Quick answers to the most common questions about our concrete calculators and about planning a concrete project. These answers are general guidance; always follow your local building codes and confirm important decisions with a qualified professional.
About Our Calculators
1. Are the calculators on this website free to use?
Yes. All calculators and articles on Online Concrete Calculators are free to use. The website is supported by advertising and occasional affiliate links, which never affect how our calculators work.
2. How accurate are the calculator results?
Results are estimates based on the numbers you enter and standard volume formulas. Real projects vary because of uneven ground, formwork movement, spillage and measurement errors, so we recommend verifying your measurements and adding a waste allowance before you order.
3. Can I rely on the results for structural or engineering decisions?
No. Our tools are for general planning and education only. For foundations, structural slabs, retaining walls, load-bearing elements or anything involving safety, consult a licensed structural engineer or contractor and follow your local codes. See our Disclaimer for details.
4. How do I convert between cubic feet, cubic yards and cubic meters?
One cubic yard equals 27 cubic feet, or about 0.765 cubic meters. One cubic meter equals about 35.3 cubic feet, or about 1.308 cubic yards. To convert cubic feet to cubic yards, divide by 27.
Calculating Your Concrete
5. How do I calculate the concrete needed for a slab?
Multiply length by width by thickness, with all three in the same unit, to get the volume. In feet, convert inches by dividing by 12, then divide cubic feet by 27 to get cubic yards. Example: a 10 ft × 10 ft slab that is 4 inches thick is 10 × 10 × (4 ÷ 12) = 33.3 cubic feet, or about 1.23 cubic yards.
6. How much extra concrete should I order?
A common allowance is 5% to 10% extra to cover uneven subgrade, spillage and small measurement errors. Use the higher end for irregular excavations, hand-dug footings or when you cannot easily get a small top-up delivery.
7. How many bags of concrete mix do I need per cubic yard?
As a rough guide, an 80 lb bag yields about 0.6 cubic feet, so one cubic yard needs about 45 bags. A 60 lb bag yields about 0.45 cubic feet (about 60 bags per yard), and a 40 lb bag about 0.3 cubic feet (about 90 bags per yard). Yields vary by brand, so check the label on the bag you buy.
8. Should I use bagged concrete or ready-mix delivered by truck?
Bagged mix suits small jobs such as fence posts, small pads and repairs, up to roughly one cubic yard. For larger pours, ready-mix is usually faster, more consistent and less physically demanding. Many suppliers have minimum order sizes or short-load fees, so ask when you call.
9. How much does a cubic yard of concrete weigh?
Normal-weight concrete weighs about 150 pounds per cubic foot, so a cubic yard weighs roughly 4,000 pounds (about 2,400 kg per cubic meter). Keep this in mind for truck access, driveways, subgrade preparation and any structure that will carry the weight.
10. How do I calculate concrete for a round column or circular footing?
Use the volume of a cylinder: π × radius² × height, with all measurements in the same unit. For example, a column with a 1 ft radius and 8 ft height holds 3.1416 × 1 × 1 × 8 ≈ 25.1 cubic feet, or about 0.93 cubic yards.
Concrete Basics
11. What is the difference between cement, concrete and mortar?
Cement is the fine powder that acts as the binder. Concrete is a mixture of cement, water, sand and coarse aggregate (gravel or crushed stone) used for slabs, footings and structures. Mortar is cement, sand and water without coarse aggregate, used to bond bricks, blocks and stones.
12. What is a standard concrete mix ratio?
Common volume ratios of cement to sand to coarse aggregate are 1:2:3 and 1:2:4, with water added for workability. Modern ready-mix concrete is usually ordered by strength (psi or MPa) rather than by ratio, so tell your supplier what the project needs.
13. What concrete strength (PSI) do I need?
Many residential sidewalks and patios use about 3,000 psi, while driveways and garage floors often use 3,500 to 4,000 psi. Freeze-thaw climates, heavy loads and structural work can require higher strength or special mixes, so confirm with your supplier, local code or engineer.
14. How thick should a concrete slab be?
A 4-inch slab is typical for patios, walkways and light-duty floors. Driveways that carry heavier vehicles are commonly 5 to 6 inches. Footings and foundations depend on soil, loads and local code, so they should be designed rather than guessed.
Pouring and Finishing
15. How long does concrete take to dry and cure?
Concrete hardens through curing, a chemical reaction that continues for weeks. It can often take light foot traffic after 24 to 48 hours and vehicles after about 7 days, and it reaches its design strength at roughly 28 days. Keeping it damp and protected during the first week helps it cure properly.
16. Can I pour concrete in cold or hot weather?
Yes, but extra care is needed. Below about 40°F (4°C), concrete gains strength slowly and can be damaged by freezing, so use protection and consider cold-weather mixes. In hot, dry or windy conditions (for example above 90°F or 32°C), water evaporates quickly, so keep the surface moist and consider pouring early or late in the day.
17. Is it okay to add extra water to make concrete easier to work?
It is better not to. Extra water makes concrete easier to place but weakens it and increases shrinkage cracking. If you need a more workable mix, ask your supplier about admixtures such as water reducers instead.
18. Do I need rebar or wire mesh in my slab?
It depends on the slab’s use, soil and local code. Reinforcement helps control crack widths and add strength, and it is commonly used in driveways, garage floors, footings and structural elements. Some light-duty slabs on stable ground are placed without it, so check local requirements or ask a professional.
19. Why does concrete crack, and can I prevent it?
All concrete shrinks as it cures, so some cracking is normal. You can reduce random cracks with a well-compacted subgrade, proper curing, not adding excess water and control joints. As a rule of thumb, space joints about 2 to 3 times the slab thickness in inches, expressed in feet (roughly 8 to 12 ft for a 4-inch slab), and cut them to about one quarter of the slab depth.
20. When can I seal my concrete?
Many manufacturers recommend waiting about 28 days so the slab has cured and excess moisture has escaped. Requirements differ by product, so follow the sealer’s label and make sure the surface is clean and dry first.
Support
21. How do I report an error or request a new calculator?
Email us at support@onlineconcretecalculators.com with the page name and a short description. We review every message and correct confirmed errors as quickly as we can. We also welcome suggestions for new calculators and topics.