Rectangular slab
Multiply length × width × depth to get cubic inches, then convert:
- Divide by 1,728 to get cubic feet
- Divide by 27 to get cubic yards
Example: a 10 ft × 12 ft slab at 4 inches (0.33 ft) deep = 10 × 12 × 0.33 = 39.6 cubic feet = 1.47 cubic yards.
Round slab or column base
Use the cylinder formula: π × r² × depth, where r is the radius (half the diameter).
Example: a round pad 3 ft across and 6 inches deep. Radius = 1.5 ft, depth = 0.5 ft. Volume = 3.14159 × 1.5² × 0.5 = 3.53 cubic feet.
Footing
A footing is a rectangular slab, usually narrow and deep. Use the same length × width × depth formula. For a continuous footing, add up all the runs before multiplying.
Concrete steps
Steps are the most tedious to calculate by hand because each tread is a different layer. The standard approach: treat each step as a rectangular slab (tread depth × width × height of that riser and all risers below it), then sum the volumes. Most free concrete calculators skip steps entirely. Mix includes them.
How many bags do you need?
Once you have cubic feet, divide by the yield per bag:
- 40 lb bag → 0.30 ft³
- 60 lb bag → 0.45 ft³
- 80 lb bag → 0.60 ft³
Always add at least 10% for waste and overage. For a 1.5 yd³ pour with 80 lb bags: 1.5 × 27 = 40.5 ft³ ÷ 0.60 = 68 bags × 1.10 = 75 bags.
Concrete mix ratios
Standard residential concrete uses a 1:2:3 mix, meaning 1 part cement, 2 parts sand, 3 parts gravel by weight. Stronger mixes (M25, M30) use less water and more cement. Mix grades M10 to M50 follow Indian standard notation; the number is the compressive strength in MPa at 28 days.
For most DIY pours (slabs, footings, posts) a 4,000 PSI mix, roughly M25, is appropriate. For steps or structural elements, go M30 or higher.