Concrete Calculator
Concrete calculator: how much concrete a slab, footing, column or post hole needs, with bag counts and ready-mix cost. Free, no sign-up, and it runs in your browser.
Dedicated page: /en/concrete-calculator
Default checked 10 Nov 2026: $150 per cubic yard (2026 US average $125 to $195 delivered). Leave it, or enter your exact price if you know it. Check the current value here.
Quick answer
A concrete calculator works out how much concrete a slab, footing, column or post hole needs. Enter the dimensions and it gives the volume in cubic yards and cubic feet, the number of 40, 60 or 80 pound bags, and the ready-mix cost.
About this Concrete Calculator
Concrete calculator: how much concrete a slab, footing, column or post hole needs, with bag counts and ready-mix cost. It is one of the Home and DIY on ToolUstad, and like every tool here it runs entirely in your browser: nothing you type, paste or open is uploaded to a server, and there is no sign-up or usage limit.
What is a concrete calculator?
Concrete is ordered by volume, in cubic yards, and mixed by the bag, in cubic feet, and the two units confuse everyone on their first pour. A cubic yard is 27 cubic feet. An 80 pound bag makes 0.6 cubic feet, a 60 pound bag makes 0.45, and a 40 pound bag makes 0.3. So one cubic yard needs 45 bags of 80 pound mix, 60 bags of 60 pound mix, or 90 bags of 40 pound mix. The calculator does those conversions from your dimensions, adds waste for the forms and the spill, and prices both options: bags from the store, or a ready-mix truck.
The choice between bags and truck is the real decision. Below about one cubic yard, bags are cheaper, because trucks charge a short-load fee. Above one yard, the truck wins on price and on your back: mixing 45 bags by hand is a full day of hard labour, while a truck pours the same amount in minutes. The calculator shows both costs, so the choice is a number, not a guess.
How does concrete maths work?
A 10 by 10 foot patio slab, 4 inches thick, is 10 times 10 times one third of a foot, which is 33.3 cubic feet. Add 10% waste and the total is 36.7 cubic feet, which is 1.36 cubic yards. At 0.6 cubic feet per 80 pound bag, that is 62 bags. At $150 per cubic yard, ready-mix costs about $204, plus any short-load fee. A round column is different: the volume is pi times the radius squared times the height, so a 12 inch diameter column 8 feet high needs about 6.3 cubic feet, or 11 bags of 80 pound mix.
When and why to use a concrete calculator
- Before ordering: a short pour stops the job halfway and leaves a cold joint, which is a visible weak line in the finished slab.
- Comparing bags and truck: the crossover is near one cubic yard, and this tool prices both.
- Post holes and footings: small round volumes are hard to estimate by eye, and the calculator handles them exactly.
- Budgeting a patio or driveway: concrete is the biggest material cost, so getting it right sets the whole budget.
- Checking a contractor's order: knowing the cubic yards yourself shows whether the order has been padded.
Best practices
- Order 10% extra, always: forms leak, ground settles, and a little always sticks to the mixer and the wheelbarrow.
- Wet the forms and the ground before the pour, so dry wood and soil do not steal water from the mix.
- Cure the concrete by keeping it damp for a week: concrete that dries too fast cracks, and concrete that stays damp gains strength.
Common mistakes to avoid
- Adding water to make the mix easier to pour: extra water weakens concrete permanently, by up to a third.
- Pouring a slab thinner than 4 inches: it will crack under its own weight and under anything parked on it.
- Forgetting reinforcement: a patio needs wire mesh or rebar, and a driveway needs both, or the first freeze will crack it.
Who is the concrete calculator for?
Homeowners pouring a patio, walkway or fence posts themselves, small builders pricing a job, and anyone ordering ready-mix who wants to check the quantity. It is also used by fence and deck installers for post hole volumes. The tool is an estimator: ground conditions and form sizes vary, so order the waste allowance rather than the exact volume.
Current value, checked 10 Nov 2026: the default ready-mix price is $150 per cubic yard (2026 US average $125 to $195 delivered). The price box is an optional override: leave the default, or enter your exact price and the cost uses it.
Learn: how to use the Concrete Calculator
- Pick the shape: slab, footing, round column or post hole.
- Enter the dimensions in feet and inches. A slab needs length, width and thickness; a column needs diameter and height.
- Read the cubic yards and the bag count, then compare the ready-mix cost against buying bags.
Features
- Slabs, footings, round columns and post holes in one tool
- Cubic yards and cubic feet, with 0, 5 or 10% waste
- Bag counts for 40, 60 and 80 pound bags (0.3, 0.45 and 0.6 cu ft each)
- Ready-mix cost from your price per cubic yard
- Clear rule of thumb for bags versus truck delivery
Worked example
A 10 by 10 foot patio slab, 4 inches thick: 10 x 10 x 0.333 feet is 33.3 cubic feet, plus 10% waste is 36.7 cubic feet, or 1.36 cubic yards, which is 62 bags of 80 lb mix. At $150 a cubic yard, ready-mix costs about $204.
Frequently asked questions
How many 80 lb bags make a cubic yard?
45 bags of 80 lb mix make one cubic yard (27 cubic feet divided by 0.6 cubic feet per bag). A 60 lb bag makes 0.45 cubic feet, so 60 bags make a yard, and a 40 lb bag makes 0.3, so 90 bags make a yard.
How thick should a patio slab be?
4 inches is standard for a patio or walkway. Driveways need 5 to 6 inches, and anything carrying heavy vehicles needs 6 inches with reinforcement.
When should I order ready-mix instead of bags?
Above about 1 cubic yard, or about 45 bags of 80 lb mix, ready-mix is usually cheaper and far less work. Below that, bags win because trucks charge a short-load fee.
Why add 10% waste?
Forms are never perfectly square, ground settles under the slab, and a little always spills or sticks to the mixer. Ordering exactly the calculated volume guarantees a short pour.
How thick should a concrete slab be?
4 inches for a patio or walkway, 5 to 6 inches for a driveway, and 6 inches with reinforcement for anything carrying heavy vehicles. Thinner slabs crack.
How long does concrete take to cure?
It sets in a day, reaches most of its strength in a week, and fully cures in 28 days. Keep it damp for the first week and do not drive on a driveway for at least a week.
Can I mix concrete in cold weather?
Below 40 degrees Fahrenheit the mix can freeze before it cures, which ruins it. Pour on a warmer day, or use a cold-weather mix with an accelerator, and cover the pour overnight.
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