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The 4-inch mistake — why concrete orders go wrong
Quick answer: most concrete ordering disasters are unit errors, not math errors. Type thickness as "4" meaning 4 feet when you mean 4 inches, and a 20×20 patio's order inflates 12×— from about 5.4 cubic yards to 59. Here is the whole calculation, done once, slowly.
The patio, worked end to end
Cubic yards = cu ft ÷ 27
A 20 × 20 ft patio at 4 inches thick. Thickness in feet: 4 ÷ 12 = 0.333 ft. Volume: 20 × 20 × 0.333 = 133.3 cu ft. In yards: 133.3 ÷ 27 = 4.94 cubic yards. Add the 10% real-world allowance (uneven subgrade, form flex, spillage): 4.94 × 1.1 = 5.43 → order 5.5 yards of readymix.
That single number is why the unit trap matters so much: a readymix truck minimum plus a short-load fee on 59 yards is a four-figure mistake made in one form field.
Bagged concrete: the same patio by hand
An 80-lb bag yields about 0.60 cu ft. The unadjusted 133.3 cu ft ÷ 0.60 = 222 bags (223 with the waste factor, which is why the readymix truck starts looking cheap around one yard). Quick reference:
| Bag | Yield | Bags per cubic yard |
|---|---|---|
| 80 lb | 0.60 cu ft | 45 |
| 60 lb | 0.45 cu ft | 60 |
| 25 kg | ≈11.6 L | ≈87 |
Thickness decides everything
| Use | Thickness | 20×20 patio needs |
|---|---|---|
| Patio / walkway | 4 in | 4.94 yd |
| Car driveway | 4–5 in | 4.94–6.17 yd |
| Truck / RV pad | 5–6 in | 6.17–7.41 yd |
One inch of slab is 20 × 20 × (1/12) ÷ 27 = 1.23 cubic yards — about 55 bags' worth. Underspecifying thickness to "save" a yard is the most expensive saving in home construction, because thin slabs crack in exactly the places loads repeat.
Three more quiet killers
- No waste allowance: the subgrade is never perfectly flat. 5% on a well-prepared base, 10% on anything real. A pour that comes up one wheelbarrow short is a cold joint — a permanent weak line.
- Water for workability: a 0.5 water-cement ratio means ~25 L per 50-kg bag. Extra water makes the mix pour easy and cured concrete weak. Compact instead of souping.
- Ignoring the short-load fee: trucks batch efficiently above ~3 yards; smaller orders often carry a fee that makes two small pours cost more than one combined one. Consolidate.
FAQ
How much concrete do I need for a 20×20 patio?
At 4 inches: 133.3 cu ft ÷ 27 = 4.94 cubic yards, plus 10% waste → order about 5.5 yards, or 222 eighty-pound bags.
Why divide by 27?
A cubic yard is 3 ft × 3 ft × 3 ft = 27 cubic feet. Readymix is sold by the cubic yard, site math usually starts in feet — the ÷27 is the bridge between the two.
Is 4 inches thick enough?
For patios and walkways carrying people and garden furniture, yes. Cars want 4–5 inches with proper base prep; anything with trucks or RVs wants 5–6 inches plus rebar.
Should I order exactly what the math says?
No — order the math plus 5–10%. Running short mid-pour means a cold joint or an emergency second truck at short-load rates; the small surplus is the cheapest insurance on the job.
Check your own pour
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