Concrete slab calculator
Use length, width and thickness, or enter a known square-foot/square-metre area plus thickness, for slabs, pads, floors, patios, driveways and sidewalks.
Concrete volume, bags & cost estimator
Concrete Calculator
Estimate how much concrete you need for a slab, pad, driveway, wall, footing, column, pier or post hole. Get cubic yards, cubic feet, cubic metres, premix bag counts, estimated weight and optional cost from one calculator.
1 yd³ = 27 ft³
Slab volume = L × W × T
Bag counts round up
Main tool
Concrete Volume & Bag Calculator
Waste, bags, weight & cost
Concrete including waste—Cubic yards
Cubic feet—
Cubic metres—
Selected bags—
Estimated placed weight—
Ready-mix cost—
Bag cost—
40 lb bags—
50 lb bags—
60 lb bags—
80 lb bags—
Bag yields are approximate manufacturer reference values. Real yield, subgrade, excavation, spillage and placement conditions vary, so confirm product coverage and supplier ordering rules before purchase.
Concrete estimator
What Does a Concrete Calculator Estimate?
A concrete calculator turns project dimensions into finished concrete volume. This tool works as a concrete yard calculator, concrete volume calculator and concrete estimator by reporting cubic feet, cubic yards and cubic metres, then estimating premix bags, placed weight and optional cost.
Concrete footing calculator
Use the same rectangular volume formula for strip footings, foundation walls and other straight-sided pours.
Concrete pier & column calculator
Use diameter and depth or height for round columns, piers, post holes and cylindrical pours.
Concrete bag & cost calculator
Convert the order volume into common bag counts and compare optional bag or ready-mix pricing. Adding a price also turns the tool into a concrete cost calculator.
Concrete vs cement
People often search for a cement calculator, but cement is only one ingredient in concrete. This page estimates finished concrete volume and premixed-bag quantities. It does not design structural concrete mixes or determine a project-specific Portland cement dosage.
Step-by-step method
How to Calculate Concrete
The simplest method is to calculate the volume of the space being filled, convert that volume into the ordering unit you need, and then apply a practical waste allowance.
Measure the project
Measure length, width and thickness for a rectangular pour, or diameter and depth for a round pour. Use actual site dimensions where possible.
Make units compatible
For imperial calculations, convert thickness in inches to feet by dividing by 12. In metric calculations, convert centimetres to metres by dividing by 100.
Calculate volume
Multiply the dimensions. Rectangular concrete uses length × width × thickness. Round concrete uses π × radius² × height or depth.
Convert and allow for waste
Divide cubic feet by 27 to get cubic yards. Add the selected waste percentage before calculating bag counts or cost.
Calculation formulas
Concrete Calculator Formulas
Rectangular volume = Length × Width × Thickness
Imperial formula
Cubic feet = L(ft) × W(ft) × [Thickness(in) ÷ 12]
Cubic yards = Cubic feet ÷ 27
Metric formula
Cubic metres = L(m) × W(m) × [Thickness(cm) ÷ 100]
Round column, pier or hole
Volume = π × (Diameter ÷ 2)² × Height
Worked example
Concrete Slab Calculator Example: 12 × 12 ft
For a 12 ft × 12 ft slab that is 4 inches thick, convert 4 inches to 0.3333 ft before multiplying.
1. Find cubic feet
12 × 12 × (4 ÷ 12) = 48 ft³
2. Convert to cubic yards
48 ÷ 27 = 1.78 yd³
3. Add 10% allowance
1.78 × 1.10 = 1.96 yd³
4. Estimate 80 lb bags
52.8 ft³ ÷ 0.60 ≈ 88 bags including the 10% allowance.
Premix & ready-mix
How Many Bags of Concrete Do I Need?
The calculator divides the waste-adjusted volume by the selected bag yield and always rounds up to a whole bag. For the common QUIKRETE Concrete Mix reference, approximate yields are 0.30 ft³ for 40 lb, 0.375 ft³ for 50 lb, 0.45 ft³ for 60 lb and 0.60 ft³ for 80 lb bags.
