Concrete
Estimator
Waste Factor NoteMost contractors order 10% extra to account for uneven subgrade, spreading forms, and minor spillage. Better to have a little extra than to run short.
Bag Requirements
PRE-MIXEDAbout This Calculator
Estimate concrete quantity and required bags for slabs, footings, and columns with precision. Features an industrial design system and real-time visual estimates.
Frequently Asked Questions
How do I calculate concrete for a slab?
The formula is (Length 脳 Width 脳 Thickness) / 27. Our calculator handles the units (feet vs. inches) and adds a recommended 10% waste factor for spills or uneven ground.
How many 80lb bags are in a cubic yard?
One cubic yard of concrete requires approximately 45 bags of 80lb premixed concrete, or about 60 bags of 60lb mix.
What is the recommended waste factor?
We recommend adding 10% to your calculation. This accounts for form deflection, uneven subgrades, and minor spills during the pour.
What concrete mix ratio should I use for a driveway versus a foundation?
Concrete mix ratios determine compressive strength (measured in PSI) and durability for specific applications. For a standard residential driveway, a 1:2:3 mix (one part cement, two parts sand, three parts gravel) yields approximately 3,000-3,500 PSI — sufficient for passenger vehicles. Use a water-to-cement ratio of 0.45-0.50. For a structural foundation, a stronger 1:1.5:3 mix or engineered ready-mix at 3,500-4,000 PSI is required. Garage floors with heavy equipment: 4,000 PSI minimum. Sidewalks and patios: 3,000 PSI is adequate. The mix ratio matters because too much water weakens concrete dramatically — every extra 1 gallon per cubic yard reduces strength by roughly 200 PSI. In freeze-thaw climates, use air-entrained concrete with 5-7% air content and a maximum 0.45 water-cement ratio to prevent surface spalling.
How much concrete do I need for a 20x20 foot patio at 4-inch depth?
Calculate: 20 ft × 20 ft × 0.333 ft (4 inches) = 133.3 cubic feet. Convert to cubic yards: 133.3 ÷ 27 = 4.94 cubic yards. Add 10% waste factor, so order 5.5 cubic yards. For bagged concrete (DIY), an 80-pound bag yields approximately 0.60 cubic feet. You would need: 133.3 ÷ 0.60 = 222 bags at roughly $8-10 per bag in 2025, totaling $1,776-$2,220. Ready-mix delivery ($150-180 per cubic yard) is more economical for anything over 1 cubic yard. For 5.5 cubic yards, ready-mix costs approximately $825-990 plus a $100-200 delivery fee, saving significant labor versus mixing 222 bags. Plan to have 2-3 helpers on pour day — a 20x20 slab must be poured, screeded, and finished before the concrete sets (30-60 minutes depending on temperature).
What happens if I add too much water to my concrete mix?
Excess water is the most common cause of weak concrete. Each extra gallon per cubic yard reduces compressive strength by approximately 200 PSI. A mix designed for 4,000 PSI can drop to 3,000 PSI with too much water, potentially failing structural requirements. Excess water also causes: increased shrinkage and cracking, surface scaling and dusting, longer cure times, reduced freeze-thaw resistance, and segregation of aggregates. The ideal water-to-cement ratio is 0.40-0.50. If the mix seems too stiff to work, use a concrete plasticizer additive ($5-$10 per bag) instead of adding water — it improves workability without weakening the concrete.
How long does concrete take to cure and reach full strength?
Concrete reaches about 50% of its design strength in 3 days, 75% in 7 days, and approximately 99% in 28 days (the standard testing benchmark). However, curing continues for months — concrete can reach 110-120% of its 28-day strength over 90 days. During the critical first 7 days, keep concrete moist (spray water or cover with plastic) to prevent premature drying. Temperature matters: below 50°F curing slows dramatically; below 25°F it essentially stops. Hot weather (above 90°F) accelerates curing but can cause surface cracking if not properly hydrated. Avoid heavy loads on new concrete for at least 7 days; park vehicles on driveways after 10-14 days.