A 10 ft × 10 ft patio at 4 inches thick needs about 56 bags of 80 lb mix before waste — closer to 62 bags if you add the usual 10%.
That is the number that empties a pickup short. You multiply length × width, forget thickness is in inches, and load forty bags. The pour still needs the full 33.3 cubic feet. Forty bags of 80 lb mix only give you about 24 cubic feet. The mixer sits empty before the last corner is struck off.
The volume formula is:
ft³ = length (ft) × width (ft) × (thickness in ÷ 12)
Then bags = ceil(ft³ ÷ bag yield). Never round down. Add 5–10% for uneven subgrade and spillage. Estimate only — not engineering or building-code advice.
| Slab | Thickness | yd³ | 60 lb | 80 lb |
|---|---|---|---|---|
| 8 × 8 ft | 4 in | 0.79 | 48 | 36 |
| 10 × 10 ft | 4 in | 1.23 | 74 | 56 |
| 12 × 12 ft | 4 in | 1.78 | 107 | 80 |
| 10 × 10 ft | 6 in | 1.85 | 111 | 84 |
| 20 × 20 ft | 4 in | 4.94 | 296 | 223 |
How many bags for a 10×10 slab?
For a 10×10 at 4 inches: 10 × 10 × (4 ÷ 12) = 33.33 ft³ ≈ 1.23 cubic yards.
Divide by bag yield. An 80 lb bag commonly yields about 0.60 ft³, so 33.33 ÷ 0.60 ≈ 55.6 → 56 bags. A 60 lb bag at about 0.45 ft³ needs 74 bags. With 10% waste you are looking at roughly 62 of the 80 lb bags or 82 of the 60 lb bags.
At 6 inches thick the same footprint is 50 ft³ ≈ 1.85 yd³ — about 84 bags of 80 lb before waste. Thickness is the multiplier people forget when they “just grab a pallet.”
The formula, on a tape
Measure length and width in feet. Convert thickness from inches to feet by dividing by 12. Multiply all three. That is cubic feet. Divide by 27 for cubic yards.
Bags = cubic feet ÷ declared yield on the bag, then round up. Typical US yields:
- 40 lb ≈ 0.30 ft³ (~90 bags per yd³)
- 60 lb ≈ 0.45 ft³ (~60 bags per yd³)
- 80 lb ≈ 0.60 ft³ (~45 bags per yd³)
UK, Australia, and New Zealand sell by kilograms — often 20 kg or 25 kg bags. Yield still comes from the label, not a US bag chart. In Concrete Mix Master pick the region first; bag sizes follow that choice.
Check the bag. Yields vary by product. Always use the number printed on the bag you will buy.
Three mistakes that short the order
Leaving thickness in inches. 10 × 10 × 4 looks like 400. It is not 400 cubic feet. Four inches is 0.333 ft. Skip the ÷ 12 and you order an absurd load — or you “fix” it by guessing and still come up short.
Rounding bags down. 55.6 becomes 55 on a calculator. The last corner still needs concrete. Always ceil to a whole bag, then add waste. Running out mid-pour means a cold joint and a second trip while the mixer sits.
Mixing bags when you should order yards. Past about one cubic yard (roughly a 10×10 at 4 in), bag-by-bag mixing is slow and inconsistent. Ready-mix short loads often win on labour even when the ticket looks higher. Use bags for pads, post holes, and small repairs. Use a truck for driveways and big slabs.
How many 80 lb bags make a cubic yard?
About 45 bags of 80 lb mix at 0.60 ft³ each (27 ÷ 0.60). Sixty-pound bags take about 60 per yard. Forty-pound bags take about 90.
That is why a 4.94 yd³ pour (20×20 at 4 in) is over 220 bags of 80 lb before waste. That is not a bag job. Call the plant.
Post holes and piers
A cylinder uses π × (diameter ÷ 2)² × depth. Count every hole. A fence line with 14 posts is 14 cylinders, not one average. Gate posts and corners often take a larger diameter and more bags than a line post.
For the post count itself, see how many fence posts do I need, then run each hole through Concrete Mix Master. Sibling tools on the EBN Build Calculators hub cover fence takeoffs, rebar, and the rest of the jobsite math.
These counts are estimates only — not engineering or building-code advice. Subgrade, slump, brand yield, frost depth, and local rules change what you should pour. Measure twice. Buy a spare bag. A leftover bag is cheaper than a cold joint.
Open Concrete Mix MasterEstimate only — not engineering or building-code advice.