Concrete calculator
Concrete is sold
by the cubic yard.
So thickness matters as much as area. Measure the slab above, set a depth, and this gives you yards for ready-mix and a bag count for the small pours — plus the point where bagging stops being sensible.
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Thickness by application
These are common conventions, not code. Your local building department has the final word, and for anything structural so does an engineer.
4 inches
Walkways, patios, garden slabs, shed bases. The default for foot traffic.
5 to 6 inches
Residential driveways and anywhere a vehicle parks. Six if you are expecting a truck, an RV, or a skip.
Thicker, or reinforced
Heavy equipment, poor subgrade, or anything holding up a structure. That is an engineering question, not a calculator question.
Subgrade allowance
A 5% waste factor covers the dips and soft spots a real excavation always has. Slabs are never as flat underneath as they are on top.
Bags or ready-mix
The crossover is around one cubic yard. An 80 lb bag yields roughly 0.60 ft³ and a 60 lb bag roughly 0.45 ft³, so a single cubic yard is about 45 bags of 80 lb mix. That is a full day of mixing and a sore back before the first one starts curing.
Ready-mix is cheaper per yard and arrives mixed correctly, but plants have a short-load minimum — often one to three yards — plus a delivery fee, and they charge for the truck's time on site. Below the minimum you are paying for concrete you do not receive.
Order a little long rather than a little short. Running out mid-pour gives you a cold joint, which is a permanent weakness and a visible line, and no amount of saved money fixes it afterwards.
What this calculator does not cover
Volume only. It does not size rebar or mesh, space control joints, or tell you whether your subgrade needs compacting — all of which matter more to whether the slab lasts than the yardage does.
Rough conventions worth knowing before you talk to a supplier: control joints are usually cut at a spacing in feet of about two to three times the slab thickness in inches, and cut within the first day. Thickened edges, footings and anything bearing a load are outside what an area calculator can answer.
Concrete is one of the few materials where getting the quantity right is the easy part.
Common questions
How many cubic yards of concrete do I need?
Multiply the slab area in square feet by the thickness in feet, then divide by 27. A 100 ft² slab at 4 inches is 100 × 0.333 = 33.3 cubic feet, which is about 1.24 cubic yards before any allowance for an uneven subgrade.
How many bags of concrete are in a cubic yard?
About 45 bags of 80 lb mix, or about 60 bags of 60 lb mix, since an 80 lb bag yields roughly 0.60 cubic feet and a 60 lb bag roughly 0.45. Check the yield printed on the bag, as it varies between products.
How thick should a concrete driveway be?
Five to six inches is the common residential convention, against four inches for a walkway or patio. Local code, soil conditions and expected vehicle weight all override a rule of thumb, so confirm with your building department.
Should I use bagged concrete or order ready-mix?
Bagged mix makes sense below about one cubic yard. Past that you are mixing roughly 45 bags by hand, and ready-mix is both cheaper and more consistent — though most plants have a short-load minimum and a delivery fee.
The rest of the toolbox
One measurement, then whichever material you are buying. Every tool reads the same ledger.