag to determine the total count.
Step 4: Add Waste Factor
Enter a waste percentage. For flat slabs on a well-prepared subgrade, 5% is usually enough. For walls, columns, or uneven terrain, 10% is safer. This covers spillage, over-excavation, and form leaks.
Step 5: Enter Unit Cost
Input the price per bag at your local hardware store, or the cost per cubic yard if ordering ready-mix. The calculator multiplies by the total quantity to give you a material cost estimate.
Step 6: Review Results
The output shows volume in cubic feet and cubic yards, the number of bags needed, and total estimated cost. For ready-mix, note that concrete is typically ordered in quarter-yard increments.
📐 Understanding the Concrete Volume Formula
The fundamental formula for rectangular concrete volume is straightforward: Volume = Length × Width × Thickness. All three dimensions must be in the same unit before you multiply. If your length and width are in feet and your thickness is in inches, divide the thickness by 12 to convert to feet first. For a 20 ft by 30 ft garage slab at 4 inches thick, the calculation is 20 × 30 × 0.333, which equals 200 cubic feet.
To convert cubic feet to cubic yards — the unit batch plants use — divide by 27, since there are 27 cubic feet in one cubic yard. So 200 divided by 27 gives 7.41 cubic yards. For circular footings or columns, the formula changes to Volume = π × radius² × depth. A round footing with a 24-inch diameter (2 feet) and 36-inch depth (3 feet) yields 3.1416 × 1² × 3, which equals 9.42 cubic feet, or about 0.35 cubic yards.
Once you have the volume, the bag count depends on the yield of the bag. A standard 80 lb bag of pre-mixed concrete produces roughly 0.6 cubic feet of wet concrete. So 200 cubic feet divided by 0.6 equals 334 bags. In practice, you round up because partial bags are not sold. The waste factor adjusts this upward: at 5% waste, 334 × 1.05 equals 351 bags.
For ready-mix, the batch plant charges by the cubic yard with a minimum load, typically 1 to 2 cubic yards. If your calculation shows 7.41 cubic yards, most contractors would order 7.5 yards to be safe, or 8 yards if the pour is in a remote location where returning for a half-yard short-load is expensive. The weight of wet concrete is approximately 150 pounds per cubic foot, which matters for formwork design and for knowing whether your truck can handle the load.
⚠️ Common Mistakes to Avoid
Not converting inches to feet before multiplying
✓ If your slab is 4 inches thick, divide 4 by 12 to get 0.333 feet. Multiplying 20 × 30 × 4 gives 2,400 cubic feet — more than ten times the correct answer of 200 cubic feet.
Forgetting the waste factor
✓ Always add 5 to 10 percent for spillage, uneven subgrade, and over-excavation. On a 7-cubic-yard order, that is less than half a yard — cheap insurance against running short mid-pour.
Ordering exact cubic yardage from the batch plant
✓ Order in quarter-yard increments and always round up. A short-load fee of 75 to 150 dollars costs more than the extra concrete you would have received.
Using the wrong bag yield
✓ An 80 lb bag yields about 0.6 cubic feet; a 60 lb bag yields about 0.45 cubic feet. Using the wrong figure gives a bag count that is off by 25 percent or more.
Ignoring the minimum order for ready-mix
✓ Most batch plants have a 1 to 2 cubic yard minimum. For small jobs, bag-mix is often cheaper than paying the short-load surcharge.
💡 Professional Tips
Always order 0.5 cubic yards more than the calculated volume for slab pours. The extra concrete is cheaper than sending the truck back for a short load.
For columns and walls, use a waste factor of 10 to 15 percent because spillage into forms is higher than on flat slabs.
On hot days above 85 degrees F, concrete sets faster. Order a retarder additive from the batch plant to keep the mix workable longer.
If the subgrade is sandy or has loose soil, compact it and add 5 percent to the volume to account for absorption into the ground.
For 4-inch slabs, use 80 lb bags for efficiency; for 2-inch patch work, 40 lb bags are easier to handle and produce less waste.
When pouring footings, stake the corners first and run a string line to check dimensions before the truck arrives.
For decorative concrete, specify 4,000 PSI minimum for driveways and 3,000 PSI for interior slabs to ensure durability.
