Concrete Calculator
Calculate bags or cubic yards of concrete needed for any project.
Concrete Calculator
What This Calculator Does
The Concrete Calculator provides a complete concrete takeoff for slabs, footings, posts, and walls. Enter your project dimensions, select the thickness, and the calculator returns cubic yards, 60-lb bags, and 80-lb bags needed - plus the recommended order quantity with 10% waste.
Concrete is the most widely used construction material in the US, with over 500 million cubic yards produced annually. It is ordered either by the cubic yard (ready-mix delivery) or by the bag (premix for smaller jobs). Getting the quantity right is critical: a short pour means a cold joint and structural weakness; over-ordering means non-returnable excess. The built-in 10% waste factor accounts for spillage, uneven subgrade, and over-excavation.
The calculator supports multiple slab thickness options from 3 to 8 inches, covering everything from light pathways to heavy foundation walls. The result includes the cubic yard figure for ready-mix quotes and bag counts for home center purchases.
How to Use
- Measure the length and width of the pour area in feet.
- Select the slab thickness from the dropdown (4 in standard, 6 in for driveways).
- Enter dimensions and click "Calculate" to get cubic yards and bag counts.
- Quote the cubic yard figure to your ready-mix supplier - add 10% for waste.
- For bagged concrete, use the bag count to order from your local home center.
How to Calculate Concrete
Concrete volume is calculated using the standard geometric formula for the shape of your pour. For rectangular slabs - the most common residential application - follow these steps:
- Measure the area. Find the length and width of the pour area in feet. Multiply to get the square footage.
- Convert thickness to feet. Divide the slab thickness in inches by 12 to convert to feet. A 4-inch slab = 0.33 ft, a 6-inch slab = 0.5 ft.
- Calculate cubic feet. Multiply area (sq ft) by thickness (ft): Area x Thickness = Cubic feet.
- Convert to cubic yards. Divide cubic feet by 27 (since 1 cubic yard = 27 cubic feet). This gives you the volume in cubic yards - the standard unit for ordering ready-mix concrete.
- Add waste. Multiply by 1.10 to add 10% for spillage, subgrade irregularities, and over-excavation.
For concrete footings, use the same formula: length x width x depth. For round columns, use the cylinder formula: pi x r squared x height, then divide by 27 for cubic yards.
Practical Measurement Tips
- Ordering ready-mix. Concrete trucks typically carry 9-11 cubic yards. Most suppliers have a 3-5 cubic yard minimum delivery charge. Plan your pour size accordingly or coordinate with a neighbor for combined delivery.
- Timing. Ready-mix concrete begins setting within 60-90 minutes of batching. Have all labor, tools, and forms ready before the truck arrives. For large pours, schedule multiple trucks in sequence.
- Slump test. Standard residential concrete has a 4-5 inch slump (workability measure). Too much water weakens concrete - never add water beyond the mix design specification.
- Curing. Concrete reaches about 70% of its design strength in 7 days and 100% in 28 days. Keep the surface moist for at least 7 days after pouring for proper hydration. Apply curing compound if you cannot wet-cure.
- Temperature. Ideal pouring temperature is 50-90 degrees F. In hot weather, use chilled water or ice in the mix. In cold weather, use accelerators and insulating blankets to prevent freeze damage.
Worked Examples
Standard Patio Slab
A 12 x 16 ft patio slab at 4 inches thick.
- →Area: 12 ft x 16 ft = 192 sq ft.
- →Thickness: 4 in / 12 = 0.33 ft.
- →Cubic feet: 192 x 0.33 = 63.4 cu ft.
- →Cubic yards: 63.4 / 27 = 2.35 cu yd.
- →With 10% waste: 2.35 x 1.10 = 2.59 cu yd (order 2.75 cu yd to be safe).
- →Bagged equivalent: 2.35 cu yd x 45 bags per yard = 106 bags of 80-lb mix.
Result: 2.35 cubic yards needed, order approximately 2.75 cubic yards with waste.
For a slab this size, ready-mix delivery is more practical than bagged concrete. At $150/yard delivered, expect to pay approximately $410 for materials.
Driveway Slab
A 20 x 24 ft two-car driveway at 5 inches thick.
