Calcs.ToolsConcrete Volume Calculator
Enter your slab length, width, thickness, and bag size to see cubic feet, cubic yards, bag counts, and weight instantly. A 5% waste buffer is built in so you order enough concrete the first time.
How to use it
- Measure the length and width of your slab or footing in feet. For odd shapes, break them into rectangles and add the totals.
- Enter thickness in inches (4-6 inches for patios, 6-8 inches for driveways, thicker for structural pads).
- Select a bag size. 80 lb bags are about 0.67 cubic feet; 60 lb bags are about 0.6 cubic feet.
- Review bags needed, cubic yards, and total weight. Add 5-10% extra if your forms or subgrade are uneven.
Example calculation
A 20 ft x 15 ft patio at 4 inches thick is 100 cubic feet (20 x 15 x 0.333). That equals 3.7 cubic yards. Using 80 lb bags (0.67 cu ft each) you need about 150 bags; with a 5% buffer, plan for 158 bags. The total wet weight is roughly 15,000 lbs, so schedule help and equipment accordingly.
Formulas and Edge Cases
Volume is calculated as length x width x depth. Depth must be in feet, so inches are divided by 12. We then convert cubic feet to cubic yards by dividing by 27. Bag counts are computed using the yield per bag and rounded up to avoid shortages.
Edge cases include sloped slabs, thickened edges, or uneven subgrades. For those projects, add extra volume or calculate each section separately and sum the totals.
References and Sources
- Concrete manufacturer bag yield charts
- Standard cubic yard conversion guidelines
FAQs
How accurate is this calculator?
It uses exact volume math and a default 5% waste buffer. Accuracy depends on your measurements and how level your forms are. Add 5-10% more if the subgrade is rough or you expect spillage.
Can I switch to ready-mix instead of bags?
For pours over 2-3 cubic yards, ready-mix delivery is usually cheaper and far faster. Small slabs and repairs are fine with bagged concrete if you have a mixer or plenty of labor.
How thick should my slab be?
Patios and sidewalks are typically 4 inches. Driveways often need 5-6 inches with proper base and steel reinforcement. Always confirm local code and soil conditions before ordering concrete.
Ordering tips
- Confirm truck access or staging space if you are ordering ready-mix.
- Keep wheelbarrows, screeds, and finishing tools ready before mixing.
- Plan helpers since concrete sets quickly, especially in warm weather.
Understanding Concrete Types and Strength
Concrete strength is measured in PSI (pounds per square inch). Standard bagged concrete typically provides 3,000-4,000 PSI after curing, which is suitable for most residential projects like patios, walkways, and shed foundations. High-strength mixes rated at 5,000+ PSI are available for driveways and areas with heavy loads or freeze-thaw cycles.
Quick-setting concrete cures faster (20-40 minutes vs. 24-48 hours for standard mixes) and is useful for setting posts or making repairs. However, the faster cure time means less working time, so plan your pour carefully and mix only what you can place immediately.
Common Concrete Bag Sizes
- 40 lb bags: About 0.30 cubic feet, easier to handle for small repairs
- 60 lb bags: About 0.45 cubic feet, good balance of weight and coverage
- 80 lb bags: About 0.60 cubic feet, most economical for larger projects
- 90 lb bags: About 0.67 cubic feet, maximum coverage per bag
Bagged Concrete vs. Ready-Mix Delivery
For projects under 1 cubic yard, bagged concrete is usually more practical. You can mix it as needed, work at your own pace, and avoid delivery minimums or scheduling constraints. The tradeoff is manual labor: mixing 45 bags of 80 lb concrete to get 1 cubic yard requires significant effort.
Ready-mix concrete delivered by truck becomes cost-effective at 2-3 cubic yards. You get consistent quality, faster placement, and less physical work. Most suppliers require a minimum order (often 1 yard) and charge extra for small loads. Schedule delivery for early morning to maximize working time before the concrete sets.
Pro Tip: For projects between 1-2 cubic yards, consider renting a mixer trailer from a concrete supplier. You get ready-mixed concrete in smaller quantities without the delivery minimums or the labor of mixing individual bags.
Concrete Curing and Weather Considerations
Concrete needs proper curing to reach full strength. Keep the surface moist for at least 7 days by covering with plastic sheeting or applying a curing compound. Avoid walking on fresh concrete for 24-48 hours and keep vehicles off for at least 7 days.
Temperature affects curing time significantly. In hot weather (above 85°F), concrete sets faster and may crack without proper hydration. In cold weather (below 50°F), curing slows dramatically, and concrete should not be poured if temperatures will drop below freezing within 24 hours. Plan your pour around favorable weather forecasts.
Related Calculators
Planning notes for concrete volume estimates
Use this calculator for slabs, footings, and pads where length, width, and thickness are known. Accuracy improves when units are consistent and waste is realistic.
If your pour has thickened edges or slopes, split it into smaller rectangles and add the volumes. For bag counts, round up and verify the bag yield printed on the product.
Quick checks
- Convert inches to feet before multiplying for volume.
- Add 5-10 percent waste for uneven forms or spillage.
- Confirm bag yield because brands can vary.
Example: A 12 ft x 18 ft slab at 4 in thick is 72 cu ft or 2.67 cu yd; with 10 percent waste plan about 2.9 cu yd.