❓ Frequently Asked Questions
For a 10 ft by 10 ft slab at 4 inches thick, the volume is 33.3 cubic feet. At 0.6 cubic feet per 80 lb bag, you need approximately 56 bags. Add 5% waste and you are looking at about 59 bags. This is a common patio size and most home improvement stores stock enough for same-day pickup.
A 20 ft by 30 ft slab at 4 inches thick is 200 cubic feet, which equals 7.41 cubic yards. Order 7.5 cubic yards from the batch plant. If you want a thicker 6-inch slab for heavier vehicles, the volume jumps to 11.1 cubic yards.
Bag mix is pre-portioned dry concrete you mix with water on site, convenient for small jobs under 2 cubic yards. Ready-mix is batched at a plant and delivered by truck, ideal for larger pours. Ready-mix is more consistent and cheaper per cubic yard, but there is a minimum order and delivery fee.
Standard 3,000 to 4,000 PSI concrete costs 125 to 165 dollars per cubic yard delivered, depending on your region and mix design. Specialty mixes like fiber-reinforced or colored concrete can run 175 to 250 dollars per yard. Bag mix costs roughly 5 to 7 dollars per 80 lb bag.
This calculator is designed for poured concrete: slabs, footings, columns, and walls. For concrete masonry units, you would need a block calculator that accounts for mortar joints. However, you can use this tool to calculate the grout fill volume inside block cores.
Concrete typically sets enough for foot traffic in 24 to 48 hours, but it takes 7 days to reach about 70% of its design strength and 28 days to fully cure. Keep the slab moist by covering it with plastic sheeting or misting it with water for the first 7 days to prevent cracking.
🏗️ Real-World Applications
Concrete is the most widely used construction material on the planet, and our calculator serves a broad range of real-world projects. For residential homeowners, the most common application is a backyard patio. A typical 12 ft by 16 ft patio at 4 inches thick requires about 2.4 cubic yards of concrete — roughly 144 bags of 80 lb mix. That is a full day of mixing and pouring for a two-person crew.
For driveway replacements, the stakes are higher because the slab must support vehicle loads. A two-car driveway, say 20 ft by 20 ft, at 6 inches thick needs about 7.4 cubic yards. This is a job that almost always calls for ready-mix delivery, and contractors in most regions specify 4,000 PSI concrete with fiber reinforcement for residential driveways. Commercial applications are even more demanding. A warehouse floor slab might be 8 to 12 inches thick and span thousands of square feet, requiring hundreds of cubic yards in a single pour.
For foundation work, the calculator helps estimate footing volumes before the walls go up. A typical spread footing for a residential home — 2 ft wide, 1 ft deep, running 60 linear feet — requires about 4.4 cubic yards. Add the stem walls and you are looking at a foundation pour of 10 to 15 cubic yards. In colder climates, insulated concrete forms (ICFs) are becoming popular for foundations because they combine structure and insulation in one step.
For DIY projects, the calculator is invaluable for bathroom renovations where a mortar bed or self-leveling underlayment is needed, for fence post footings (typically 12-inch diameter by 36 inches deep, about 0.3 cubic feet per hole), and for setting mailbox posts and basketball hoop anchors. The cost estimator helps homeowners decide whether to DIY or hire a contractor by giving them a realistic material budget to compare against bid prices.
Our free concrete calculator is the most accurate online tool for estimating concrete volume, bag count, and project cost. Whether you need a concrete slab calculator for your patio, a concrete bag calculator for a small repair, or a ready-mix concrete calculator for a foundation pour, this tool handles it all. Use it as a concrete yard calculator, a concrete volume calculator, or a general construction calculator for any project that involves pouring concrete. It works for rectangular slabs, circular footings, columns, and walls. The calculator supports both Imperial and metric units, making it useful for contractors and DIYers worldwide. Save time, save money, and avoid costly mistakes by using our free online concrete estimate tool before your next pour.
✅Method, Sources & Review
Maintained by theBuildingCalc methods desk· Last reviewedSeptember 23, 2026. This calculator implements the standards and references below; every figure can be rechecked against the formulas and worked example on this page.
Results are estimates, not engineering advice. Verify against local codes and licensed professionals for structural, electrical, and gas work. See ourTerms. Spotted an error? Report it— corrections ship with the next review.