Concrete Calculator
Work out cubic yards, cubic metres and 40, 60 or 80 lb bags of concrete for slabs, footings, post holes, walls, curbs and steps, with waste and cost.
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About this toolWhat is the Concrete Calculator?
Concrete is sold by volume but measured on site in a mix of feet and inches, which is where most ordering mistakes come from. A 10 by 10 foot patio poured 4 inches thick holds 33.3 cubic feet, or 1.23 cubic yards, and the difference between those two numbers is a factor of 27 that is easy to forget at the supply yard. This calculator takes the dimensions the way you measure them, converts thickness from inches (or centimetres) for you, and reports cubic yards, cubic feet, cubic metres and litres at once.
Pick the shape you are pouring: a slab or driveway, a square footing, round columns and post holes, a wall, a curb and gutter run, or a set of steps. Add a quantity for repeated pours such as 20 fence posts, set the extra allowance for spillage and uneven subgrade, and the result shows how many premix bags of each common size to buy and how much ready-mix to order, rounded up the way suppliers sell it.
A cost panel turns both options into money so you can see where bagged concrete stops being worth the labour. Nothing is sent anywhere; the arithmetic runs in your browser and the Share button copies a link with your inputs.
- Six pour shapes: slab, footing, round column or hole, wall, curb and gutter, and steps with an optional landing
- Imperial and metric inputs that convert in place when you switch
- Bag counts for 40, 50, 60 and 80 lb and 20 or 25 kg bags from published yields
- Ready-mix order rounded to an editable increment, plus cost for bags and ready-mix
- Formula panel, copy and share link; runs entirely in the browser
How to use the Concrete Calculator
- Choose imperial or metric. Long dimensions are in feet or metres; thickness, diameter and step sizes are in inches or centimetres.
- Pick the shape you are pouring and enter its dimensions. For several identical pours, such as fence post holes, set the quantity.
- Set the extra allowance. 5% suits formed slabs on a flat, compacted base; use 10% or more for dug footings and holes.
- Read the volume, the bag table and the ready-mix order. The highlighted row is the bag size used for the cost estimate.
- Enter local prices for a bag and for a cubic yard or metre of ready-mix to compare the two, then copy or share the result.
Worked examples
10 x 10 ft patio, 4 in thick
Input
Slab 10 ft x 10 ft x 4 in, 10% extra
Output
1.36 yd³ (1.04 m³); 62 bags of 80 lb or 82 of 60 lb; ready-mix order 1.5 yd³
The net volume is 1.23 yd³. With 10% added the bag count passes 60, which is nearly 5,000 lb of dry mix to lift; at this size a short load of ready-mix is usually the better buy.
Twenty fence post holes
Input
Round hole 10 in diameter x 3 ft deep, quantity 20, 10% extra
Output
36.0 ft³; 60 bags of 80 lb, 80 of 60 lb or 120 of 40 lb
Each hole holds 1.64 ft³. The post itself displaces some concrete (a 4 x 4 takes about 0.25 ft³ over 3 ft), so the extra allowance mostly covers belled hole bottoms.
Three front steps
Input
Steps: 7 in rise, 11 in run, 36 in wide, 3 steps, 10% extra
Output
9.63 ft³ before extra, 0.39 yd³ with it; 18 bags of 80 lb
Each step is a solid block from the ground up, so the top step holds three times the concrete of the bottom one.
Metric garden slab
Input
Slab 4 m x 3 m x 10 cm, 10% extra
Output
1.32 m³; 113 bags of 25 kg; ready-mix order 1.5 m³ at a 0.25 increment
Switching to metric converts the inputs too, so the slab stays the same size.
