Quick answer
Calculate slab volume, then yards or bags
Multiply length by width by thickness after converting every dimension to feet. Divide cubic feet by 27 for cubic yards. If you are using bagged mix, apply your project-specific allowance, divide by the yield printed for the exact product, and round the final bag count upward.
Calculate my slab nowThe formula
Calculate concrete for a slab step by step
length (ft) × width (ft) × thickness (ft)Net cubic yardsnet cubic feet ÷ 27Order cubic feetnet volume × (1 + allowance ÷ 100)Complete bagsround up (order volume ÷ yield per bag)When thickness is measured in inches, divide it by 12 before multiplying. Keep full precision through the calculation and round only the displayed result or final package quantity.
Product input
Use the yield for the exact mix
As a documented example, Sakrete High-Strength Concrete Mix lists yields of 0.30 ft³ for a 40 lb bag, 0.45 ft³ for a 60 lb bag, and 0.60 ft³ for an 80 lb bag. Other mixes and products can differ, so the current bag or technical data sheet controls.
Reference table
Common 4-inch slabs with 10% allowance
These examples assume a rectangular slab, uniform 4-inch thickness, and the example yields above. They are planning references—not substitutes for measuring the actual project.
| Slab | Order yd³ | 40 lb | 60 lb | 80 lb |
|---|---|---|---|---|
| 5 × 5 ft | 0.340 | 31 | 21 | 16 |
| 10 × 10 ft | 1.358 | 123 | 82 | 62 |
| 12 × 12 ft | 1.956 | 176 | 118 | 88 |
| 20 × 20 ft | 5.432 | 489 | 326 | 245 |
A high bag count can make ready-mix delivery more practical. Compare delivery minimums, placement time, labor, mixing capacity, and supplier recommendations before deciding.
Worked example
A 9 ft × 9 ft slab, 4 inches thick
- Convert thickness: 4 in ÷ 12 = 0.333333 ft.
- Find net volume: 9 × 9 × 0.333333 = 27 ft³.
- With 0% allowance, the result is exactly 1 yd³.
- For the 80 lb example: 27 ÷ 0.60 = 45 complete bags.
If you add an allowance, apply it to the 27 ft³ net volume before dividing by the bag yield.
Before purchase
Verify what a calculator cannot know
- Re-measure the formed length, width, and actual thickness.
- Confirm that the selected mix is suitable for the application and depth.
- Use the current yield printed for the exact product and bag size.
- Choose an allowance based on the subgrade, forms, spillage, and site conditions.
- Check structural, reinforcement, code, weather, curing, and supplier requirements with a qualified professional.
Common questions
Concrete slab FAQ
How much concrete do I need for a 10 × 10 slab at 4 inches thick?
A 10 ft × 10 ft slab at 4 inches thick is about 1.235 cubic yards before waste. With a 10% allowance it is about 1.358 cubic yards. Using an 80 lb bag yield of 0.60 cubic feet, that order volume is 62 complete bags.
How many 80 lb bags make one cubic yard?
Using a published yield of 0.60 cubic feet per 80 lb bag, one cubic yard requires 45 bags before any waste allowance because 27 divided by 0.60 equals 45.
Should I add 10% extra concrete?
Ten percent is a common starting allowance, not a universal rule. Form accuracy, subgrade, spillage, supplier increments, and site conditions can require a different amount.
When should I order ready-mix instead of bags?
Large bag counts can be slow and labor-intensive to mix consistently. Compare the total bag quantity, mixing capacity, placement time, delivery minimums, and supplier advice before choosing.
Open the BuildMeasure Concrete Calculator for cubic yards and bags and keep the product yield adjustable.