Concrete Calculator
Yards and 80-lb bag count for any slab, footing, or column
What is concrete calculator?
A concrete calculator does two jobs: tell you how many cubic yards your project needs, and translate that into the bag count at the store. The volume math is simple — area × thickness for slabs, π·r²·h for columns — but bag yields are empirical, not derived from cured-slab density.
Why two density numbers? Cured concrete is 150 lb/ft³. That’s the right number for “how heavy is my finished slab.” But a single 80-lb bag of concrete mix doesn’t yield 80/150 = 0.53 ft³. The actual yield is 0.60 ft³ because wet mix incorporates water and air voids that compact during cure. The Quikrete and Sakrete High-Strength TDS both publish 0.60 ft³ per 80-lb bag — identical, despite the brand-loyalty wars.
This calculator uses bag yield directly when you select a bag size, and the cured-150 figure for the total-weight readout. Both numbers are correct for their respective use.
Slab thickness rules of thumb: 4 in for patios, walkways, and shed pads; 6 in for driveways and garages; 8+ in for parking pads or under heavy machinery. Always pour over at least 4 in of compacted gravel — the calculator’s gravel-calculator sibling sizes that for you.
Bags vs ready-mix: at about 1 cubic yard (~50 bags), ready-mix becomes cheaper than bags on a per-yard basis, and it’s vastly faster. Most ready-mix companies have a 1-yard minimum and a short-load fee for anything under 3 yards. For a 10×10 patio at 4 in (about 1.5 yd³), you’re squarely in the “call ready-mix” zone.
When to use a concrete calculator
- Backyard patio slab — 10 ft × 12 ft × 4 in is the textbook backyard pour. The slab tab handles it directly; the result tells you bags or whether to call ready-mix.
- Deck or fence post footing — Switch to the Footing tab for a single rectangular footing, or use Column for a sonotube. Multiply the result by your post count.
- Bag vs ready-mix decision — Past about 1 cubic yard (50+ 80-lb bags), ready-mix delivery saves time and money. Below that, mixing in a wheelbarrow or mixer is cheaper.
How to use the Concrete Calculator
- Pick a shape preset — Slab/Pad for flat pours, Footing for rectangular footings, Column for sonotubes/piers, Wall for vertical pours, Stairs for staircases.
- Enter dimensions — Slab thickness for residential is 4 in (driveway 6 in, garage 6 in). Footing depth is set by frost line — typically 36-48 in in cold climates.
- Pick your bag size — 80-lb bags are the standard at most yards (0.6 ft³ per bag). 60-lb (0.45 ft³) and 40-lb (0.3 ft³) are easier to lift but cost more per yard.
- Decide bags vs ready-mix — Past 1 cubic yard, ready-mix delivery beats bags on price, time, and consistency. Below that, bags are usually fine.
Worked examples
10×10 ft × 4 in slab
Input: Slab tab: 10 ft × 10 ft × 4 in
Output: 1.36 cu yd / 62 of 80-lb bags (with 10% waste)12 in × 4 ft sonotube column
Input: Column tab: 12 in dia × 4 ft height
Output: 0.13 cu yd / 6 of 80-lb bags20 ft × 8 ft × 8 in wall
Input: Wall tab: 20 ft × 8 ft × 8 in
Output: 4.34 cu yd — ready-mix territory