Concrete orders fail in two directions: short pours that force cold joints, and leftover bags that harden in the truck bed. Volume is simple math — L × W × thickness in feet, ÷ 27 for yards — then add waste and decide bags vs ready-mix.
The Volume Formula
Cubic feet = length (ft) × width (ft) × thickness (ft)
Cubic yards = cubic feet ÷ 27
Convert inches to feet before multiplying (4″ = 0.333 ft). For cylinders (piers/columns): π × r² × height. For footings, use the trench length × width × depth, not the slab footprint above.
Thickness = 4/12 = 0.333 ft. Volume = 10 × 12 × 0.333 = 40 ft³ ≈ 1.48 yd³. With 10% waste ≈ 1.63 yd³. That is roughly 74 bags of 80 lb mix — usually a ready-mix call.
Bags vs Ready-Mix
Bag yield from the calculator:
| Bag | Approx. yield | Bags per yd³ | Best for |
|---|---|---|---|
| 40 lb | 0.30 ft³ | ~90 | Patches, small repairs |
| 60 lb | 0.45 ft³ | ~60 | Posts, small pads |
| 80 lb | 0.60 ft³ | ~45 | Most DIY pours |
Above about 1–1.5 yd³, ready-mix is usually faster and more consistent than bagging and mixing by hand. Below that, bags win on convenience if you can stage water and a mixer.
Waste Factor
Add 5–10% for grade irregularities, form bow, spill, and pump/chute loss. Complex forms, rocky subgrade, or long chute runs may need more. Shorting a pour mid-slab creates a cold joint — overage is cheaper than a callback.
Shape Notes
- Slabs: Use finished thickness, not “nominal 4″” if you float thick spots.
- Footings: Measure trench depth to the bottom of the footing, not frost depth alone.
- Columns / piers: Use actual form diameter; cardboard tubes often run oversize.
- Stairs: Sum tread×riser volumes or treat as a stepped prism — do not use slab area alone.
Fence post holes are a different takeoff — cylinder minus the post, then bags. Size that job with the fence post spacing calculator and the spacing and concrete method. Whether to pour at all is a separate call: concrete vs gravel vs packed dirt.
Run the numbers for your shape
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