Concrete Calculator
Buy this many
74 eighty-pound bags
That is 1.48 cubic yards, or 40 cubic feet, before the 10% allowance. Below about one yard, bags win on price and hassle; above two, ready-mix delivered is normally cheaper — 2 cubic yards here.
- 80 lb bags
- 74
- 60 lb bags
- 98
- 40 lb bags
- 147
- Or ready-mix, ordered
- 2 cubic yards
Bag counts include your waste allowance and always round up to a whole bag. Being one bag short stops a pour; a spare bag costs a few dollars. Ready-mix rounds up from the bare volume instead, without the allowance added first — ordering a whole extra cubic yard is already a bigger margin than any percentage would add.
How this is calculated
Volume is straightforward: length times width times thickness for a slab, or the area of a circle times height for a column, converted to cubic feet and then to cubic yards by dividing by twenty-seven. The part every other calculator leaves you to work out yourself is what to actually buy. An eighty-pound bag of mix yields about six tenths of a cubic foot, a sixty-pound bag about forty-five hundredths, and a forty-pound bag about three tenths, so the same slab is a different number of trips to the car depending on which you pick. The waste allowance is applied before that division and the result always rounds up to a whole bag, because a part-used bag is fine and being one bag short in the middle of a pour is not — concrete does not wait while you drive back. Ready-mix is bought by the whole cubic yard instead, and rounds up from the raw volume without the allowance added first, since a spare cubic yard delivered is already a bigger margin than any percentage would add. The crossover worth knowing is roughly one cubic yard: below it, bags are usually cheaper and always easier; above two, mixing that many by hand is a serious afternoon and ready-mix delivered is normally both cheaper and better concrete, because it is mixed consistently.
Worked example
Ten by twelve feet at four inches deep is 40 cubic feet, which is 1.48 cubic yards. Adding ten percent brings it to 44 cubic feet, and at six tenths of a cubic foot per eighty-pound bag that is 74 bags — about a ton and a half of mix, and a long day. That is the judgement call the FAQ describes, and what usually tips it at this size is not the volume but the lifting: seventy-four bags is roughly a ton and a half to carry, open and mix, against one delivery. Two cubic yards ordered covers the same slab with room to spare.
Inputs: A slab 10 feet by 12 feet, 4 inches thick, with a 10% waste allowance
Result: 1.48 cubic yards — 74 eighty-pound bags, or 2 cubic yards ready-mixed
What this assumes
- Bag yields are the usual figures for standard mix — 0.6, 0.45 and 0.3 cubic feet. Check your bag, because they vary a little by brand and mix type.
- The waste allowance covers spillage, uneven ground and the concrete that never quite comes out of the barrow, and it is built into the bag counts above. Ten percent is a normal starting point; rough or sloping ground wants more. Ready-mix is ordered in whole cubic yards and rounds up from the raw volume instead — a spare cubic yard delivered is already a bigger margin than any percentage would add.
- This is a volume calculation only. It says nothing about thickness, reinforcement or footing depth, all of which are structural questions with local code requirements.
Questions
- How thick should a slab be?
- That depends on what is going on it and on your local code, which is why this calculator asks rather than assumes. Four inches is common for a patio or shed base and six for a driveway, but frost depth, soil and load all matter, and none of that is something a volume calculator can tell you.
- Bags or ready-mix?
- Under about a cubic yard, bags — cheaper, no minimum order, and you mix what you need when you need it. Over about two, ready-mix, because mixing forty-plus bags by hand is a genuinely hard day and the result is less consistent than a truck's. Between the two it is a judgement call about how much help you have.
- Why does it always round up?
- Because the two mistakes are not equal. A spare bag costs a few dollars and sits in the garage. Being one bag short stops the pour with wet concrete already down, and a cold joint in the middle of a slab is a permanent weakness rather than an inconvenience.
- What will the concrete cost?
- Type your merchant's price for an 80 lb bag, or their delivered price per cubic yard, and the result card costs the purchase this page recommends — waste allowance and round-up included, not the bare volume. Fill both in and you can see the bags-versus-ready-mix decision in your own money rather than as a rule of thumb. The prices are yours, never ours, because bag prices vary by merchant, region and week, and a figure we supplied would be wrong for most people while looking authoritative.