Concrete Slab Calculator
Stop guessing at the hardware store. Accurately calculate the exact volume of concrete you need for slabs, patios, and driveways, instantly converting dimensions into cubic yards and bag quantities.
Project Estimate
Whether you are planning a small backyard patio or quoting a massive 20×30 concrete slab for a new detached garage, accuracy is everything in masonry. Ordering too much concrete wastes hundreds of dollars, while coming up short halfway through a pour creates weak “cold joints” that compromise the structural integrity of the entire slab.
Our interactive concrete slab calculator removes the guesswork. By inputting your length, width, and thickness, you can instantly determine exactly how much is a yard of concrete needed for your project, and exactly how many 60lb or 80lb premixed bags it translates to.
How to Calculate Concrete for a Slab
Before you load up your truck at Home Depot, you must understand how concrete volume is measured. In North America, bulk concrete is ordered in cubic yards, while bagged concrete is calculated in cubic feet.
The foundation of the calculation is converting all your dimensions into a single unit (feet) and multiplying them together to find the geometric volume.
The Concrete Slab Formula
Volume (Cubic Feet) = Length (ft) × Width (ft) × Depth (ft)
Note on Depth: If your slab is 4 inches thick, you must divide by 12 to convert it to feet (4 / 12 = 0.333 ft).
Once you have your total in cubic feet, you simply divide that number by 27 to find the total cubic yards (because there are exactly 27 cubic feet in one cubic yard).
Example: 10×12 Concrete Slab Calculator
Let’s calculate the volume for a standard 10 ft by 12 ft patio poured at 4 inches thick.
- Convert thickness to feet: 4 ÷ 12 = 0.333 ft.
- Multiply dimensions: 10 ft × 12 ft × 0.333 ft = 39.96 cubic feet.
- Convert to yards: 39.96 ÷ 27 = 1.48 cubic yards.
Understanding Area vs. Volume: How to Calculate Sq Ft of a Slab
Many homeowners confuse square footage (area) with cubic footage (volume). Square footage only measures the flat, 2D surface of the ground you are covering. It does not account for how thick the concrete will be.
How to calculate sq ft of a slab: Simply multiply the Length (in feet) by the Width (in feet). A 10×10 concrete slab is exactly 100 square feet. A 20×30 concrete slab is 600 square feet.
While you need the square footage to buy finishing sealers or surface tiles, you must calculate volume (cubic yards) to know how much wet concrete to pour.
Concrete Bag Conversions (40lb, 60lb, 80lb)
If your project requires less than 1.5 cubic yards, you will likely be using a bag concrete calculator method and mixing it yourself. A common mistake is assuming that bag weight equals coverage area. You must divide your total required cubic feet by the specific yield of the bag size you are buying.
| Bag Size | Volume Yield (Cubic Feet) | Bags to make 1 Cubic Yard |
|---|---|---|
| 40 lb | ~ 0.30 ft³ | 90 bags |
| 50 lb (Fast Set) | ~ 0.375 ft³ | 72 bags |
| 60 lb | ~ 0.45 ft³ | 60 bags |
| 80 lb | ~ 0.60 ft³ | 45 bags |
How many 60lb bags of concrete for 10×10 slab?
A 100 sq ft slab poured at 4 inches deep requires 33.3 cubic feet of concrete. A standard 60lb bag yields 0.45 cubic feet. Therefore, 33.3 ÷ 0.45 = 74 bags. Adding a 10% waste factor means you should buy about 82 bags.
Structural Needs: Thickness, Rebar, and Gravel
A perfectly calculated slab will still crack and fail if the subgrade and reinforcement are inadequate.
Do I need gravel under a concrete slab?
Yes. You should almost always install a 4-inch to 6-inch compacted base of crushed gravel or road base beneath your slab. This base provides crucial drainage, prevents water from pooling against the bottom of the concrete, and protects against the severe lifting forces of winter frost heave.
Does a 4 inch concrete slab need rebar?
The standard 4-inch thickness is meant for pedestrian traffic—patios, sidewalks, and shed floors. For these light loads, heavy steel rebar is usually overkill. However, you should install welded wire mesh to control temperature shrinkage and keep the surface from cracking apart.
Does a 6 inch concrete slab need rebar?
Yes, absolutely. If you are pouring a 6-inch slab, it is because you intend to park vehicles (RVs, trucks) or place heavy machinery on it. A 6-inch slab requires a tied grid of #3 (3/8″) or #4 (1/2″) rebar, suspended perfectly in the middle third of the concrete’s depth, to provide the tensile strength needed to resist cracking under massive weight.
Concrete Slab Cost Estimates
Determining your concrete slab cost involves more than just the price of the wet mix. You must factor in form boards, gravel base, rebar, delivery fees, and labor.
- How much does a 20×20 slab cost? A 400 sq ft slab requires about 5 yards of concrete. Factoring in delivery, rebar, site prep, and professional labor, expect to pay between $2,400 and $4,000.
- How much does it cost to pour a 40×60 slab of concrete? A massive 2,400 sq ft monolithic slab (often for commercial steel buildings) requires roughly 30 cubic yards of concrete and extensive heavy machinery for grading. Total turnkey costs generally range from $14,000 to $24,000.
Bags vs. Ready-Mix Delivery
Is it cheaper to mix and pour your own concrete? The answer depends strictly on the volume.
If your project is under 1.5 cubic yards (about 65 bags of 80lb concrete), buying bags and renting a concrete mixer is cheaper because ready-mix companies charge hefty “short load” fees for small deliveries.
However, if your project exceeds 2 cubic yards, ordering a delivery truck is highly recommended. The cost per concrete yard drops significantly, you guarantee a continuous monolithic pour (avoiding cold joints), and you save yourself the brutal physical labor of manually lifting and mixing 100+ bags.
Frequently Asked Questions
Reference: https://en.wikipedia.org/wiki/Concrete_slab