- →Area: 20 ft x 24 ft = 480 sq ft.
- →Thickness: 5 in / 12 = 0.417 ft.
- →Cubic feet: 480 x 0.417 = 200 cu ft.
- →Cubic yards: 200 / 27 = 7.41 cu yd.
- →With 10% waste: 7.41 x 1.10 = 8.15 cu yd.
- →This fills most of a standard 10-yard truck.
Result: Approximately 7.4 cubic yards of concrete needed for a 2-car driveway.
Driveways require 4,000 PSI concrete with proper reinforcement. Use #4 rebar at 12-inch spacing or welded wire mesh. Order a full 8.5 yards to account for waste and grade variations.
Waste Factors by Material
Always order more than your exact calculated quantity. Material suppliers typically do not accept returns on cut or opened materials. The waste factor accounts for cuts at walls, corners, defects, and installation error.
| Application | Recommended Waste | Notes | Related Calculator |
|---|---|---|---|
| Slab-on-grade (patios, floors) | 10% | Spillage, uneven subgrade, minor over-excavation | Concrete Slab Cost Calculator |
| Driveways | 10-15% | Grade variations, thicker edges for load-bearing | Driveway Cost Calculator |
| Footings and foundations | 10% | Trench width variations, over-dig in soft soil | Concrete Slab Cost Calculator |
| Columns and posts | 5-10% | Sonotube overlap and spillage at top | Concrete Volume Calculator |
| Sidewalks and walkways | 10% | Form leakage and surface variations | Patio Cost Calculator |
Reference Table
| Slab Size | Thickness | Cubic Yards | 60-lb Bags | 80-lb Bags | Cost Range |
|---|---|---|---|---|---|
| 10 x 10 ft | 4 in | 1.23 | 55 | 45 | $160-$250 |
| 12 x 16 ft | 4 in | 2.37 | 107 | 88 | $310-$475 |
| 20 x 20 ft | 4 in | 4.94 | 222 | 183 | $640-$990 |
| 20 x 20 ft | 6 in | 7.41 | 333 | 274 | $960-$1,480 |
| 24 x 24 ft | 4 in | 7.11 | 320 | 263 | $920-$1,420 |
| 30 x 40 ft | 5 in | 18.52 | 833 | 685 | $2,400-$3,700 |
How We Calculate
Volume Calculation
The calculator uses the standard geometric formula for rectangular prism volume: Volume = Length x Width x Thickness. All dimensions are converted to feet before calculation. The result in cubic feet is divided by 27 (the number of cubic feet in one cubic yard) to give the standard US concrete ordering unit.
Bag Count Conversion
Bag counts are calculated using standard industry yields: a 60-lb bag produces approximately 0.45 cubic feet of concrete, and an 80-lb bag produces 0.60 cubic feet. The formula is: Total cubic feet / cubic feet per bag, rounded up to the nearest whole bag.
Waste Factor
A 10% waste factor is applied to all results by default. This accounts for spillage during placement, uneven subgrade conditions, minor over-excavation, and the small amount that remains in the truck drum after discharge. For driveways and complex pours, experienced contractors often use 15%.
References and Data Sources
ASTM C94 - Standard Specification for Ready-Mixed Concrete
The industry standard governing production, delivery, and testing of ready-mixed concrete in the United States. All ready-mix suppliers operate to ASTM C94 specifications, which define acceptable slump ranges, sampling procedures, and strength testing protocols referenced in this calculator.
ACI 318 - Building Code Requirements for Structural Concrete
The American Concrete Institute code that sets minimum requirements for design and construction of structural concrete. Rebar spacing, cover requirements, minimum slab thicknesses, and PSI recommendations in the worked examples are based on ACI 318 residential provisions.
PCA - Portland Cement Association Design and Control of Concrete Mixtures
The reference manual for concrete mix design, including water-cement ratio guidelines, curing requirements, and aggregate specifications. Standard yields for 60-lb and 80-lb concrete bags (0.45 and 0.60 cu ft respectively) are based on PCA published data.
All references are used for general estimation guidance only. BuildCalcHub does not claim certification, endorsement, or partnership with any listed organization. Always consult a licensed professional for your specific project requirements.
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