Concrete Calculator for Slabs and Footings
This calculator sizes a rectangular slab or footing and tells you what to order. Enter the length, width and thickness, add a waste percent for spillage and uneven ground, and it returns the volume in cubic feet and cubic yards plus the number of 60 lb and 80 lb bags. Bag counts assume a 60 lb bag yields 0.45 cubic feet and an 80 lb bag yields 0.60 cubic feet, and both are rounded up.
How to estimate concrete
- Enter the slab or footing length in feet in the 'Length (ft)' field.
- Enter the 'Width (ft)' the same way.
- Enter the 'Thickness (in)' in inches (4 inches is common for a slab).
- Set the 'Waste (%)' to cover spillage and uneven subgrade (the default is 5).
- Read the cubic yards, cubic feet, and 60 lb and 80 lb bag counts in the result cards.
- Click 'Copy estimate' to copy the summary for your supplier.
Examples
10 by 10 ft slab, 4 inches thick, no waste
Length 10 ft, width 10 ft, thickness 4 in, 0% waste
33.33 cubic feet, 1.2346 cubic yards, 75 sixty-lb bags or 56 eighty-lb bags
Same slab with a 5% waste allowance
Length 10 ft, width 10 ft, thickness 4 in, 5% waste
35 cubic feet, 1.2963 cubic yards, 78 sixty-lb bags or 59 eighty-lb bags
20 by 20 ft driveway, 6 inches thick
Length 20 ft, width 20 ft, thickness 6 in, 5% waste
210 cubic feet, 7.7778 cubic yards, 467 sixty-lb bags or 350 eighty-lb bags
Frequently asked questions
How does the calculator work out the volume?
It multiplies length by width by thickness, converting the thickness from inches to feet by dividing by 12, then adds the waste percent. The result is shown in cubic feet and in cubic yards (cubic feet divided by 27).
How many 60 lb bags are in a cubic yard?
About 60 bags. A 60 lb bag yields roughly 0.45 cubic feet, and a cubic yard is 27 cubic feet, so 27 divided by 0.45 is 60 bags. An 80 lb bag yields about 0.60 cubic feet, so it takes about 45 of those.
Why does the tool round the bag count up?
You cannot buy a fraction of a bag, so the count is always rounded up to the next whole bag. That guards against a short pour, since concrete cannot be topped up cleanly once it starts to set.
When should I order ready-mix instead of bags?
Bags are practical for small jobs. Past roughly 1 cubic yard the bag count climbs fast, so a ready-mix truck is usually cheaper, faster and gives a single continuous pour.
How much waste should I add?
Five to ten percent is typical, and the field defaults to 5. Uneven subgrade, spillage and over-excavated footings all consume extra concrete, so ordering a little more avoids a short pour.
What thickness should a slab be?
Four inches suits patios and walkways. Driveways and areas that carry vehicles are usually five to six inches thick. Enter your figure in the thickness field to see the effect.
Can I calculate a footing or a non-square shape?
Yes for a rectangular footing, since it is length by width by depth. For an L shape or a circle, split it into rectangles and add the results, or use a shape-specific calculator.
How do I convert cubic feet to cubic yards?
Divide cubic feet by 27, since one cubic yard equals 27 cubic feet. This tool does the conversion for you and shows both figures.
Is my data private?
Yes. The calculation runs entirely in your browser and nothing is sent to a server, so your project dimensions stay on your device.
Learn more
- Concrete slabs: bags, ready-mix and how much to order
From slab dimensions to cubic yards and bag counts, why bag numbers climb fast, and when a ready-mix truck wins on cost and time.
Related tools
Paint Calculator
Estimate how many gallons of paint you need for a room or wall. Enter dimensions, coats, doors and windows to get paintable area and gallons.
Tile Calculator
Calculate how many tiles you need for a floor or wall. Enter the area and tile size with a waste allowance to get tiles and boxes to buy.
Gravel Calculator
Estimate gravel or aggregate for an area by volume and weight. Enter length, width, depth and density to get cubic feet, cubic yards, pounds and tons.
Appliance Energy Calculator
Find what an appliance costs to run from its wattage. Enter watts, hours per day and your rate to see kWh and cost per day, month and year.
Aquarium Volume Calculator
Calculate your fish tank volume in gallons and liters from inside dimensions in inches. Set a fill level to estimate the actual water it holds.
Baseboard Calculator
Estimate linear feet and pieces of baseboard or trim for a room. Enter length and width, subtract doorways, set piece length and add waste.