Concrete Calculator
Work out how much concrete to buy for a slab, footing, column (sonotube), wall, round pad, steps or fence posts. You get cubic yards, the number of 40, 60 and 80 lb bags, an approximate weight and, once you enter your local prices, a bagged vs ready-mix cost.
How to use the concrete calculator
- Pick what you are pouring: slab, footing, column / tube, wall, circle, steps or post holes.
- Enter the dimensions. Lengths are in feet; thickness, footing size and tube diameters are in inches. Switch to metric at the top if you measure in metres and centimetres.
- Set How many if you are pouring several identical pieces (for example 8 deck footings), and adjust the waste allowance. 10% is the default.
- Read the cubic yards and the bag counts for 80, 60 and 40 lb bags. Open Cost estimate and enter your local bag and ready-mix prices to compare the two.
- Use Copy link to save your exact inputs, or Print to take the numbers to the store.
How the concrete calculator works
Every shape comes down to volume in cubic feet, which is then divided by 27 to get cubic yards (1 yd³ = 3 ft × 3 ft × 3 ft = 27 ft³). Thicknesses are entered in inches, so they are divided by 12 first.
- Slab: length × width × thickness ÷ 12
- Footing: length × (width ÷ 12) × (depth ÷ 12), with width and depth in inches
- Column or sonotube: π × (diameter ÷ 24)² × height, with the diameter in inches and the height in feet
- Wall: length × height × thickness ÷ 12
- Round slab: π × (diameter ÷ 2)² × thickness ÷ 12
- Steps: each step is a block that reaches down to the ground, so the volume is width × tread × rise × (1 + 2 + … + number of steps), plus an optional top platform
- Post holes: the hole cylinder minus the part filled by the post, times the number of posts
The waste allowance is then added, and bags are always rounded up: you can't buy 0.1 of a bag.
Worked example: a 10 ft × 10 ft slab, 4 in thick
This is the example loaded in the calculator above.
- Volume: 10 × 10 × (4 ÷ 12) = 33.33 ft³, or 1.23 yd³ before waste.
- Add 10% waste: 33.33 × 1.10 = 36.67 ft³, which is 1.36 yd³.
- Bags: an 80 lb bag yields about 0.60 ft³, so 36.67 ÷ 0.60 = 61.1, rounded up to 62 bags. With 60 lb bags (0.45 ft³) you need 82; with 40 lb bags (0.30 ft³), 123.
- Weight: 33.33 ft³ × 150 lb/ft³ ≈ 5,000 lb of finished concrete, which matters if you are hauling bags in a pickup or trailer.
Bags or ready-mix?
Quikrete's data sheet gives these yields for its Concrete Mix: about 0.30 ft³ per 40 lb bag, 0.45 ft³ per 60 lb bag and 0.60 ft³ per 80 lb bag. Sakrete lists the same figures. That works out to 90, 60 and 45 bags per cubic yard.
Below about 1 yd³, bags are usually the practical choice: a handful of post holes, a small pad or a set of steps. From about 1 yd³ upward, a ready-mix truck is usually easier than mixing 45 or more heavy bags, and the concrete arrives evenly mixed for one continuous pour. Many plants charge a short-load fee for small orders, so ask for the full delivered price. The calculator doesn't pre-fill any prices because they vary a lot by region; enter the prices you are quoted to compare both options side by side.
Concrete for common slab sizes (4 in thick)
| Slab size (4 in thick) | Cubic yards | Yd³ + 10% waste | 80 lb bags | 60 lb bags |
|---|---|---|---|---|
| 8 × 8 ft | 0.79 | 0.87 | 40 | 53 |
| 10 × 10 ft | 1.23 | 1.36 | 62 | 82 |
| 10 × 12 ft | 1.48 | 1.63 | 74 | 98 |
| 12 × 12 ft | 1.78 | 1.96 | 88 | 118 |
| 12 × 16 ft | 2.37 | 2.61 | 118 | 157 |
| 16 × 16 ft | 3.16 | 3.48 | 157 | 209 |
| 20 × 20 ft | 4.94 | 5.43 | 245 | 326 |
| 24 × 24 ft | 7.11 | 7.82 | 352 | 470 |
Bag counts include 10% waste. For 5 in slabs multiply the volume by 1.25; for 6 in, by 1.5.
Tips before you order
- Check the real thickness. Ground is rarely flat. A slab that is 4 in at the forms but 5 in over a low spot uses 25% more concrete there. Level and compact the base first, then measure in several places.
- Keep the waste allowance. 5–10% covers spillage, form bulges and a slightly uneven base. Running short in the middle of a pour leaves a cold joint, which Quikrete's instructions warn can weaken the slab.
- Check your local code and frost depth. Footings for decks, porches and walls usually have to reach below the frost line, and many towns set minimum slab thicknesses or require a permit. The calculator does the arithmetic; your local building department sets the rules.
- Fence and deck posts: Quikrete suggests a hole about three times the post's width and about one third of the post's total length deep. Use the Post holes option with your post size to get bags per post.
Frequently asked questions
How many 80 lb bags of concrete are in a yard?
About 45. A cubic yard is 27 ft³ and an 80 lb bag yields about 0.60 ft³, so 27 ÷ 0.60 = 45. For 60 lb bags (0.45 ft³) it is 60 bags, and for 40 lb bags (0.30 ft³) it is 90 bags. Add your waste allowance on top.
How much concrete do I need for a 10×10 slab?
At 4 in thick, a 10 × 10 ft slab is 33.33 ft³, or about 1.23 yd³. With a 10% allowance that is about 1.36 yd³, or 62 bags of 80 lb mix. At 6 in thick you need half as much again.
How thick should a concrete slab be?
Patios, walks and shed bases are commonly poured 4 in thick. Driveways and slabs that carry vehicles or heavy loads are usually 5–6 in, often with reinforcement. Your local building code may set a minimum, so check before you pour.
How much does a yard of concrete weigh?
Normal-weight concrete is usually taken as about 150 lb per ft³, so one cubic yard weighs roughly 4,050 lb (just over 2 US tons). Quikrete gives a unit weight of about 140 lb per ft³ for its bagged mix, which works out closer to 3,800 lb per yard.
When should I order ready-mix instead of bags?
As a rule of thumb, from about 1 cubic yard (45 bags of 80 lb mix) upward. Ready-mix arrives mixed and lets you pour in one go. For smaller jobs bags are usually simpler, especially as many plants add a short-load fee.
How do I work out concrete for a sonotube?
Use π × (diameter ÷ 24)² × height, with the diameter in inches and the height in feet. A 12 in tube 4 ft deep holds π × 0.5² × 4 = 3.14 ft³ (0.116 yd³), which is 6 bags of 80 lb mix after rounding up. Choose Column / tube above and set How many for several tubes.
More concrete calculators
- Concrete Slab CalculatorSlab volume and bags, with a size table from 8×8 to 30×40 ft at 4, 5 and 6 in.
- Concrete Slab Cost CalculatorPrice a slab with bags or ready-mix using your local prices.
- Concrete Footing CalculatorStrip and pad footings: cubic yards and bags per footing or per run.
- Sonotube CalculatorConcrete per tube for 6–24 in form tubes, and bags for every tube.
- Concrete Steps CalculatorConcrete for solid steps on grade, with an optional top landing.
- Fence Post Concrete CalculatorBags of concrete per post for 4×4 and 6×6 posts by hole size.
- Concrete Bags CalculatorHow many 40, 60 or 80 lb bags make a yard, and how many per pallet.
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