Concrete Calculator
Estimate concrete volume, bag counts, and ready-mix quantities for slabs, footings, columns, circular slabs, and stairs.
Pricing (optional)
10 ft × 10 ft × (4/12) ft = 33.3 ft³
+ 10% waste = 36.7 ft³ (1.36 yd³)
How It Works
- 1
Choose the shape and enter dimensions
Pick slab, footing, column, circular slab, or stairs. Enter the relevant dimensions in feet and inches. For multiple identical pours, set quantity above 1.
- 2
Set the waste factor
A 10% default matches typical ordering practice for bag work and small ready-mix jobs. Drop to 5% for flat, well-prepped slabs. Bump to 15% for footings on uneven or over-excavated subgrade.
- 3
Compare bags versus ready-mix
Review volume in cubic yards, cubic feet, and cubic meters, plus bag counts at 40, 60, and 80 lb sizes. Add optional prices per cubic yard and per 80 lb bag for a side-by-side cost comparison. Under 1 cubic yard, bags usually win on price; above that, ready-mix wins on both price and labor.
How to Calculate Concrete Volume and Bags
Concrete is priced and ordered by the cubic yard for ready-mix deliveries and by the bag for smaller pours. The math is dimensional: multiply length by width by thickness (converted to the same unit) to get volume. One cubic yard equals 27 cubic feet, and one cubic meter equals about 35.31 cubic feet. A 4-inch slab 10 feet by 10 feet contains 33.3 cubic feet, which is 1.23 cubic yards. Bag yields are standardized by manufacturers. An 80 lb Quikrete or Sakrete bag yields roughly 0.60 cubic feet of mixed concrete. A 60 lb bag yields about 0.45 cubic feet, and a 40 lb bag yields 0.30 cubic feet. That means one cubic yard needs 45 bags of 80 lb, 60 bags of 60 lb, or 90 bags of 40 lb. Normal-weight concrete weighs about 150 lbs per cubic foot, or 4,050 lbs (just over 2 tons) per cubic yard. Industry practice adds a 5 to 10 percent waste factor to account for spills, over-excavation, and uneven subgrade. For pours under one cubic yard, bagged mix is usually cheaper per unit than a short-load ready-mix delivery, which often carries a minimum charge. For pours of one cubic yard or more, ready-mix wins on both price per yard and labor, since mixing that much by hand is impractical. Columns, footings, circular slabs, and stairs use their own volume formulas: cylinder volume for columns, sum-of-steps volume for staircases.
Common pitfalls
Slab thickness is measured in inches, but length and width are in feet. Forgetting to convert thickness to feet (divide by 12) is the most common source of a 12x error in volume estimates.
Ready-mix suppliers typically charge a short-load fee for deliveries under 3 cubic yards, sometimes on top of a fixed minimum. For small pours, always get a quote before assuming ready-mix is cheaper than bags.
Over-excavated footings waste concrete fast. A trench dug 2 inches wider and 2 inches deeper than spec on a 20-foot footing adds roughly 0.25 cubic yards of concrete. Plan for a 10 to 15% waste factor on hand-dug footings.
Concrete sets on its own schedule, not yours. Once a ready-mix truck arrives, you have 60 to 90 minutes to place and finish before the mix stiffens. ASTM C94 historically set a 90-minute default from batching to discharge (replaced in 2021 by a producer/purchaser-negotiated limit), and the same window is still the working rule in the field, shorter in hot weather. Have forms, screeds, and help ready before the truck shows up.
Bag yield varies slightly by manufacturer and by how wet you mix. The 0.60 cu ft figure for an 80 lb bag is the published Quikrete yield at recommended water content. Over-watering reduces yield and strength.
Frequently Asked Questions
How do I calculate concrete volume?
Multiply length × width × thickness, converting to a single unit. A 10 ft × 10 ft × 4 in slab is 10 × 10 × (4/12) = 33.33 cubic feet, or 1.23 cubic yards. One cubic yard equals 27 cubic feet. For columns use π × r² × height. For circular slabs use π × r² × thickness. For stairs sum each step's volume: width × run × (rise × step number), summed from step 1 to N.
How many bags of concrete are in a cubic yard?
An 80 lb bag yields about 0.60 cubic feet of mixed concrete, a 60 lb bag yields 0.45 cu ft, and a 40 lb bag yields 0.30 cu ft. One cubic yard equals 27 cu ft, so a cubic yard takes 45 × 80 lb bags, 60 × 60 lb bags, or 90 × 40 lb bags. Manufacturer yields match this: Quikrete publishes 0.60, 0.45, and 0.30 cu ft respectively.
Bags or ready-mix — which is cheaper?
Below roughly one cubic yard, bagged mix is almost always cheaper because most ready-mix plants charge a short-load fee and a minimum delivery charge. Above one cubic yard, ready-mix wins on both price per yard and labor — mixing 45 bags by hand is impractical. For pours right at the boundary, get quotes: the short-load fee varies by region and supplier.
What waste factor should I use?
Industry rule-of-thumb is 5 to 10% for standard ready-mix and bag work. Use 5% for well-prepped, flat slabs on level subgrade. Use 10% as a safe default. Use 15% for hand-dug footings on uneven or over-excavated ground, where a trench 2 inches wider and deeper than spec can add 10% quickly. Err high when ordering ready-mix; running short means a second short-load fee.
How much does concrete weigh?
Normal-weight concrete weighs about 150 pounds per cubic foot, or roughly 4,050 lbs (just over 2 tons) per cubic yard. This is the standard density ACI 318 uses for design calculations. Actual density ranges from about 140 to 150 pcf depending on aggregate. Lightweight concrete mixes run 90 to 115 pcf.
How long do I have to place ready-mix after it leaves the plant?
ASTM C94 historically set a 90-minute default from batching to discharge, often cited alongside a 300-drum-revolution limit. The 2021 edition shifted to a time limit negotiated between producer and purchaser, but 60 to 90 minutes remains the working rule of thumb in practice. In hot weather or without retarders, the window shrinks. Have forms, screeds, finishing tools, and a full crew ready before the truck arrives. Once concrete begins to set, it cannot be re-tempered with water without compromising strength.
What thickness should a slab be?
Residential patios and walkways typically use 4 inches. Driveways for cars use 4 inches; for heavier vehicles 5 to 6 inches. Garage floors start at 4 inches; 6 inches for shops. Footings follow local code and frost depth requirements — often 12 to 16 inches wide by 8 to 12 inches thick, with bearing below the frost line. Always check your local building code.