Concrete Calculator

How to Use the Concrete Calculator

Whether you are pouring a small patio, a driveway, footings for a deck, or a full foundation, knowing exactly how much concrete you need is essential. Ordering too little means an interrupted pour and a potential cold joint. Ordering too much means paying for material you do not need. This calculator gives you an accurate estimate in both cubic yards and bag counts.

Steps:

1. Enter the length, width, and depth of your pour in feet or inches
2. Set the number of identical pours (e.g., 4 footings of the same size)
3. Set the waste factor (typically 5--10%) to account for spillage and variations
4. Review the results: area, volume in cubic feet and cubic yards, bag counts, and estimated cost

The calculator automatically converts between feet and inches, so you can enter measurements in whichever unit is most convenient. For example, a typical patio might be 12 feet long, 10 feet wide, and 4 inches thick.

How to Measure for Concrete

Accurate measurements are the foundation of a good concrete estimate. Here is how to measure for common project types.

• Slabs and patios: Measure the length and width at their longest points. For irregular shapes, break the area into rectangles and triangles, calculate each separately, then add them together. Measure the depth (thickness) at several points and use the average.
• Footings:Measure the length, width, and depth of each footing trench. If you have multiple identical footings, enter the count in the “Number of Pours” field.
• Columns or piers:For round columns, calculate the area using the circle formula (π × radius²) and multiply by the height. Convert the result to cubic yards by dividing by 27.
• Driveways: Measure the total length and width of the driveway. If the thickness varies (e.g., thicker at the edges), use the average depth or calculate sections separately.

Always measure twice and round up. It is better to have a slight overestimate than to run short in the middle of a pour. Remember to account for any sub-base or gravel that will be displaced by the concrete.

Concrete Calculator Formula

The basic formula for calculating concrete volume is straightforward:

Volume (cubic feet) = Length × Width × Depth

Since depth is usually measured in inches, you need to convert it to feet first by dividing by 12:

Volume (cu ft) = Length (ft) × Width (ft) × (Depth (in) ÷ 12)

To convert cubic feet to cubic yards, divide by 27 (since 1 cubic yard = 27 cubic feet):

Volume (cu yd) = Volume (cu ft) ÷ 27

To account for waste, multiply by (1 + waste percentage / 100):

Final Volume = Volume (cu yd) × (1 + waste% / 100)

For example, a 10×10-foot slab that is 4 inches thick:

10 × 10 × (4/12) = 33.33 cu ft ÷ 27 = 1.23 cu yd. With 10% waste: 1.23 × 1.10 = 1.36 cu yd.

How Many Bags of Concrete Do You Need?

If you are mixing concrete on-site using bagged concrete mix, here is how many bags you need per cubic yard:

These numbers are approximate because different brands and mixes have slightly different yields. Always check the yield printed on the bag, which is usually listed as “covers X sq ft at Y inches thick.” For small projects (under 1 cubic yard), bags are often the most practical option. For larger projects, ready-mix delivery becomes more economical.

Tip: For a 4-inch thick slab, one 80 lb bag covers approximately 2 square feet. One 60 lb bag covers approximately 1.5 square feet. This can help you do a quick mental check of your order.

Ready-Mix vs Bags

Choosing between ready-mix concrete delivered by truck and bagged concrete you mix on-site depends on the size of your project, your timeline, and your budget.

For most DIY projects like small patios, walkways, and footings, bagged concrete is perfectly fine. You can work at your own pace, and the total cost difference for a small project is manageable. For anything over about 2 cubic yards (a 10×10 slab at 6 inches thick, for example), ready-mix becomes the more practical and cost-effective choice.

Tips for Ordering Concrete

Ordering concrete is not as simple as calling and requesting a number. Here are tips to ensure your order goes smoothly and your pour is successful.

• Order 5--10% extra. This is the single most important rule. Running short in the middle of a pour can ruin the entire project. The extra cost is minimal compared to the risk and expense of a failed pour.
• Order at least 24--48 hours in advance. Ready-mix plants need time to schedule your delivery, especially during busy construction seasons. Last-minute orders may not be available or may carry a surcharge.
• Have your site prepared before the truck arrives. Forms should be built, reinforced, and leveled. The truck will start its timer as soon as it arrives, and you pay for waiting time after the first few minutes.
• Consider access for the truck. A fully loaded concrete truck weighs about 30,000 pounds (15 tons). Make sure your driveway, yard, or job site can support this weight without damage. If the truck cannot reach your pour site, you may need a concrete pump or wheelbarrows.
• Specify the right mix. Different projects require different concrete strengths. Standard sidewalks and patios use 3,000 PSI, while driveways and garage floors typically use 4,000 PSI. Ask your supplier which mix is right for your project.
• Pour all concrete from the same batch when possible. Using concrete from different batches can create visible lines and color differences. Plan your pour so you can complete each section without stopping.

Frequently Asked Questions

How many yards of concrete do I need for a 10×10 slab?

It depends on the thickness. At 4 inches thick: 10 × 10 × (4/12) = 33.33 cu ft = 1.23 cu yd. At 6 inches thick: 10 × 10 × (6/12) = 50 cu ft = 1.85 cu yd. Always add a waste factor of 5--10% when ordering.

How much does a yard of concrete cost?

Ready-mix concrete typically costs between $125 and $175 per cubic yard, depending on your location, the mix design, and delivery distance. There may also be a short-load fee for orders under a certain minimum (usually 1--3 cubic yards). Bagged concrete mix costs about $4--$7 per 80 lb bag, which works out to roughly $200--$350 per cubic yard when you factor in the number of bags needed.

What PSI concrete should I use?

The required strength depends on the project. Use 2,500 PSI for non-structural fill and footings. Use 3,000 PSI for standard sidewalks, patios, and slabs. Use 4,000 PSI for driveways, garage floors, and structural slabs. Use 5,000+ PSI for heavy-duty applications like industrial floors or foundations. When in doubt, go one strength level higher than you think you need.

How thick should a concrete slab be?

Minimum thickness varies by application: 4 inches for sidewalks and patios, 5--6 inches for driveways, 6--8 inches for garage floors, and 8--12 inches for structural footings. Local building codes may have specific requirements, so always check with your municipality before pouring. Thicker slabs are stronger and more durable but cost significantly more in materials.

Can I pour concrete in cold weather?

Concrete can be poured in temperatures down to about 35°F, but special precautions are needed. Use hot water in the mix, cover the pour with insulating blankets, and avoid pouring on frozen ground. Below 35°F, the risk of the concrete freezing before it cures properly is very high. Most professionals recommend waiting until temperatures are consistently above 40°F for best results. If you must pour in cold weather, consider using a concrete mix with accelerating admixtures.

How long does concrete take to cure?

Concrete reaches about 70% of its design strength in 7 days and full strength in 28 days. You can walk on it after 24--48 hours and drive on it after 7 days. Keep the surface moist for the first 7 days by covering it with plastic sheeting, wet burlap, or applying a curing compound. In hot or windy weather, concrete can cure too quickly and develop cracks, so proper moisture management is essential.