Show the math
- Convert to feet (inches ÷ 12, centimeters ÷ 30.48): Length 20 ft; Width 20 ft; Thickness 4 in = 0.3333 ft
- Volume = 20 × 20 × 0.3333 × 1 = 133.3333 ft³
- Net volume = 133.3333 ft³ ÷ 27 = 4.9383 yd³ (× 0.0283168 = 3.7756 m³)
- Order quantity = 4.9383 yd³ × (1 + 10%) = 5.4321 yd³ = 146.6667 ft³
- 80 lb bags = 146.6667 ft³ ÷ 0.6 ft³ per bag = 244.44 → 245 bags (always round up)
- Weight = 146.6667 ft³ × 150 lb/ft³ = 22,000 lb
What this slab is used for
Twenty by twenty feet (400 ft²) is the smallest two-car garage slab, a large driveway section, a workshop floor or a pool deck. It is the size where the cost and logistics of a pour become a project, with a truck, a crew and a finishing plan rather than a weekend job with bags.
Four inches is the minimum for a garage floor, and 5 or 6 in is more typical where trucks or heavy tools are expected. Reinforce it with rebar or welded wire on chairs, and place a compacted stone base under it. A thickened edge is common around the perimeter.
Concrete for a 20×20 ft slab at each thickness
A 4 in slab is 5.43 yd³ with 10% waste. At 5 in it is 6.79 yd³ and at 6 in 8.15 yd³. All of them fit in a single truck, but the order falls just under the common short-load threshold of 7 yd³, so ask for the price of the extra half yard. The slab weighs 11.00 tons including waste, which a firm base needs to carry.
| Thickness | Net yd³ | Order yd³ | Order m³ | 80 lb bags | 60 lb bags | Weight |
|---|---|---|---|---|---|---|
| 3 in | 3.70 | 4.07 | 3.11 | 184 | 245 | 16,500 lb |
| 4 in | 4.94 | 5.43 | 4.15 | 245 | 326 | 22,000 lb |
| 5 in | 6.17 | 6.79 | 5.19 | 306 | 408 | 27,500 lb |
| 6 in | 7.41 | 8.15 | 6.23 | 367 | 489 | 33,000 lb |
The formula is 20 × 20 × thickness ÷ 12 ÷ 27 for the net volume. The order adds 10% for waste and rounds the bag counts up. Use the calculator above for another thickness or waste allowance.
Bags or ready-mix for 20×20 ft?
Bags: 245 bags of 80 lb mix at 4 in, a total of 19,600 lb of dry product. Ready-mix: 5.43 yd³, material about $729–$1,459 at the published range, last verified 2026-10-04, before delivery and short-load fees. Bagged concrete for this slab costs about $1,348–$1,960. Because the order is above two cubic yards, a truck is usually the practical choice. See bagged vs ready-mix.
What to watch for
- No joint plan. Cut joints at 10 ft in each direction for a 4 in slab, which gives four 10×10 ft panels.
- Placing from one truck position only. Plan how the chute reaches every corner or book a pump.
- Leaving no time for finishing. A slab this size sets in sections, so bring a crew.
- Forgetting the apron, the step at the door or the thickened edge.
Layout and joints
A 20×20 ft slab is large enough for the six-step order of work in the slab guide to matter: base, forms, steel, pour, finish and cure.
For a 4 in slab, joint spacing of 8 to 12 ft is the usual range, with a 15 ft maximum for any thickness. Read the control joints guide for depth and timing.
Before you order
- Measure the forms, not the excavation, and check the thickness in several places.
- Add the volume of any thickened edge, apron or step separately.
- Plan the base: use the gravel base calculator to estimate stone for 400 ft².
- If the design calls for steel, estimate it with the rebar calculator.
- Put the quantity on the Pour List to combine it with footings and steps.
Frequently asked questions
How many yards of concrete for a 20×20 slab?
4.94 yd³ net at 4 in, 5.43 yd³ with 10% waste.
How many bags of concrete for a 20×20 slab?
245 bags of 80 lb mix or 326 bags of 60 lb mix at 4 in with 10% waste.
How much does a 20×20 slab weigh?
About 22,000 lb (11.00 US tons) at 4 in with 10% waste, using 150 lb/ft³.
How many yards for a 20×20 slab at 6 in?
8.15 yd³ with 10% waste.
How much does a 20×20 concrete slab cost in materials?
About $729–$1,459 for ready-mix concrete at 4 in, before delivery fees, based on the ranges on the cost page (verified 2026-10-04).
Sources
Estimate only. Results depend on the numbers you enter, ground conditions and your supplier. For structural work consult a licensed engineer and follow local codes. Brand names such as Quikrete and Sakrete are used only to cite published product data. This site is not affiliated with any brand.