How Many Bags for a 20x20 Slab? 245 80-lb Bags, 5.43 yd3

A 20x20 concrete slab is 400 square feet. At 4 inches thick with a 10% waste allowance, it needs about 5.43 yd3, equivalent to 245 80-lb bags, 326 60-lb bags, or 489 40-lb bags using the yields below. That is a multi-pallet handling job: compare ready-mix, mini-mix, or volumetric delivery before buying bags for a continuous pour.

Keep full precision from geometry through the allowance and price calculation; only display yards and currency rounded, and round bags up once at the end. Pallet quantities and product yields are planning assumptions, not the retailer's actual SKU or delivery promise.

Use the Concrete Bag Calculator to adjust thickness, product yield, waste, and bag price, then compare delivery in the Ready-Mix vs Bags Calculator. If this is a garage floor, use the garage slab guide for vapor barrier, apron, edge, and inspection scope.

Sources and checks

This page uses common planning yields and a simple flat-slab formula. Always confirm the printed yield on the exact bag and the written ready-mix quote before ordering.

Concrete Estimator Hub is independent and is not affiliated with these manufacturers, retailers, or ready-mix suppliers.

Thickness changes the bag order

These estimates use common bag yields:

Approximate whole bags for a 20 ft by 20 ft slab:

All values are rounded up to whole bags. Check the yield printed on the bag you plan to buy, especially if you use a fast-setting or specialty mix.

Formula for a 20x20 slab

Start with slab volume:

cubic feet =
  length ft x width ft
  x thickness inches / 12

For a 20 ft by 20 ft slab at 4 inches:

20 x 20 x 4 / 12 = 133.333333... ft3

Then divide by the bag yield:

bags needed = cubic feet / bag yield

Add waste before rounding up:

bags to buy =
  round up(
    bags needed x (1 + waste percentage)
  )

The example uses one 10% allowance. Re-measure uneven base and form depth, and estimate thickened edges separately; an allowance does not replace missing geometry or professional confirmation of the design.

Example: 20x20 slab at 4 inches

Volume:

20 x 20 x 4 / 12 = 133.333333... ft3

Using 40 lb bags:

(20 x 20 x 4 / 12) / 0.30 = 444.444444... bags
(20 x 20 x 4 / 12) x 1.10 / 0.30 = 488.888888... bags
round up to 489 bags

Using 60 lb bags:

(20 x 20 x 4 / 12) / 0.45 = 296.296296... bags
(20 x 20 x 4 / 12) x 1.10 / 0.45 = 325.925925... bags
round up to 326 bags

Using 80 lb bags:

(20 x 20 x 4 / 12) / 0.60 = 222.222222... bags
(20 x 20 x 4 / 12) x 1.10 / 0.60 = 244.444444... bags
round up to 245 bags

For one-bag square-foot coverage by thickness, use the 40, 60, and 80 lb concrete bag coverage chart. For smaller slab comparisons, see the Concrete Bags for a 16x16 Slab Guide and the Concrete Bags for an 18x18 Slab Guide.

Cost and delivery check for a 20x20 slab

Compare both material paths with hypothetical USD inputs, not local market rates. For this 4 in example, assume an 80-lb bag costs $6.50:

245 bags x $6.50 = $1,592.50

For ready-mix, assume $165 per yd3 and one $125 delivery or short-load fee. Use the original geometry, not the displayed 5.43 yd3, to calculate the price:

order yd3 = 20 x 20 x 4 / 12 x 1.10 / 27 = 5.432098765...
material USD = order yd3 x 165 = 896.296296...
material and delivery USD = material USD + 125 = 1021.296296...
display total: $1,021.30

This comparison excludes tax, supplier minimum billing, bag delivery, mixer rental, placement labor, and finishing. Replace both unit prices and fees with dated local quotes; the example does not establish which option is cheaper for your site.

Use the ready-mix vs bags break-even chart to see where delivered concrete starts beating bagged concrete by bag count. If you are comparing installed prices instead of supplier-only material, put each scope into the Concrete Contractor Bid Comparison so prep, reinforcement, access, cleanup, and payment terms are not hidden inside a single lump-sum number.

Pallet and labor reality check

At this size, the bag count becomes a logistics warning.

Those numbers do not include pallet weight, wrap, tools, mixer rental, water, crew time, cleanup, or the risk of inconsistent batches. Use the dry weights and pallet assumptions to plan delivery and staging, then confirm actual retailer pallet counts and the crew's placement capacity.

For pallet count assumptions, use How Many Bags of Concrete Are on a Pallet?.

How many cubic yards is a 20x20 slab?

A 20x20 slab at 4 inches is about 4.94 cubic yards before waste:

20 x 20 x 4 / 12 / 27 = 4.938271604... yd3
display before waste: 4.94 yd3

With 10% waste, that is about 5.43 cubic yards. That is the main reason a 20x20 slab should normally be priced as a ready-mix delivery before you consider buying bags.

Use the How Many Yards of Concrete Do I Need? guide for cubic-yard planning, then compare material and delivery cost in the Concrete Cost Calculator.

Is bagged concrete practical for a 20x20 slab?

Usually no. Even with 80 lb bags and a 10% waste factor, a 20x20 slab at 4 inches takes about 245 bags. That is roughly 19,600 lb of dry mix before water, not counting pallets, handling, mixing time, or the risk of cold joints while you place and finish the slab.

