Concrete Calculator (Slabs, Columns & Bags)
Slab, round-column, and wall volumes with a waste margin, converted to cubic yards, cubic feet, and the exact bag count for 80 lb and 60 lb bags.
Concrete — interactive calculator, computed instantly in your browser.
How Much Concrete Do You Need? Yards, Bags, and Waste Margins
12 ft × 20 ft × 4 in = 80 ft³ = 2.96 yd³ (+10% = 3.26)
Ordering concrete is volume math with a delivery window attached: too little and the pour stops mid-slab (a full re-pour seam is a cold joint), too much and you pay for a truck's minimum load. This calculator handles slabs, round columns, and walls in feet or meters, adds a waste margin, and converts the result into cubic yards, cubic feet, and ready-to-carry bag counts.
1. Volume by Shape
- Slab: 12 ft × 20 ft × 4 in (0.333 ft) = 80 ft³
- Round column: 12 in diameter (r = 0.5 ft), 8 ft high → 3.14 × 0.25 × 8 = 6.28 ft³ each
- Wall: length × height × thickness — same product, different orientation
- Thickness converts first: inches ÷ 12, centimeters ÷ 30.48, before multiplying
2. Cubic Feet to Cubic Yards
- That 80 ft³ slab = 2.96 yd³ — concrete is sold by the cubic yard in the US
- Metric orders use m³ (1 yd³ ≈ 0.7646 m³); the calculator shows both units side by side
- Mixed units are the classic job-site error: a 4-inch slab over 100 m² is 9.26 m³, not 92.6
- Order the calculated number rounded up to the next quarter yard — trucks batch in increments
3. Bags vs Ready-Mix
- One cubic yard = 45 bags of 80 lb (each yields ≈ 0.6 ft³) or 60 bags of 60 lb (≈ 0.45 ft³)
- Ready-mix beats bags above ~1 yd³ on price, but suppliers enforce minimum loads plus per-mile charges
- Bags fit patios, footings, and repairs where a truck cannot reach — buy 10% extra regardless
- Mix small batches only as fast as you can place them; partial bags set up cold and crack later
4. The Waste Margin
Add 5–10% to the raw volume: subgrades are never perfectly level, forms flex, and some mix stays in the chute. The calculator builds the margin in as a field — 10% is the default for uneven ground, 5% for a formed pad on compacted base. Under-ordering by even a few percent forces a second delivery, and the seam between pours (the cold joint) becomes the slab's weakest line.
🔗 Project Math Cross-References
Concrete is volume math with a delivery window — the same unit conversions run through the 🔄 Conversion Hub tools.
Order by the cubic yard, measure by the square foot: pair this with the room-area tools below when the pour sits under a floor you still have to cover.
Frequently Asked Questions (Concrete)
One cubic yard equals 27 cubic feet: 45 bags of 80 lb (each yields about 0.6 ft³) or 60 bags of 60 lb (about 0.45 ft³). Buy 5–10% extra for spillage and uneven ground — the calculator prints the exact bag count for your volume.
12 × 20 × (4 ÷ 12) = 80 cubic feet, which is 80 ÷ 27 = 2.96 cubic yards. With a 10% waste margin, order 3.26 yards — practically, suppliers round up to the next quarter yard, so plan on 3.25 to 3.5.
No — order 5–10% extra. Subgrade unevenness, form flex, and chute residue all consume volume, and a short pour forces a second delivery that creates a weak cold joint at the seam. It is cheaper to return a partial load than to demolish and re-pour.
Standard residential driveways use 4–6 inches of 4,000 psi mix (6 inches for heavy trucks or poor base), patios 4 inches on compacted gravel, and walkways 3–4 inches. Footings follow load and frost depth — the calculator converts any thickness you enter into volume.