Background and insight
Bag counts use published yields rather than weight. The QUIKRETE Concrete Mix data sheet lists about 0.30 cubic feet for a 40 lb bag, 0.45 for 60 lb and 0.60 for 80 lb, and Sakrete gives the same 0.6 cubic feet for its 80 lb bag. Those yields are for mixed concrete, so a cubic yard (27 cubic feet) takes 45 bags of 80 lb or 60 bags of 60 lb. Kilogram bags sold outside the US are scaled from the 80 lb figure by weight, roughly 11.7 litres per 25 kg bag; the yield printed on your bag wins if it differs.
Ready-mix plants batch by the cubic yard or cubic metre and many dispatch in quarter-yard steps, so the calculator rounds up to an increment you can change. Small loads often carry a short-load fee, which is why a 1.4 yard patio can cost nearly as much as a 3 yard one. Normal-weight concrete weighs about 150 lb per cubic foot (2,400 kg per cubic metre), which matters if you are hauling bags in a car or checking a trailer limit.
Common use cases
Patios, sheds and hot-tub pads
Size a slab and see whether 60-odd bags or one short ready-mix load makes more sense for the job.
Fence and deck posts
Multiply one hole by the number of posts to buy the right pallet quantity in one trip.
Footings and piers
Price square pads and round sonotube piers for a deck or small extension before calling the plant.
Curbs, edging and steps
Handle the awkward L-shaped curb and gutter section and stacked steps without sketching volumes by hand.
Tips and limitations
- Measure thickness in several places. A slab specified at 4 inches over a base that dips to 5 inches needs 25% more concrete in that area.
- Order ready-mix slightly over. Running short mid-pour leaves a cold joint, while a little left over can fill a few post holes or splash blocks.
- Bag yields assume the right amount of water. Adding extra water to make mixing easier weakens the concrete without adding much volume.
- Above roughly one cubic yard, mixing by hand is a long day for two people; a mixer rental or ready-mix truck saves time and gives a more even pour.
- The weight estimate uses 150 lb per cubic foot. Check vehicle payload before loading dozens of 80 lb bags.
Frequently asked questions
How many 80 lb bags of concrete make a cubic yard?
About 45. An 80 lb bag of QUIKRETE or Sakrete concrete mix yields roughly 0.60 cubic feet, and a cubic yard is 27 cubic feet, so 27 / 0.6 = 45 bags. For 60 lb bags it is 60, and for 40 lb bags 90.
How do I calculate concrete for a slab in cubic yards?
Multiply length by width in feet, multiply by the thickness in inches divided by 12, and divide by 27. A 12 x 12 ft slab at 4 inches is 12 x 12 x 0.333 = 48 cubic feet, or 1.78 cubic yards, before any waste.
How much extra concrete should I order?
Most contractors add 5 to 10%. Use the low end for a formed slab on a level, compacted base and the high end, or more, for excavated footings, post holes and uneven ground, where the hole is rarely the exact size you planned.
Is it cheaper to buy bags or ready-mix concrete?
For under about half a cubic yard, bags usually win because ready-mix loads carry minimum charges or short-load fees. Past one cubic yard, ready-mix is normally cheaper per yard and far less work. Enter your local prices in the cost panel to see the crossover for your job.
How many bags of concrete do I need per fence post?
A 10 inch hole 3 feet deep holds about 1.6 cubic feet, so it takes three 80 lb bags or four 60 lb bags with a little to spare once the post is in. Wider or deeper holes scale with the square of the diameter, so a 12 inch hole needs about 44% more.
How thick should a concrete patio or driveway be?
Patios and walkways are commonly 4 inches thick. Driveways for cars are usually 4 to 5 inches, and 6 inches is typical where heavier vehicles park. Local codes and soil conditions decide the final figure.
What does a yard of concrete weigh?
About 4,000 lb (roughly 2 short tons). Normal-weight concrete is around 150 lb per cubic foot, and 150 x 27 = 4,050 lb. In metric, a cubic metre weighs about 2,400 kg.
Is there an in-depth guide to the Concrete Calculator?
Yes. “Estimating Concrete, Flooring, Tile and Gravel Without Running Short” covers the background, how it works, common pitfalls and worked examples. Read the full guide.
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