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 × 15 ft patio at 4 inches thick is 100 cubic feet (20 × 15 × 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.
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—concrete sets quickly, especially in warm weather.