# Concrete Calculator Calculate cubic yards, cubic feet, bags needed, and estimated cost for any concrete shape - slabs, columns, stairs, rings, and more. Canonical page: https://invoicemama.com/calculators/concrete-calculator This is a free browser calculator. Treat results as planning aids and confirm figures against your own measurements, codes, and professional judgment. ## How to use this concrete calculator Enter slab or footing dimensions and thickness to estimate cubic yards, bag counts, and material quantities before you order ready-mix or bags. ### 1. Define the pour shape Enter length, width, and thickness for slabs, or use the footing and wall inputs the calculator provides. Break odd shapes into rectangles and add volumes. ### 2. Choose units Work in feet and inches or meters consistently so thickness matches depth in the same unit system. ### 3. Add waste and round up Include a small waste factor for spillage and uneven subgrade so ordered yards cover the whole pour. ### 4. Plan delivery or bags Use cubic yards for ready-mix tickets or convert to bag counts for small pours. Confirm slump, PSI, and fiber requirements with your supplier. ## Frequently asked questions ### How much does a cubic yard of concrete cover? One cubic yard of concrete (27 cubic feet) covers 81 square feet at a 4-inch thickness, 54 square feet at 6 inches, or 324 square feet at a 1-inch thickness. The formula is: coverage = 27 divided by thickness in feet. So for a standard 4-inch driveway slab, divide 27 by 0.333 = 81 sq ft per yard. ### How many bags of concrete do I need? It depends on the bag size. An 80 lb bag yields about 0.60 cubic feet, a 60 lb bag yields about 0.45 cubic feet, and a 40 lb bag yields about 0.30 cubic feet. Divide your total cubic feet by the yield per bag and always round up. For example, a 10 ft x 10 ft slab at 4 inches thick = 33.3 cubic feet, which requires about 56 bags of 60 lb concrete. ### What is a waste factor and why do I need it? A waste factor accounts for spillage, uneven subgrade, form overfill, and mixing losses that always happen on a real job site. The standard industry waste factor is 5-10% for slabs and footings, and up to 15% for complex shapes like stairs or curbs. If you skip the waste factor and run short, you'll have to order more concrete, often at a premium rush delivery cost. Always better to order a little more. ### When should I use ready-mix concrete vs. pre-mix bags? Use ready-mix concrete (delivered by truck) for large pours over 1-2 cubic yards. It's faster, more consistent, and usually cheaper per yard at scale. Use pre-mix bags for small projects under 1 cubic yard like fence posts, small slabs, or repairs. Minimum ready-mix orders are typically 1 cubic yard, and most plants charge a short-load fee for orders under 3-5 yards. For anything above that, call your local batch plant for a quote. ### How do I calculate concrete for a circular slab or round patio? Use the formula: Volume = pi x radius squared x thickness. For example, a circular patio 10 feet in diameter and 4 inches thick: radius = 5 ft, so Volume = 3.14159 x 25 x 0.333 = 26.2 cubic feet = 0.97 cubic yards. Our calculator handles this automatically - just enter the diameter and thickness. ### How do I calculate concrete for stairs? Stair calculations are more complex because each step represents an additional wedge of concrete. The formula is: Volume = width x run x rise x n(n+1)/2, where n = number of steps. For 5 steps that are 4 ft wide, 12 inches deep, and 7 inches tall: Volume = 4 x 1 x 0.583 x 15 = 35 cubic feet. Add a platform and use a 10% waste factor. Our calculator does all of this automatically. ### How thick should a concrete slab be? Typical slab thicknesses are: 4 inches for residential driveways and patios, 5-6 inches for driveways with heavy vehicle traffic or commercial use, 3.5-4 inches for sidewalks and walkways, and 6-8 inches for industrial floors or heavy equipment areas. Thicker slabs require more concrete and rebar but carry significantly higher loads. ### What does concrete cost per cubic yard? Ready-mix concrete typically costs between $120 and $200 per cubic yard in the US, depending on your location, mix design, and current market conditions. Fiber-reinforced or high-strength mixes cost more. You'll also have delivery fees, short-load charges, and pump truck costs if needed. Pre-mix bags cost about $5-$8 per 80 lb bag, which works out to roughly $240-$360 per cubic yard, significantly more expensive than ready-mix for large pours. ### Do I need rebar or wire mesh in my concrete? For most residential slabs 4 inches thick or less, fiber mesh mixed into the concrete or welded wire fabric (6x6 WWF) is sufficient. For structural elements, driveways, or anything carrying vehicular loads, rebar is recommended. Typical rebar patterns: #4 bars on 12-inch centers for driveways, #3 bars on 18-inch centers for walkways and patios. Always check local building codes, as many jurisdictions have specific requirements for reinforcement. ### How long does concrete take to cure? Concrete gains strength gradually over time. It reaches about 70% of its final strength after 7 days and over 90% by 28 days, the standard cure period referenced in engineering specs. However, you can typically walk on it after 24-48 hours and drive on it after 7 days. Full cure for heavy loads takes 28+ days. Keep concrete moist during the first week by covering it with plastic sheeting or burlap to prevent cracking from rapid evaporation. ### Can I mix concrete myself instead of ordering it? Yes, for small projects. A standard hand-mix ratio is 1 part cement : 2 parts sand : 3 parts gravel, plus enough water to achieve a workable consistency. However, for anything over about 0.5 cubic yards, using pre-mix bags or ordering ready-mix is more practical and gives better results. Machine mixing (a rented drum mixer) produces more consistent concrete than hand mixing. ### What is the minimum concrete order for a ready-mix truck? Most ready-mix suppliers require a minimum order of 1 cubic yard, though some will deliver as little as 0.5 yards with a short-load surcharge. Orders under 3 yards typically incur a short-load fee of $50-$150. If you only need a small amount, pre-mix bags are often more economical despite the higher cost per yard. ## Concrete Contractors: Get Paid Faster Invoice Mama helps concrete and construction contractors create professional invoices and estimates in seconds. From the job site, on your phone, bill clients before the truck even leaves. - [Create your first invoice free](https://app.invoicemama.com/sign-up) --- Invoice Mama is invoicing software for contractors, freelancers, and small businesses. It is not a tax, legal, or accounting firm. For current pricing, use the live pricing page. **More:** [Home](https://invoicemama.com/) - [Pricing](https://invoicemama.com/pricing) - [Guides](https://invoicemama.com/guides) - [Tools](https://invoicemama.com/tools) - [Calculators hub](https://invoicemama.com/calculators)