How to use it
- Pick what you're pouring. The calculator fills in the usual thickness for that kind of job, and you can change it.
- Enter the length and width in feet. Add inches if you have them, so 10 ft 6 in is 10 and 6. Working in meters? Switch Units to Metric first.
- Set the number of identical slabs and the waste factor. 10% is a good place to start.
- Turn on the sections you need: bag counts, a gravel base or a rebar grid.
- To price the job, type the prices you see at the store or your supplier in the cost section.
Results update as you type. Copy result gives you a plain-text material list for a text or email, and the page address saves your numbers so you can bookmark the job.
The formula
Cubic feet = Length × Width × (Thickness ÷ 12) × Number of slabs
Cubic yards = Cubic feet ÷ 27
With waste = Cubic yards × (1 + Waste % ÷ 100)
Bags = Cubic feet with waste ÷ Bag yield, rounded up
Gravel tons = Length × Width × (Depth ÷ 12) ÷ 27 × Tons per cubic yard
A cubic yard is 27 cubic feet (3 ft × 3 ft × 3 ft). Thickness is divided by 12 to turn inches into feet. Bags always round up, because you can't buy part of a bag.
For rebar, the bars running the length are spaced across the width: (width in inches − 2 × edge cover) ÷ spacing, rounded down, plus one. Each one is as long as the slab minus the cover at both ends. The bars running the width work the same way. Sticks are the total footage plus waste, divided by the stick length and rounded up. Laps, chairs and cut waste aren't included, so allow for them on bigger jobs.
Worked example: a 10 ft × 12 ft patio, 4 in thick
- Volume: 10 × 12 × (4 ÷ 12) = 40.00 cu ft, which is 40 ÷ 27 = 1.48 cu yd
- With 10% waste: 1.63 cu yd (44 cu ft)
- Bags: 44 ÷ 0.60 = 73.3, so 74 bags of 80 lb. Or 98 of 60 lb, or 147 of 40 lb
- Gravel base, 4 in deep: 1.48 cu yd, about 2.07 tons at 1.4 tons per yard
- Rebar at 12 in on center with 3 in cover: 12 bars at 9 ft 6 in and 10 bars at 11 ft 6 in = 229 ft. With 10% waste that's 251.9 ft, so 13 sticks of 20 ft
For a feel of the work involved: 74 bags of 80 lb mix weigh about 5,920 lb, close to three tons, all lifted and mixed by hand. That's why many people call a ready-mix truck once a job gets past a yard or so.
Common mistakes
- Ordering with no extra. Ground is never perfectly flat and forms bow a little. Running short in the middle of a pour is far worse than having a few bags left over.
- Ignoring low spots. If the ground under a 10 × 12 slab averages half an inch low, that's 5 more cubic feet of concrete. Check the depth in several places before you pour.
- Mixing up cubic feet and cubic yards. Bags are sold by cubic feet of yield, and trucks by cubic yards. Dividing by 27 is the step people skip.
- Measuring the outside of the forms. Measure inside the forms, where the concrete actually goes.
- Trusting a bag yield you haven't checked. Yields differ by brand and mix. The numbers printed on your bag are the ones to use.
Questions people ask
How many bags of concrete do I need for a 10 × 10 slab?
At 4 inches thick, a 10 × 10 slab is 33.3 cubic feet. With 10% waste that's 36.7 cubic feet, or 62 bags of 80 lb mix at 0.60 cubic feet per bag. Enter your own size above for an exact count.
Should I use bags or ready-mix?
Bags make sense for small pours like post holes, steps, pads and short walks. For a full yard or more, a ready-mix truck usually saves a lot of labor and gives a more consistent pour. Compare both in the cost section by switching how you buy it.
How thick should a concrete slab be?
Four inches is the usual thickness for patios, walks and shed pads. Driveways that carry heavy trucks, RVs or equipment are often poured at 5 to 6 inches. Your local building code or the job's plans have the final say.
Do I need rebar or wire mesh?
Reinforcement doesn't keep concrete from cracking, but it helps hold cracks tight and keeps the pieces from shifting. Slabs that carry weight, like driveways, usually get rebar or mesh. Follow your plans and local code, and use chairs so the steel sits in the middle of the slab, not on the dirt.
How do I figure the cost?
Use the cost section above. Type the price per bag (or per yard for ready-mix), per ton of gravel and per stick of rebar, plus tax and any labor or delivery. Prices change often and vary a lot by area, so a quick check at your store or a call to your supplier gives the most accurate total.
Why add a waste factor?
Some concrete always ends up in the mixer, on the ground or filling low spots in the base. Estimators add a percentage so the job doesn't come up short. Use more waste for rough ground or first-time pours and less for tight, well-prepared forms.