Concrete Slab Calculator
Area 1
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Area __n__
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No result yet. Enter length, width and thickness.
Buying options
Starts at 10%, the top of the 4% to 10% that NRMCA, the ready-mix industry association, says to order over the plan dimensions. That is one source, so treat 10% as a starting value and change it to suit the job.
Bag sizes are US bags, sold by the pound.
How this is calculated
How to calculate concrete for a slab
Multiply length by width to get the area in square feet; for a round slab the area is π × (diameter ÷ 2)², and with Known area it is the figure you type. Multiply the area by the thickness in feet (inches ÷ 12) to get the volume in cubic feet. Divide cubic feet by 27 to get cubic yards, the unit ready-mix is ordered in.
Add waste to the volume. Divide the result by the volume one bag makes, called its yield, and round up to a whole bag.
The bag counts are for standard concrete mix. Specialty mixes yield a different volume. The yield printed on the bag is the number to trust: if it differs from the one used here, go by the bag.
Examples
Patio: 12 ft × 10 ft × 4 in
- Area: 12 ft × 10 ft = 120 sq ft = 11.15 m²
- Volume: 120 sq ft × 4 in = 40.0 cu ft = 1.48 cu yd
- With 10% waste: 40.0 cu ft + 10% = 44.0 cu ft = 1.63 cu yd
- Ready-mix order: 44.0 cu ft ÷ 27, rounded up to 0.01 cu yd = 1.63 cu yd
- Bags: 44.0 cu ft ÷ 0.60 cu ft per bag, rounded up = 74
- Weight of the bags: 74 bags × 80 lb = 5,920 lb
- Cost: 74 bags × $6.50 = $481.00
74 80 lb bags with 10% waste. Ready-mix order: 1.63 cu yd. Weight of the bags: 5,920 lb. Estimated cost: $481.00.
Walkway: 24 ft × 3 ft 6 in × 4 in
- Area: 24 ft × 3 ft 6 in = 84 sq ft = 7.80 m²
- Volume: 84 sq ft × 4 in = 28.0 cu ft = 1.04 cu yd
- With 10% waste: 28.0 cu ft + 10% = 30.8 cu ft = 1.14 cu yd
- Ready-mix order: 30.8 cu ft ÷ 27, rounded up to 0.01 cu yd = 1.15 cu yd
- Bags: 30.8 cu ft ÷ 0.45 cu ft per bag, rounded up = 69
- Weight of the bags: 69 bags × 60 lb = 4,140 lb
69 60 lb bags with 10% waste. Ready-mix order: 1.15 cu yd. Weight of the bags: 4,140 lb.
Round pad: 10 ft × 4 in
- Area: π × (10 ft ÷ 2)² = 78.5 sq ft = 7.30 m²
- Volume: 78.5 sq ft × 4 in = 26.2 cu ft = 0.97 cu yd
- With 10% waste: 26.2 cu ft + 10% = 28.8 cu ft = 1.07 cu yd
- Ready-mix order: 28.8 cu ft ÷ 27, rounded up to 0.01 cu yd = 1.07 cu yd
- Bags: 28.8 cu ft ÷ 0.60 cu ft per bag, rounded up = 48
- Weight of the bags: 48 bags × 80 lb = 3,840 lb
48 80 lb bags with 10% waste. Ready-mix order: 1.07 cu yd. Weight of the bags: 3,840 lb.
Area already measured: 150 sq ft × 4 in
- Area: 150 sq ft = 150 sq ft = 13.94 m²
- Volume: 150 sq ft × 4 in = 50.0 cu ft = 1.85 cu yd
- With 10% waste: 50.0 cu ft + 10% = 55.0 cu ft = 2.04 cu yd
- Ready-mix order: 55.0 cu ft ÷ 27, rounded up to 0.01 cu yd = 2.04 cu yd
- Bags: 55.0 cu ft ÷ 0.60 cu ft per bag, rounded up = 92
- Weight of the bags: 92 bags × 80 lb = 7,360 lb
92 80 lb bags with 10% waste. Ready-mix order: 2.04 cu yd. Weight of the bags: 7,360 lb.