Bagged concrete may still be considered when truck access is impossible, when the slab is poured in controlled sections, or when delivery minimums make a small remote job unusually expensive. For a continuous slab, ready-mix is usually the cleaner planning path.

Use the Ready-Mix vs Bags Calculator before buying pallets of bagged concrete. If the slab is for a driveway, garage, foundation, or heavy load, get project-specific guidance rather than sizing the job from bag count alone. For bagged delivery planning, see How Many Bags of Concrete Are on a Pallet?.

Turn the 20x20 result into the next decision

For a 20x20 slab, the useful answer is rarely "245 bags" by itself. The useful answer is what to quote, what to compare, and what to keep as a fallback if truck access fails.

If a quote only says "20x20 slab" without thickness, waste, base prep, reinforcement, edge detail, access, delivery, and finishing scope, the number is not ready to approve yet. A slab this size is large enough that scope gaps cost more than small arithmetic errors.

Ready-mix-first quote workflow

Before buying bags for a 20x20 slab, collect a written ready-mix quote with the same assumptions:

1. Tell the supplier the slab is 20 ft by 20 ft at 4 inches, about 5.43 yd3 with 10% waste.
2. Ask whether the quote bills actual yards, minimum billable yards, or a short-load fee.
3. Confirm delivery, fuel, tax, wait time, chute reach, pump, buggy, and access charges.
4. Compare the delivered total with the 245-bag material price, pallet delivery, mixer rental, and labor.
5. Save the estimate or quote comparison before ordering so bag count, ready-mix yards, and scope assumptions stay together.

For small-order fee checks, use Concrete Short-Load Fee. For a line-item quote review, use the Concrete Contractor Bid Comparison.

Adjust for thickened edges and uneven base

The table assumes a flat slab with a consistent thickness. On a 20x20 slab, small depth errors become large material errors.

For thickness planning, see the Concrete Slab Thickness Guide. For cost comparison, use the Ready-Mix vs Bags Cost Guide.

40 lb vs 60 lb vs 80 lb bags

For a 20x20 slab, the bag size mainly changes how many bags you must move. It does not change the total volume of concrete needed.

40 lb bags are easier to lift, but the count becomes extremely high. See the 40 lb Concrete Bag Calculator if handling weight is the limiting factor.

60 lb bags reduce the count compared with 40 lb bags, but still require hundreds of bags for a 20x20 slab. See the 60 lb Concrete Bag Calculator for yield and coverage details.

80 lb bags usually give the lowest bag count for store-bought concrete mix. See the 80 lb Concrete Bag Calculator before choosing them for a large slab.

For thickness planning, see the Concrete Slab Thickness Guide.

Buying checklist

• Confirm the slab is actually 20 ft by 20 ft after forms are set.
• Confirm the planned thickness at several points.
• Check whether the slab has thickened edges, footings, or a sloped base.
• Add waste before rounding to whole bags or cubic yards.
• Compare ready-mix delivery before buying bagged concrete.
• Plan placement, finishing, joints, reinforcement, and help before the pour.

FAQ

How much concrete do I need for a 20x20 slab?

At 4 inches thick, a 20x20 slab needs about 133.33 cubic feet, or 4.94 cubic yards before waste. With a 10% waste factor, plan around 5.43 cubic yards.

How many bags of concrete do I need for a 20x20 slab?

At 4 inches thick with 10% waste, a 20x20 slab needs about 489 40 lb bags, 326 60 lb bags, or 245 80 lb bags. Without waste, the same slab is about 445 40 lb bags, 297 60 lb bags, or 223 80 lb bags.

How many 80 lb bags for a 20x20 slab at 4 inches?

A 20x20 slab at 4 inches needs about 223 bags of 80 lb concrete before waste or about 245 bags with a 10% waste factor.

How many 60 lb bags for a 20x20 slab at 4 inches?

A 20x20 slab at 4 inches needs about 297 bags of 60 lb concrete before waste or about 326 bags with a 10% waste factor.

How many 40 lb bags for a 20x20 slab at 4 inches?

A 20x20 slab at 4 inches needs about 445 bags of 40 lb concrete before waste or about 489 bags with a 10% waste factor.

How many cubic yards is a 20x20 slab at 4 inches?

A 20x20 slab at 4 inches is about 4.94 cubic yards before waste. With 10% waste, it is about 5.43 cubic yards.

Should I use bags or ready-mix for a 20x20 slab?

For a 20x20 slab, compare ready-mix first. Bagged concrete is usually too slow and labor-heavy for a continuous slab this size unless access or delivery constraints force a different plan.

Can I pour a 20x20 slab with bagged concrete?

Only in unusual situations, such as no truck access or a planned sectional pour. For a continuous slab, the bag count, mixing time, and finishing window make ready-mix the more practical first option.

How much does a 20x20 slab weigh in dry bags?

With 10% waste, the common 80 lb bag estimate is about 245 bags, or about 19,600 lb of dry mix before water. Confirm delivery and storage before buying.

Next step

Open the Ready-Mix vs Bags Calculator to compare delivery against bagged concrete. Then use the Concrete Cost Calculator to estimate the total material cost before buying. If your slab size may change, use the Concrete Bag Calculator to adjust dimensions, thickness, bag size, waste, and bag price.