How to Calculate How Much Concrete You Need
Whether you're pouring a driveway slab, a footing, or a set of steps, the math is the same: volume first, then bags. Here's how to do it by hand — and how to skip all of it with the Mix app.
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 don't include steps — Mix does.
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 — 1 part cement, 2 parts sand, 3 parts gravel by weight. Stronger mixes (like M25 or M30) use less water and more cement. Mix grades M10–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 (roughly M25) bagged mix is appropriate. For steps or structural elements, go M30 or higher.
Skip the math
The Mix – Concrete Calculator app for Android handles all five shapes (slab, round slab, footing, column, steps), lets you select a mix grade or enter a custom ratio, and shows you bag count, material breakdown, and estimated cost in one tap. It's free.