Before you buy anything, decide whether you are mixing bags or ordering a truck. That one choice changes the price, the labor, and how many trips you make to the building center. A pour under half a yard is reasonable by hand. A pour over one yard is a truck job whether you like it or not.
The bag versus truck line
A standard 80 lb bag holds 0.6 cu ft of concrete mix. A 60 lb bag holds 0.45 cu ft, and a 40 lb bag holds 0.30 cu ft. To put a yard of concrete in perspective, that is 27 cu ft, which works out to about 45 eighty-pound bags, 60 sixty-pound bags, or 90 forty-pound bags. Most people who try to mix 45 bags by hand quit around bag number twenty. The practical line is one cubic yard. Below it, bags are fine for a single slab or a few post holes. Above it, call the ready-mix plant and save your back.
Bag coverage at a glance
| Bag size | Volume per bag | Bags per cubic yard |
|---|---|---|
| 80 lb | 0.60 cu ft | about 45 |
| 60 lb | 0.45 cu ft | about 60 |
| 40 lb | 0.30 cu ft | about 90 |
At a 4 inch pour, one 80 lb bag spreads over about 1.8 sq ft. A 60 lb bag covers closer to 1.35 sq ft, and a 40 lb bag covers under a square foot. That is why the big bag is almost always the cheapest per cubic foot even if it costs more at the register.
Choose the thickness first
Foot traffic and shed floors get a 4 inch slab. That is the default and the number this page starts from. A driveway that sees a car goes 5 to 6 inches, because a 4 inch slab cracks under tire loads in the first winter. Footings below frost must extend past the local frost line, which runs 12 inches in mild parts of the country and over 4 feet in the cold north.
Walk a 10 by 10 slab
Picture a shed pad that measures 10 ft by 10 ft, poured 4 inches thick. The area is 100 sq ft. The volume lands near 1.23 cu yd before waste. That single number tells the story. It is just over one yard, which means a ready-mix truck is the sane choice, and the bag count underneath is about 56 bags of 80 lb, 75 bags of 60 lb, or 110 bags of 40 lb. With 10 percent waste on top, add a few bags or a quarter yard to the truck order.
Shape it right
The calculator handles three shapes. A rectangular slab uses length, width and thickness. A strip footing under a wall uses the long narrow trench footprint. A round sonotube post footing uses the diameter and the depth below grade. Subtract any area you do not want concrete on, like an opening left for a door, because the number assumes the whole rectangle fills solid.
Place the order
For a 10 by 10 by 4 inch pad, order about 1.25 yards ready-mix, or buy 56 to 60 eighty-pound bags if the truck minimum is too high. Add wire mesh or rebar, 2 inches of compacted gravel under the slab, and form boards screwed to stakes. Have a helper on pour day, because concrete sets in about two hours and there is no coming back once the trowel comes out.
A slab lasts decades when the thickness matches the load, the base is compacted, and the order covers the waste. Pitting bags against a truck before you drive anywhere is the cheapest decision on the whole job.
Concrete Pad Calculator
Pick the delivery method first, pour a 4 inch slab, and see how far one 80 lb bag goes, how many bags fill a 10 by 10 pad, and when one cubic yard flips you to a truck.
How to use
- Choose slab, strip footing or round sonotube column
- Enter the length, width and thickness in feet and inches
- Decide bag mix versus ready-mix truck
- Set the waste factor, 10 percent on most pours
- Read cubic yards and all three bag sizes at once
Supported formats and limits
- Slab and footing share length times width times thickness
- Default 10 percent waste already added
- US feet and inches only, no metric
- Truck threshold starts near 1 cubic yard
Examples
10 by 10 ft slab poured 4 in thick needs about 1.23 cu yd, or roughly 56 bags of 80 lb concrete.Common errors
- Length and width must be positive for a slab
- Diameter and height must be positive for a column
- Thickness must sit between 2 and 12 inches
Privacy and browser support
All calculations run locally in your browser. No measurements are uploaded.
FAQ
How much area does one 80 lb bag cover
One 80 lb bag holds 0.6 cu ft, so at a 4 inch pour it covers about 1.8 sq ft before waste.
How many 80 lb bags fill a 10 by 10 slab
A 10 by 10 ft slab at 4 in thick uses about 56 bags of 80 lb concrete. At that volume, a truck wins on price.
How thick should a patio slab be
Four inches for foot traffic and shed bases. Driveways go 5 to 6 inches on a compacted gravel base.
When should I call a ready-mix truck
Over 1 cubic yard. One yard is about 45 bags of 80 lb, and carrying that many bags is back-breaking labor most days.
What do the other bag sizes cover
A 60 lb bag holds 0.45 cu ft and a 40 lb bag holds 0.30 cu ft, so the same slab needs more of the lighter bags.
Do I need rebar inside the slab
Walkways and patios get wire mesh. Driveways and shed floors want rebar on a gravel base, especially in cold winters.