Primary material
62 bags
80 lb concrete mix
Use the calculated bag count as your buying starting point, then verify yield and pallet limits with the store.
20 x 20 concrete slab
How much concrete for a 20x20 slab?
A 20 ft by 20 ft slab is 400 square feet. At 4 inches thick it needs about 5.43 cubic yards with 10% waste — a ready-mix order, not a bag job. Adjust thickness, waste, and price below, or see the 4/5/6 inch breakdown.
20 x 20 x 4 in
5.43 yd³
ready-mix order
Area
400 sq ft
20 ft x 20 ft
Material cost (4 in)
$815
ready-mix, material only
20x20 by thickness
Concrete for a 20x20 slab at 4, 5, and 6 inches
Thickness drives the volume. A patio pad can be 4 inches; a garage, shop, or vehicle pad usually steps up to 5 or 6 inches with reinforcement. All figures include the default 10% waste.
| Thickness | Cubic yards | 80 lb bags | 60 lb bags | Ready-mix material |
|---|---|---|---|---|
| 4 in | 5.43 yd³ | 245 | 326 | $815 |
| 5 in | 6.79 yd³ | 306 | 408 | $1,019 |
| 6 in | 8.15 yd³ | 367 | 489 | $1,222 |
Bag counts are shown for reference only — at this size the bag totals confirm it is a ready-mix pour.
Want installed cost?
The figures here are material only. For labor, base prep, forms, reinforcement, and finishing, the concrete cost calculator gives a full installed range.
Shopping list
Ready to price and order
Use the calculated quantities as a starting cart, then compare store availability, delivery fees, pallet limits, and local ready-mix minimums before buying.
Large pours
Ready-mix quote check
For high bag counts, compare the bagged route against local ready-mix minimums, delivery, access, and unload time.
Optional base
Gravel base
Base depth is project-specific. Estimate it separately before adding bags or bulk gravel to the order.
Optional forms
Forms and stakes
Plan lumber, stakes, screws, release, and bracing around the perimeter and any step-downs or curves.
Store buttons open retailer search results for now. Confirm exact product yield, pickup, delivery, and pallet rules before ordering.
Formula shown
How the 20x20 estimate works
1. Area is fixed at 400 sq ft
20 ft x 20 ft = 400 sq ft
2. Volume in cubic feet
400 sq ft x (thickness in / 12)
3. Yards and waste
yd³ = ft³ / 27, then add the waste allowance and round up to the load
Plan the pour
A 5+ yard pour needs the forms, base, and reinforcement ready and enough hands to place and finish before it sets. Confirm the ready-mix minimum, truck access, and whether you need a pump for a back-yard slab.
Sources
- Quikrete product yields — bagged concrete mix yield references
- Sakrete bagged concrete references — common bag sizes and concrete mix guidance
- ConcreteNetwork concrete price guide — ready-mix and installed concrete cost context
Reviewed for estimating accuracy
Last updated: June 29, 2026
Estimate only.
This calculator is not a quote. Verify product yield, price, delivery minimums, and local conditions with your supplier or contractor before buying materials.
FAQ
20x20 concrete slab questions
Cubic yards, ready-mix loads, thickness, cost, and reinforcement for a 20 ft by 20 ft slab.
How much concrete do I need for a 20x20 slab? + -
A 20 ft by 20 ft slab is 400 square feet. At 4 inches thick it needs about 5.43 cubic yards with the default 10% waste — a ready-mix order. At 5 inches it rises to 6.79 yd³ and at 6 inches to 8.15 yd³.
How many yards of concrete for a 20x20 garage slab? + -
About 5.43 cubic yards at 4 inches, 6.79 yd³ at 5 inches, and 8.15 yd³ at 6 inches, all including 10% waste. Many garage and shop slabs use 5 to 6 inches, especially where vehicles park.
Can I pour a 20x20 slab with bags? + -
It is about 245 bags of 80 lb mix at 4 inches — far too many to hand-mix in a single pour. A slab this size is a ready-mix job. Bags only make sense for a small patch or if a truck genuinely cannot reach the site.
How thick should a 20x20 slab be? + -
Four inches suits a patio or light shed floor. A garage, shop, or anything holding vehicles or heavy equipment usually steps up to 5 or 6 inches, often with rebar or wire and a proper compacted base. Confirm thickness and reinforcement for your load.
How much does a 20x20 concrete slab cost? + -
Material alone for a 4 inch 20x20 slab is roughly $815 in ready-mix at typical prices. Installed cost adds labor, base prep, forms, reinforcement, and finishing and varies by region — use the concrete cost calculator for a full installed range.