How to use the concrete calculator
- Choose what you are pouring. Use Slab, pad or footing for anything rectangular. Use Round column or hole for tube forms, piers and post holes.
- Enter the size. Length and width in feet, thickness or depth in inches. For several identical pads or holes, set How many.
- Pick your bag size. The table in the result shows the count for every common bag size so you can compare.
- Add prices for bags and for ready-mix to see which is cheaper for your job.
The concrete formula
Slab: cubic feet = length × width × (thickness in inches ÷ 12)
Round form: cubic feet = π × (diameter ÷ 2)² × depth, all in feet
With extra = cubic feet × (1 + waste)
Cubic yards = cubic feet with extra ÷ 27
Bags = cubic feet with extra ÷ yield per bag, rounded up
Worked example
A 10 ft × 10 ft patio slab, 4 inches thick
- Volume = 10 × 10 × (4 ÷ 12) = 33.33 cu ft.
- Add 10%: 33.33 × 1.10 = 36.67 cu ft.
- Cubic yards = 36.67 ÷ 27 = 1.36 cu yd.
- 80 lb bags at 0.60 cu ft each: 36.67 ÷ 0.60 = 61.1, so 62 bags (about 4,960 lb).
That is well over a cubic yard, so the calculator suggests comparing the price of ready-mix.
Concrete bag yields
| Bag size | Typical yield | Bags per cubic yard | Bags for 1 sq ft at 4 in |
|---|---|---|---|
| 40 lb | 0.30 cu ft | 90 | 1.11 |
| 50 lb | 0.375 cu ft | 72 | 0.89 |
| 60 lb | 0.45 cu ft | 60 | 0.74 |
| 80 lb | 0.60 cu ft | 45 | 0.56 |
Yields are for standard concrete mix. High-strength, fast-setting and fiber-reinforced mixes can differ, so always check the bag.
Bags or ready-mix?
Bags are ideal for post holes, small pads, steps and repairs: you buy exactly what you need and mix as you go. Once a pour reaches about a cubic yard, three things favor ready-mix:
- Labor. One yard is 45 bags of 80 lb, over 3,600 lb to lift, carry and mix.
- Quality. A slab should be poured in one go. Hand-mixing a large slab risks cold joints where batches meet.
- Price. Ready-mix is usually cheaper per yard than bags, although small loads often carry a short-load fee. Enter both prices to compare.
Common mistakes to avoid
- Using inches as feet. A 4-inch slab is 0.333 ft thick. Entering 4 as feet gives twelve times too much.
- No extra. Forms bow, ground dips and some concrete is always lost. Add at least 5 to 10%.
- Ignoring the gravel base. Most slabs sit on a few inches of compacted gravel, which you order separately.
- Assuming every bag yields the same. 80 lb bags of different products can yield different volumes. Read the label.
- Forgetting the weather. Concrete needs the right temperature to cure. Very hot, very cold or rainy days need precautions or a different day.
Frequently asked questions
How many 80 lb bags of concrete make a cubic yard?
An 80 lb bag of standard concrete mix yields about 0.6 cubic feet, so a cubic yard (27 cubic feet) takes 45 bags. With 60 lb bags (0.45 cu ft) it takes 60, and with 40 lb bags (0.3 cu ft) it takes 90.
How many bags of concrete do I need for a 10 × 10 slab?
At 4 inches thick, a 10 × 10 ft slab is 33.3 cubic feet. With 10% extra that is 36.7 cubic feet, or 62 bags of 80 lb concrete (1.36 cubic yards). At that size, ready-mix is often the better choice.
When should I order ready-mix instead of bags?
As a rule of thumb, once a pour is around a cubic yard or more. That is 45 or more 80 lb bags, over 3,600 lb to lift and mix, and a large slab must be poured in one go. Ready-mix trucks often charge extra for small loads, so compare both with your local prices.
How thick should a concrete slab be?
Patios, walkways and shed bases are commonly 4 inches thick. Driveways and slabs for vehicles are often 5 to 6 inches. Anything structural, or anything your local code covers, should follow the code or an engineer.
How much extra concrete should I order?
About 5 to 10%. Ground is never perfectly flat, forms bow slightly and some concrete is always spilled. Running out in the middle of a pour leaves a weak cold joint.
Is concrete mix the same as cement?
No. Cement is the gray powder that binds concrete. Concrete mix is cement already blended with sand and gravel; you only add water. This calculator is for concrete mix.