How Much Concrete Do I Need for a 20x20 Slab?
A 20x20 slab takes 4.94 yd³ of concrete at 4 in thick, or 5.43 yd³ to order once a waste allowance of 10%[1] is added; at 6 in thick the order is 8.15 yd³.
Estimates for planning, not professional advice. Read the full notice.
Work it out for your own sizes
Work out the yards for your own slabExample to try: Length 20 ft, Width 20 ft, Thickness 4 in
Yards of concrete for a 20x20 slab, by thickness
Cubic yards for a 20x20 slab at three thicknesses: the slab itself, the amount to order, truck loads and bags
| Slab thickness | Concrete in a 20 ft × 20 ft slab | To order, with the waste allowance | Truck loads at the smallest truck (8 yd³) | Truck loads at the largest truck (12 yd³) | 80 lb bags for the same order |
|---|---|---|---|---|---|
| 4 in | 4.94 yd³ | 5.43 yd³ | 1 load | 1 load | 245 × 80 lb bags |
| 5 in | 6.17 yd³ | 6.79 yd³ | 1 load | 1 load | 306 × 80 lb bags |
| 6 in | 7.41 yd³ | 8.15 yd³ | 2 loads | 1 load | 367 × 80 lb bags |
Estimate for planning, from what you entered and the product data shown. Check product labels and your supplier before you buy.
The order adds the waste allowance once, on the total. Truck loads and bags are each rounded up once, from that order.
The same pour in bags, by bag size
Bags to buy and their total weight for this slab at the first thickness, by bag size, with the waste allowance added
| Bag size | Bags to buy | Weight of the bags | Yield per bag |
|---|---|---|---|
| 40 lb | 489 × 40 lb bags | 9.78 tons | approx. 0.3 ft³ |
| 50 lb | 392 × 50 lb bags | 9.8 tons | approx. 0.375 ft³ |
| 60 lb | 326 × 60 lb bags | 9.78 tons | approx. 0.45 ft³ |
| 80 lb | 245 × 80 lb bags | 9.8 tons | approx. 0.6 ft³ |
| 90 lb | 218 × 90 lb bags | 9.81 tons | approx. 0.675 ft³ |
Figures in the Yield per bag column are per QUIKRETE's data sheet, checked Nov 12, 2026.
Product names are trademarks of their owners. HoneyDoMath is not affiliated with, sponsored by or endorsed by these makers. Figures come from each maker's published sheets and are shown for information only.
Source: [2]
Each count is rounded up once, to whole bags. The weight is the dry mix alone, before water.
Bags or a truck at this size
This footprint sits where a bag job turns into a truck job. At 4 in thick the order is 5.43 yd³, which is 1 load counted at the smallest truck, 8 yd³; truck mixers carry 8 yd³[1] to 12 yd³[1], and the table shows the largest beside it, so the count never under-buys. At 6 in the order grows to 8.15 yd³, or 2 loads at the smallest truck and 1 load at the largest.
Poured from bags, the 4 in slab is 245 × 80 lb bags: 9.8 tons of dry mix to carry, open and mix. The second table gives the count and the weight for every bag size.
A patio and a two-car parking pad can share this footprint and differ in thickness. Each added inch over this area is another 1.23 yd³ before waste, so the thickness changes the number you give the plant.
The number to give the ready-mix plant
A plant takes the order in cubic yards. The first column of the table is the slab exactly as measured; the second adds 10%[1], the top of the range the ready-mix industry body gives, which runs from 4%[1]. An uneven subgrade or forms that bow outward use up that margin.
Quantity of concrete ordered should be 4% to 10% more than an estimate calculated from the plan dimensions to account for contingencies.
The capacity of truck mixers varies between 8 to 12 cubic yards (5 to 9 m3).
For other footprints, the common slab sizes table lists a dozen sizes. The slab calculator takes a slab that is not a plain rectangle.
Questions
How many yards of concrete do I need for a 20x20 slab at four inches thick?
The first row of the table gives it twice: the cubic yards in the slab itself, and the cubic yards to order with the waste allowance added. The rows below do the same for thicker slabs.
How many bags of concrete is that?
The last column of the first table counts the bags for each thickness, and the second table counts them for every bag size, with the total weight of the bags beside each count.
Do the yards include waste?
The first column does not; the order column does. The yards calculator adds the same allowance by default and lets you change it.
How many concrete trucks does a 20x20 slab take?
The truck loads column answers it for each thickness, counted at the smallest truck capacity the ready-mix industry body gives, with the largest beside it. A plant's own trucks may carry more or less than the capacities used in the count.
Calculate it for your project
Enter your own sizes to get the amount to buy or the layout figures, with the working shown and a page to print.
Check your sizes in the yards calculatorSources (2)
-
CIP 31, Ordering Ready Mixed Concrete (opens in new tab), National Ready Mixed Concrete Association, 2000, 2016, 2023.
Standards or industry body.
“Quantity of concrete ordered should be 4% to 10% more than an estimate calculated from the plan dimensions to account for contingencies.”
“The capacity of truck mixers varies between 8 to 12 cubic yards (5 to 9 m3).”
Checked November 12, 2026. Figures and codes change; verify with the source. Back to where this is used (Source 1) -
QUIKRETE Concrete Mix No. 1101 data sheet (opens in new tab), The Quikrete Companies, Revised October 2022.
Manufacturer data.
“A 40 lb (18.1 kg) bag yields approximately 0.30 ft3 (8.5 L)”
“A 50 lb (22.6 kg) bag yields approximately 0.375 ft3 (10.6 L)”
“A 60 lb (27.2 kg) bag yields approximately 0.45 ft3 (12.7 L)”
“An 80 lb (36.2 kg) bag yields approximately 0.60 ft3 (17 L)”
“A 90 lb (40.8 kg) bag yields approximately 0.675 ft3 (19.1 L)”
Per QUIKRETE's data sheet, checked Nov 12, 2026. Figures and codes change; verify with the source. Back to where this is used (Source 2)
These are summaries of what the sources say, not professional advice. Codes and product data change. Check with your building department and the product label about your own project.