You're staring at your backyard and picturing a new patio-but first you need to know: how much concrete actually goes into this thing?
Whether you're pouring a small walkway or a substantial garage foundation, ordering too little concrete means expensive return trips or incomplete work. Too much and you're throwing money away. Our concrete calculator eliminates the guesswork, giving you the exact cubic yards (or bag count) you need in seconds.
What This Calculator Does
This calculator converts your slab dimensions into cubic yards of concrete-the industry standard measurement. You input the length, width, and thickness of your project, and we handle the math that accounts for depth in inches and volume in cubic yards. We also calculate how many standard 60-lb or 80-lb bags you'd need if you're mixing concrete manually, and apply a realistic 10% waste factor for spillage, settling, and edge variations.
How to Use This Calculator
Step 1: Measure your slab length and width in feet. Be as precise as possible-a tape measure works best, but even pacing it out gives a reasonable estimate.
Step 2: Decide on your slab thickness. Most residential slabs run 4 inches (standard for driveways and patios), but foundation work, garage floors, and high-traffic areas often call for 5–6 inches. Check local codes or talk to a concrete supplier if you're unsure.
Step 3: Enter these dimensions into the calculator. It instantly converts everything into cubic yards, the measurement concrete trucks deliver.
Step 4: Note the waste factor included. Concrete settles, splashes, and fills tiny surface imperfections. The extra 10% accounts for reality, not just perfect geometry.
Step 5: If you're hand-mixing bags instead of ordering ready-mix, the calculator shows how many bags you'll need. Keep this number handy when you order from your home center or landscaping supplier.
The Formula Behind the Math
Here's the actual equation our calculator uses:
Volume (cubic yards) = (Length ft × Width ft × Thickness inches ÷ 12) ÷ 27
Let's walk through a real example: You're pouring a 20-foot by 12-foot patio at 4 inches thick.
If you're mixing bags instead, one 80-lb bag yields roughly 0.6 cubic feet. So 80 cubic feet ÷ 0.6 = 133 bags (before the waste factor). With 10% added, you'd order about 146 bags.
Our calculator does all of this instantly-but now you understand exactly what it's computing.
Residential Driveway (4" Thick Standard)
A typical single-car driveway runs about 9 feet wide by 20 feet long. At 4 inches thick:
A double-car driveway (18 feet wide × 20 feet long) needs about 4.4 cubic yards with waste. Most suppliers deliver a 5-cubic-yard truck for jobs this size.
Backyard Patio (4" Thick, Irregular Shape)
If your patio isn't a perfect rectangle, break it into smaller rectangles, calculate each, then add them together. A patio shaped like an "L" might be a 12 × 8 section plus a 6 × 4 section. Calculate both, sum them, and add 10% waste. This method works for any non-standard shape without complex geometry.
Foundation or Garage Floor (5–6" Thick)
Foundations and heavy-use garage floors demand thicker concrete for load-bearing strength. A 20 × 20 garage floor at 5 inches thick:
For a full basement foundation with walls, you'll also need cubic yardage for footings and other elements. This calculator handles the slab; consult a contractor for complete foundation specs.
Tips and Things to Watch Out For
Order from a ready-mix supplier when possible. A concrete truck delivers far faster than hand-mixing hundreds of bags, and it's often cheaper per cubic yard, especially for jobs over 3 cubic yards. Plus, one truckload pours as one solid mass-uniformly mixed and compacted. Bagged concrete is best for small repairs, thin overtopping, or projects under 1 cubic yard.
Know your local concrete standards. Building codes vary by region. Some areas require 5 inches for driveways in freezing climates, others accept 4 inches. Check with your local building department or a concrete contractor before placing an order. Undersizing thickness creates cracking and failure faster than a year or two in.
Factor in the slope. Proper drainage keeps water from pooling on your slab. If you're including a slope (crucial for driveways), the slope's height adds a bit of extra volume. Measure the average thickness across the slab if it's not uniform-this calculator assumes even thickness, so adjust if your plan includes a crown or pitch.
Always order 10–15% more material than your calculation to account for waste, cuts, breakage, and measurement errors. This is non-negotiable. Concrete shrinks slightly as it cures, spillage happens during the pour, and your tape measure reading might be off by an inch or two over 20 feet. Running short mid-pour forces you to stop, clean the surface, order more concrete, and create a cold joint-a visible seam that's a stress point for cracks.
Prepare the subgrade. Concrete poured on soft, uncompacted soil will sink and crack. Compact and level the soil, add 2–4 inches of gravel base for drainage, and tamp that down too. This foundation layer isn't about volume calculation, but it is about keeping your concrete intact for years.
Check the weather. Concrete sets through a chemical reaction (hydration) that's sensitive to temperature. Pour in freezing conditions and the curing process stalls. Extreme heat speeds it up too fast. Ideal conditions are 50–75°F with low wind. If conditions are outside this range, a concrete contractor can add admixtures to adjust curing time.
Frequently Asked Questions
How many bags of concrete do I need for a 10×10 patio?
A 10-by-10-foot patio at 4 inches thick needs 1.23 cubic yards, or about 2.05 cubic yards with 10% waste. That's roughly 55 eighty-pound bags. A 60-lb bag count would be higher. Always check the coverage rate on the bag-products vary.
Can I use concrete calculator results for ordering ready-mix?
Yes. Call your concrete supplier, tell them the cubic yardage, thickness, and intended use (driveway, patio, foundation), and they'll confirm the mix and delivery truck size. Most ready-mix trucks hold 8–10 cubic yards, so smaller residential jobs fit easily.
What if my slab isn't perfectly square?
Divide it into rectangles. Calculate each section separately, add the results, then apply the 10% waste factor once at the end. A 20-by-12 space with a 5-by-4 cutout becomes (20 × 12) − (5 × 4) = 220 square feet. Then proceed with thickness conversion as normal.
Do I need to add extra concrete for slopes or crowns?
If the slope is gradual (standard drainage pitch for a driveway), the extra volume is minimal-usually under 3% of your total. Use the average thickness across the slab. For steep slopes or variable thickness, calculate sections separately at their respective thicknesses.
How long does concrete take to cure before I can use it?
Concrete reaches about 70% strength in 7 days and full strength around 28 days, depending on temperature and mix design. Most contractors recommend waiting at least 7 days before heavy foot traffic or vehicle use. Check with your concrete supplier for their specific product timeline.
What's the difference between 60-lb and 80-lb bags?
An 80-lb bag covers slightly more volume (0.6 cubic feet) than a 60-lb bag (0.45 cubic feet). Eighty-pound bags are more efficient if you're mixing by hand-fewer bags to handle-but they're heavier. The price difference is usually small, so check both at your supplier.
Can the concrete calculator account for decorative finishes?
This calculator measures structural concrete volume only. Decorative finishes like stains, stamps, or polished surfaces don't change the amount of concrete needed-only how it looks afterward. Calculate structural thickness as usual, then plan finishing work on top.
Related Calculators
If you're building a complete outdoor living space or foundation project, the square footage calculator helps you measure irregular areas precisely, and the rebar calculator determines reinforcement specs if your project requires it. The gravel calculator estimates base preparation, and the material cost estimator can forecast total project spending once you know your concrete needs.