The yield number decides it, and it is printed on the bag.
By David Liu, founder of DIY9lives. Last updated November 2, 2026.
Concrete is sold two ways: by the bag at a hardware store and by the cubic yard from a plant. The bag is easier to buy and harder to carry, and the arithmetic between them is one multiplication. This guide works a slab through the same steps the concrete calculator uses, so you can price both routes before you decide which one your back is willing to support.
Volume is length times width times thickness, with every measurement in feet. The trap is thickness: it is usually quoted in inches because that is how slabs are discussed, and 4 inches is 0.333 feet, not 4.
A 10 by 12 foot patio at 4 inches thick is 10 × 12 × 0.333, which is 40 cubic feet. At 6 inches the same slab is 60 cubic feet, a 50% increase for two extra inches of concrete — which is why the thickness decision is the expensive one.
Add 10% for an uneven base and the bits you cannot recover from the wheelbarrow. Forty cubic feet becomes 44.
An 80 lb bag of concrete mix yields about 0.60 cubic feet, according to QUIKRETE's concrete-mix data sheet. The other common sizes scale with it:
| Bag size | Yield per bag | Bags for 44 cu ft |
|---|---|---|
| 40 lb | about 0.30 cu ft | 147 |
| 60 lb | about 0.45 cu ft | 98 |
| 80 lb | about 0.60 cu ft | 74 |
Seventy-four bags. That is nearly two tons of material, and it is the moment most people stop thinking of this as a weekend errand. The calculator prints both the bag count and the yardage so the comparison is in front of you before you start loading a cart.
A cubic yard is 27 cubic feet, so 44 cubic feet is 1.63 cubic yards. Ready-mix is sold in whole or partial yards, and most plants have a minimum order — often one yard, sometimes more — plus a short-load charge when you order less than a full truck. Ask about both before you compare prices, because the fee can swallow the saving.
A single cubic yard is roughly 4,000 pounds. That is the practical dividing line: below about a yard, bags are tiring but manageable in a day; above it, you are mixing for hours and the concrete in your first wheelbarrow is already setting while you mix the fifth batch. A truck also delivers consistent mix, which matters more than it sounds when you are finishing a single slab that has to look like one pour.
| Slab | Volume | 80 lb bags | Yards |
|---|---|---|---|
| 8 × 8 ft at 4 in | 21.3 cu ft + 10% | 40 | 0.87 |
| 10 × 10 ft at 4 in | 33.3 cu ft + 10% | 62 | 1.36 |
| 10 × 12 ft at 4 in | 44 cu ft | 74 | 1.63 |
| 12 × 16 ft at 6 in | 105.6 cu ft | 177 | 3.91 |
Anything in the last row is not a bag job. Look at the yardage, not the bag count: 3.91 yards is a truck, a couple of helpers, and a plan for where the truck can stand.
Concrete is strong in compression and weak in bending, which is why it needs support underneath. Four inches of compacted gravel under a patio slab is standard practice, and the base has to be dug out, graded and tamped before any concrete arrives. If you skip that, the slab's thickness was never the thing holding it up.
For anything that carries a vehicle, thickness goes up and reinforcement stops being optional. A driveway is typically 5 to 6 inches with rebar or mesh; a shed floor for a small shed may not need reinforcement at all. Thickness and reinforcement are design decisions, and a bag count cannot make them for you.
Mix to the consistency on the bag, not to what looks right. Adding water makes concrete easier to move and weaker when it sets; the bag states a quantity, often around three quarts per 80 lb bag, and the strength figures on the bag assume you did not add more.
Temperature matters more than most first-timers expect. Concrete that freezes before it cures is damaged permanently, and concrete poured in high heat sets fast enough that finishing becomes a race. If the forecast is near or below freezing, wait. If it is very hot, work early, shade the slab and keep the surface damp while it cures.
Curing is not drying. The slab wants to stay moist for several days to reach its rated strength, which is why covering it or misting it is worth the effort. Walking on it after 24 hours is normal; driving a car on it is a matter of days, not hours.
L-shaped patios and slabs with a cut-out for a step are the common case. Split them into rectangles, work out each one, and add the results. An L that is 10 by 12 with a 4 by 6 notch is 120 square feet minus 24, which is 96, and at 4 inches that is 32 cubic feet. Measuring the longest side of each arm and multiplying is the mistake that produces an order 20% too large.
Round slabs and pads are rare but not unknown. A circle's area is 3.14 times the radius squared, so a 10 foot diameter pad is 78.5 square feet. A post hole is a cylinder: 3.14 times the radius squared times the depth, which is why a 12 inch auger at 3 feet deep takes about 2 cubic feet per hole.
A wheelbarrow and a hoe will mix one or two bags at a time, which is fine for setting posts and hopeless for a patio. A small electric mixer handles two or three bags per batch and turns a two-day job into an afternoon. Beyond about half a yard, the mixing itself becomes the bottleneck rather than the concrete.
Mix only what you can place and finish before it starts to stiffen. Concrete does not wait, and a batch that sits in the barrow while you screed the previous one is a batch you will throw away. Working in small batches along a straight edge is slower and produces a better slab than racing a large one.
For the patio above, that is 74 bags or about 1.63 yards. Most people who have done both will tell you the same thing: the bags are cheaper on paper and more expensive on the day.
Last updated: November 2, 2026. Estimates only. Not tax, legal, financial, or professional advice. See the terms.