Bags by slab size
| Shape and size | Volume | 40 lb | 60 lb | 80 lb |
|---|---|---|---|---|
| Slab: 8 ft × 8 ft × 4 in | 0.87 cu yd | 79 | 53 | 40 |
| Slab: 10 ft × 10 ft × 4 in | 1.36 cu yd | 123 | 82 | 62 |
| Slab: 12 ft × 10 ft × 4 in | 1.63 cu yd | 147 | 98 | 74 |
| Slab: 12 ft × 12 ft × 4 in | 1.96 cu yd | 176 | 118 | 88 |
| Slab: 20 ft × 10 ft × 4 in | 2.72 cu yd | 245 | 163 | 123 |
| Slab: 20 ft × 20 ft × 4 in | 5.44 cu yd | 489 | 326 | 245 |
Bags needed for each bag size, with 10% waste. The volume includes the waste.
Questions
Why is the bag count rounded up?
You can't buy part of a bag. The calculator divides the volume by the volume one bag makes and rounds any fraction up to the next whole bag.
What does waste cover?
Spills, mix left in the wheelbarrow, and ground that isn't level, which makes the slab thicker in places. The calculator starts at 10%, the top of the 4% to 10% that the ready-mix association NRMCA advises ordering over the plan dimensions. Change it to suit your job, or set it to 0 to see the bare amount.
Does the bag count work for any concrete mix?
No. It is for standard concrete mix, using the yields Quikrete and Sakrete publish for it. Fast-setting, high-yield and other specialty mixes make a different volume per bag. Read the yield printed on your bag. If it differs from the yield shown under "How this is calculated", divide the volume with waste by the number on the bag.
What is the ready-mix number for?
Ready-mix concrete is ordered by the cubic yard. The ready-mix line is the same volume, waste included, in cubic yards rounded up to 0.01. Switch "Buy as" to ready-mix to enter a price per cubic yard and compare the delivery with buying bags. Each mode keeps its own price.
What does the weight line tell me?
How heavy the load is. In bags it is the count times the bag weight, 80 lb for an 80 lb bag. As ready-mix it is the volume times 150 lb per cubic foot, normal-weight concrete. Use it to check a trailer or truck rating.
How is the cost worked out?
The estimated cost is the number of bags, waste included, times the price per bag you enter, or the ready-mix cubic yards times the price per cubic yard. The site has no prices of its own, so use the shelf or supplier's price.
My slab isn't a simple rectangle. What do I do?
Split it into rectangles and add each one with "Add another area". The calculator adds the volumes and gives one bag count for the whole pour. A round pad has its own shape, Round slab, which takes the diameter. If you already know the area in square feet, from a plan or a survey, choose Known area and type it in with the thickness.
Why is thickness in inches when length is in feet?
Slab thickness is normally given in inches, so the field takes inches and the calculator converts. If the bag count looks far too low or far too high, check that thickness was entered in inches, not feet.
Related tools
Guides
Sources
The area and volume formulas are plain geometry. Area is length × width. Volume is area × thickness. One cubic yard is 27 cubic feet by definition. The volume one bag makes is a reference value; each one used here is published by both Quikrete and Sakrete. The ready-mix weight uses 150 lb per cubic foot, inside the 140 to 155 lb FHWA gives and the figure an Ohio University table of building material weights lists. The unit definitions are from NIST.
- Quikrete Concrete Mix, Product No. 1101, data sheet (October 2022), section Yield, accessed October 7, 2026
- Sakrete High Strength Concrete Mix, product page, yield per bag, accessed October 7, 2026
- NRMCA, CIP 31, Ordering Ready Mixed Concrete, accessed October 8, 2026
- NRMCA, CIP 8, Discrepancies in Yield, accessed October 8, 2026
- FHWA, HRT-05-057, Appendix C, Proposed Revisions to the AASHTO Standard Specifications for Highway Bridges, unit weight of normal weight concrete, accessed October 8, 2026
- Ohio University, H-20, Weights of Building Materials, Agricultural Commodities, and Floor Loads for Buildings, Table 1, accessed October 8, 2026
- NIST Handbook 44 (2024), Appendix C, General Tables of Units of Measurement, accessed October 8, 2026
- NIST Special Publication 811, Appendix B.9, conversion factors listed by kind of quantity, accessed October 8, 2026
Last reviewed: October 8, 2026