Concrete Slab Calculator
Work out the concrete volume for a rectangular slab and how many bags of premix to buy.
Last reviewed September 19, 2026
Result
Enter length and width to see the result.
https://calcgallery.com/concrete/concrete-slab-calculator/
Results are estimates for planning only. Actual needs vary with site conditions, products and workmanship. Always confirm quantities with your supplier and follow local building codes and manufacturer instructions. Read the full disclaimer
How to use it
- Measure the length and width of the slab. For a shape that is not a rectangle, split it into rectangles and add the results.
- Enter the thickness. Most residential slabs fall between 100 and 150 mm, or 4 and 6 in.
- Pick the bag size you can actually buy. The list follows your region.
- Read the bag count. It already includes a 10% allowance, which you can change under More options.
How it is calculated
The volume of a rectangular slab is its three dimensions multiplied together:
Volume = length × width × thickness × number of slabsAll three have to be in the same unit before they are multiplied, which is why entering a thickness in inches next to a length in feet is safe here: the conversion happens before the multiplication, not after.
The waste allowance is applied to that volume, and only then is the bag count rounded up:
Bags = ceil( volume × (1 + waste) ÷ yield per bag )Rounding up last matters. Rounding each step would quietly add a bag or two that you do not need.
The yield is how much set concrete one bag of dry premix makes. It is not the bag’s weight: dry mix is denser than the finished slab, and a standard 80 lb bag makes about 0.60 ft³. If your bag states a different figure, put it in under More options and everything downstream follows it.
Practical tips
Buy one extra bag, not ten. The 10% allowance covers an uneven sub-base. It does not cover a bag that splits, or a wheelbarrow that tips. One spare bag is cheap insurance; unopened premix keeps for months if you store it dry and off the floor.
Mixing by hand is slower than you think. A single 80 lb bag is a few minutes of work with a mixer and rather more with a shovel. Twenty bags is most of a day. Work out the bag count before you decide how you are pouring, not after.
Pour a slab in one go where you can. Concrete placed against concrete that has already started to set forms a cold joint, which is a weakness. If the count says two days of mixing, that is a strong argument for ready-mix.
The sub-base is not optional. Concrete is strong in compression and weak in tension, so it cracks where the ground beneath it moves. A compacted gravel base gives it something stable to sit on and lets water drain away.
Measure the depth in several places. Excavations are deeper in the middle than at the edges more often than not. Measuring in four or five spots and using the largest figure is more reliable than measuring once.
Local codes vary. Thickness, reinforcement, sub-base depth and the need for a permit all differ by jurisdiction, and for anything structural they matter. Check your local building code and speak to a professional if the slab is carrying a building.
With your numbers
- Volume = 10 ft × 12 ft × 0.3333 ft = 40 ft³
- With 10% waste = 44 ft³
- Bags = 44 ft³ ÷ 0.6 ft³ per bag, rounded up to 74
Quick reference
Ground rarely sits level, so slabs run deeper than planned. This includes 10% extra.
| Job | Bags to buy |
|---|---|
| 8 × 8 ft shed base, 4 in thick | 40 |
| 10 × 10 ft patio, 4 in thick | 62 |
| 10 × 12 ft slab, 4 in thick | 74 |
| 12 × 12 ft slab, 4 in thick | 88 |
| 20 × 20 ft garage, 6 in thick | 367 |
| 3 × 3 m slab, 100 mm thick | 106 |
| 4 × 4 m slab, 100 mm thick | 188 |
| 6 × 4 m garage, 150 mm thick | 423 |
Common questions
- How many 80 lb bags of concrete are in a cubic yard?
- About 45. A cubic yard is 27 ft³ and an 80 lb bag of premix makes roughly 0.60 ft³, so 27 ÷ 0.60 comes to 45 bags. At that point ready-mix is usually cheaper and far less work.
- When should I order ready-mix instead of bags?
- Somewhere around a cubic yard, or about 45 bags of 80 lb. Below that, bags are convenient and you can mix at your own pace. Above it you are mixing for hours, and a truck arrives with consistent concrete in one pour, which matters because a slab poured in stages can develop cold joints.
- How thick should a concrete slab be?
- Residential slabs are commonly 100 mm (4 in) for patios, shed bases and footpaths, and 150 mm (6 in) where vehicles drive. Thickness requirements depend on the load, the ground underneath and your local building code, so treat those as typical ranges and check what applies where you are.
- Why does the calculator add 10%?
- Ground is rarely as level as it looks, so a slab almost always ends up deeper in places than the number you measured. Ten percent is a common allowance for a slab on ground. You can set it to zero, or to your own figure, under More options.
- Does this include the gravel base?
- No. This works out the concrete only. A slab normally sits on a compacted sub-base of 100 to 150 mm of gravel, which you should calculate separately.
- What if my slab is not a rectangle?
- Break it into rectangles, calculate each one, and add the bag counts. Round up once at the end rather than for each piece, or you will buy more than you need.