Concrete calculator
Enter a slab, footing, or column and this sizes the concrete: the volume in cubic yards for a ready-mix order, the cubic feet, and how many bags of each size to buy. It runs the same volume math as the CutOnce app, which also does the rebar schedule and the pour weight.
Updated
Volume is the pour reduced to cubic feet, then cubic yards for a ready-mix order. Bags is the volume plus waste divided by the bag yield, rounded up. Waste covers spillage and subgrade that is not dead level; 10% is the usual start.
- Cubic feet
- 36.7
- 80 lb bags
- 62
- Cubic meters
- 1.04
At 1 cu yd or more, ordering ready-mix by the truck usually beats bagging it.
How the volume works
Concrete is sold by volume, so every shape reduces to cubic feet first. A slab or wall is length times width times thickness, with the thickness in inches divided by twelve to keep the units honest. A footing is its length times the cross-section (width times depth), times how many you pour. A column is a cylinder: pi times the radius squared times the height. Cubic feet divided by twenty-seven is cubic yards, which is what a ready-mix truck is ordered in. Bags come last: the volume plus a waste factor, divided by what one bag yields, rounded up to whole bags.
A worked example
A 10 ft by 10 ft slab, 4 in thick, with 10% waste, counted in 80 lb bags. Here is the volume and the bag count, and where each number comes from.
| Slab volume | 33.33 cu ft | 10 x 10 ft x (4 in / 12) |
| Volume with waste | 36.67 cu ft | 33.33 x 1.10 |
| Cubic yards | 1.36 cu yd | 36.67 / 27 |
| 80 lb bags | 62 | ceil(36.67 / 0.60 per bag) |
| Cubic meters | 1.04 cu m | 36.67 x 0.0283 |
Slab, footing, or column
The three shapes take their dimensions differently, which is why the calculator asks for the units it does. Match your pour to one of these.
| Shape | It takes | Volume |
|---|---|---|
| Slab or wall | Length and width in feet, thickness in inches | L x W x (T / 12) |
| Footing | Length in feet, width and depth in inches, and a count | L x (W/12) x (D/12) x count |
| Column or pier | Diameter and height in inches, and a quantity | pi x (D/24)^2 x (H/12) x qty |
How much each bag yields
A bag of concrete mix yields far less than its weight suggests, because most of the volume is water you add on site. These are the yields the bag count uses. Bigger bags cost less per cubic foot but are heavier to haul.
| Bag | Yield |
|---|---|
| 80 lb | 0.60 cu ft |
| 60 lb | 0.45 cu ft |
| 50 lb | 0.38 cu ft |
| 40 lb | 0.30 cu ft |
Waste and when to order ready-mix
Ten percent waste is the usual start for a slab. Add more when the subgrade is not dead level, the forms are rough, or you are pumping, because the low spots and the spillage add up fast. The other call is bags versus a truck. Bagging a small pour is fine, but the bag count climbs quickly: a 10 by 10 slab is already sixty-plus bags to mix by hand. Once the volume passes about one cubic yard, a ready-mix truck is usually less work and less money, so the calculator flags that line.
What the app adds
This page sizes the pour volume and the bags. The app carries it further: the rebar schedule for a footing or slab, the pour weight for load and transport, and a footing takeoff across a whole foundation. It prints the result as a material list you take to the yard.
Common questions
- How much concrete do I need for a 10x10 slab?
- A 10 ft by 10 ft slab at 4 in thick is 33.3 cubic feet, or 1.23 cubic yards, before waste. Add 10% and it rounds to 1.36 cubic yards, which is 62 of the 80 lb bags. Change the thickness and it scales straight up: the same slab at 6 in is half again as much.
- How many bags of concrete are in a cubic yard?
- A cubic yard is 27 cubic feet. An 80 lb bag yields 0.6 cubic feet, so a full yard is 45 bags; 60 lb bags yield 0.45 each, so it takes 60 of them. That is a lot of mixing, which is why a yard is the rough line where a ready-mix truck starts to make sense.
- What thickness should a slab be?
- Four inches is standard for a patio, a shed floor, or a walk. Go to five or six inches for a driveway or anything a vehicle sits on, and thicken the edges into a footing at the perimeter. The calculator takes the thickness in inches so you can compare both.
- How do I figure concrete for round column footings?
- Pick the column shape and enter the tube diameter and the height in inches, plus how many piers you are pouring. A 12 in tube 48 in deep is 0.12 cubic yards each, so eight of them is close to a full yard. Sonotube sizes are diameters, which is what the field asks for.
- Does the bag count include waste?
- Yes. The bag count is figured on the volume after the waste factor, then rounded up to whole bags, because you buy bags whole and you would rather finish the pour than come up short. The cubic yard and cubic foot figures shown are also the with-waste numbers.
Size the pour, schedule the steel
CutOnce sizes the concrete like this page, then lays out the rebar, figures the pour weight, and totals the footings across the whole foundation, on the jobsite.
Free to download, with a 7-day trial of everything. This page sizes the pour; the app prints the full material list and the rebar schedule.
See also