Bag yield matters more than bag weight alone
Different products can have different yields even at similar package weights. Check the bag or manufacturer data sheet and use the custom-yield option when your product differs. The published QUIKRETE Concrete Mix yields used as defaults are available in the manufacturer data sheet.
Mix terminology
Concrete Mix Calculator vs Concrete Quantity Calculator
This page is primarily a concrete quantity calculator: it estimates finished volume, yards, bags, weight and cost. A structural concrete mix calculator is a different task because cement, water, fine aggregate, coarse aggregate, admixtures, exposure and required strength all affect mix design.
The commonly searched “4-2-1 rule” is usually a reversed way of referring to the traditional 1:2:4 proportion of cement : sand : coarse aggregate. Treat that as a general nominal-mix reference, not a universal structural specification. Follow the approved project mix, local code and product instructions for structural work.
Quick reference
Concrete Slab Yardage & Bag Table
This table shows common slab areas at 4-inch and 6-inch thicknesses before any waste allowance. The 80 lb bag column uses an approximate 0.60 ft³ yield per bag.
| Slab Area | 4 in Thick | 6 in Thick | 80 lb Bags at 4 in |
|---|---|---|---|
| 50 ft² | 0.62 yd³ | 0.93 yd³ | 28 |
| 100 ft² | 1.23 yd³ | 1.85 yd³ | 56 |
| 144 ft² | 1.78 yd³ | 2.67 yd³ | 80 |
| 200 ft² | 2.47 yd³ | 3.70 yd³ | 112 |
| 300 ft² | 3.70 yd³ | 5.56 yd³ | 167 |
| 400 ft² | 4.94 yd³ | 7.41 yd³ | 223 |
| 500 ft² | 6.17 yd³ | 9.26 yd³ | 278 |
| 600 ft² | 7.41 yd³ | 11.11 yd³ | 334 |
| 800 ft² | 9.88 yd³ | 14.81 yd³ | 445 |
| 1,000 ft² | 12.35 yd³ | 18.52 yd³ | 556 |
Common questions
Concrete Calculator FAQs
Closely overlapping questions are merged so the answers stay useful instead of repeating the same sentence with slightly different wording.
What's the simplest way to calculate concrete?
For a rectangular slab, wall or footing, multiply length × width × thickness after putting the dimensions in compatible units. If the result is in cubic feet, divide by 27 for cubic yards. Then add the waste allowance you plan to use.
How many 80lb bags of concrete do I need for a 12x12 slab?
Thickness must be known. At 4 inches thick, a 12 ft × 12 ft slab is 48 ft³. Using an approximate 0.60 ft³ yield per 80 lb bag, that is 80 bags before waste, or 88 bags with a 10% allowance. At 6 inches thick, the same slab needs more concrete.
What is the 4 2 1 or 421 rule for concrete?
This phrase is commonly used for the nominal 1:2:4 proportion of cement, sand and coarse aggregate, sometimes stated in reverse order as “4-2-1.” It is not a universal mix design. Structural concrete should follow the specified mix and applicable requirements.
What happens if you use too much water in concrete?
Excess water can alter workability and can reduce concrete quality by affecting segregation, strength and durability. Use the water amount specified by the product or approved mix design rather than adding water by guesswork.
Methodology
How This Concrete Estimator Works
The calculator first converts the entered geometry to cubic feet, then converts that volume to cubic yards and cubic metres. Waste is applied to the calculated volume before bag counts and costs are estimated. Bag counts are always rounded up because partial bags are not practical ordering units. The default weight estimate uses 145 lb/ft³ as a planning density and is explicitly an estimate; actual concrete density varies by mixture.
Dimensions, subgrade conditions, excavation shape, compaction, spillage and supplier policies can all change the quantity you actually need. This tool is for material planning and does not replace structural drawings, an approved mix design or professional engineering requirements.