How Much Concrete Do I Need?
From a shape and a measurement to an order quantity, step by step
The short answer
Work out the volume of the shape you're pouring, convert it to cubic yards (cubic feet ÷ 27), add a waste allowance on top of that exact figure, then round up to a quantity you can actually buy — a ready-mix increment or a whole number of bags.
Match the formula to the shape
The shape determines the formula. Measure in one consistent unit and calculate the exact volume — the amount of concrete the shape mathematically contains, with no allowance yet for anything that happens on pour day. Each shape has its own calculator:
- Rectangular slab or pad: length × width × depth — Concrete Calculator
- Round column or sonotube: π × radius² × height — Sonotube Concrete Calculator, Concrete Column Calculator or Concrete Pier Calculator
- Footing: length × width × depth of each rectangular part, added together — Concrete Footing Calculator; the footings guide covers strip, pad and stepped footings
- Post holes: hole volume minus the post's volume, times the number of posts — Concrete for Fence Posts Calculator; the fence post guide covers hole sizes and bags per post
- Steps: the stepped profile, made up of several blocks — Concrete Stairs Calculator
- Wall: length × height × thickness — Wall Concrete Calculator
Typical results by shape
To show how far the answer moves with the shape, the table puts each calculator's own worked example side by side. The exact figure is the geometry; the recommended figure adds the 10% default and rounds up to the next 0.01 yd³, as the calculators do. Your own pour will differ, so treat these as a sense of scale, not an order.
| Pour | Example | Exact | With 10% |
|---|---|---|---|
| Slab | 12 × 12 ft, 4 in thick | 1.78 yd³ | 1.96 yd³ |
| Strip footing | 60 ft long, 16 in wide, 8 in thick | 1.98 yd³ | 2.18 yd³ |
| Round columns | 4 columns, 12 in diameter, 9 ft high | 1.05 yd³ | 1.16 yd³ |
| Sonotubes | 4 tubes, 12 in diameter, 4 ft high | 0.47 yd³ | 0.52 yd³ |
| Fence posts | 6 posts, 10 in holes 24 in deep, 4 × 4 posts | 0.20 yd³ | 0.23 yd³ |
Convert to cubic yards
Ready-mix is ordered by the cubic yard in the US, and 1 cubic yard = 27 cubic feet, so divide a volume in cubic feet by 27. In metric, concrete is ordered by the cubic metre, and 1 yd³ ≈ 0.765 m³. Depth is the measurement most often entered in the wrong unit; How to Calculate Concrete Volume for a Slab covers the unit conversions and irregular slabs in more detail.
Add a waste allowance
Real pours lose material to an uneven subgrade, spillage, over-excavation, and formwork that isn't perfectly plumb. A 10% allowance is a common rule of thumb, added on top of the exact volume rather than baked into it silently. How to Calculate Concrete With Waste covers choosing the percentage, and Exact Quantity vs Recommended Order Quantity explains why both figures should stay visible.
Ready-mix or bags?
For bagged concrete, bags needed = required volume ÷ the yield of one bag. Yield varies by product and bag size, so use the yield printed on the bag you're buying. As one example, the table below gives the yields from Quikrete's data sheet for Concrete Mix 1101.
At 0.60 ft³ per 80 lb bag, one cubic yard (27 ft³) takes 27 ÷ 0.60 = 45 bags of that product. If you're mixing from separate cement, sand and gravel instead, the Cement Calculator splits a volume into each ingredient, and Concrete vs Cement: What's the Difference? explains what each bag contains.
| Bag size | Yield per bag |
|---|---|
| 40 lb | 0.30 ft³ |
| 50 lb | 0.375 ft³ |
| 60 lb | 0.45 ft³ |
| 80 lb | 0.60 ft³ |
| 90 lb | 0.675 ft³ |
Round to what you can actually order
Ready-mix is usually sold in fractions of a cubic yard, often ¼-yard increments, and some suppliers add a short-load or delivery fee to small orders. Increments, minimums and fees vary by supplier, so confirm them before you order. Bags are bought whole, so round the bag count up. To price an order including fixed fees, use the Concrete Cost Calculator.
Worked Example
A 12 ft × 12 ft slab, 4 inches thick, with a 10% rule-of-thumb waste allowance. The bag figures use the 0.60 ft³ yield of an 80 lb bag of Quikrete Concrete Mix 1101 — check the yield printed on the product you actually buy.
Volume: 12 × 12 × (4 ÷ 12) = 48 ft³
Cubic yards: 48 ÷ 27 = 1.78 yd³ exact
With 10%: 1.78 × 1.10 = 1.96 yd³
Ready-mix in ¼ yd³ increments: order 2.0 yd³
Bags (Quikrete 1101, 80 lb at 0.60 ft³): 48 ÷ 0.60 = 80 bags exact
Bags with 10%: 48 × 1.10 = 52.8 ft³ ÷ 0.60 = 88 bags
Assumptions & Limitations
- Waste-allowance ranges here are general estimating practice, not a fixed rule any supplier enforces.
- Short-load fees and order increments vary by region and supplier — confirm both before finalizing an order.
- Bag yields are product-specific; the figures shown are from Quikrete's Concrete Mix 1101 data sheet.
FAQ
- Should I always add the same waste percentage?
- No — a smooth, formed slab on graded ground can use less (5%), while a footing poured into a hand-dug, irregular trench often needs more (10%+). The BuildNumerics calculators let you change the percentage and always show the exact figure alongside it.
- Is it cheaper to under-order and get a second small load if I run short?
- Usually not. Another delivery can add its own fees and delays the pour while you wait for it, so it's generally better to round up to the next order increment the first time.
- How many 80 lb bags make a cubic yard?
- 45 bags for Quikrete Concrete Mix 1101, using its 0.60 ft³ yield per 80 lb bag (27 ÷ 0.60 = 45). Yield differs between products, so always check the yield printed on the bag you're buying.
Related Calculators
Related Guides
From the Blog
Sources
NIST SP 811
Unit conversion reference (cubic feet, cubic yards, cubic metres).
NIST SP 811 — referenceQuikrete Concrete Mix 1101 data sheet
Manufacturer yield per bag: 40 lb 0.30 ft³, 50 lb 0.375 ft³, 60 lb 0.45 ft³, 80 lb 0.60 ft³, 90 lb 0.675 ft³.
Quikrete Concrete Mix 1101 data sheet — reference