18x18 Concrete Slab Bags: 198 80-lb Bags or 4.40 yd3

An 18x18 concrete slab is 324 square feet. At 4 inches thick, plan on about 360 bags of 40 lb concrete, 240 bags of 60 lb concrete, or 180 bags of 80 lb concrete before waste. With 10% waste, that becomes about 396 bags of 40 lb, 264 bags of 60 lb, or 198 bags of 80 lb concrete.

At a glance:

• 18x18 slab area: 324 sq ft.
• 4 inch concrete volume: 4.00 yd3 before waste.
• With 10% waste: about 4.40 yd3.
• 80 lb bags with 10% waste: about 198 bags.
• 60 lb bags with 10% waste: about 264 bags.
• 40 lb bags with 10% waste: about 396 bags.

Quick answer: an 18 ft by 18 ft slab at 4 inches thick is usually a ready-mix quote signal, not a normal bagged-concrete shopping trip.

At the common 4 inch slab thickness, the most searched quick answer is: buy about 198 80 lb bags, 264 60 lb bags, or 396 40 lb bags after adding a 10% waste factor. If you are asking how many bags of concrete for an 18x18 slab, also check delivery cost because the dry bagged mix is about 15,840 lb.

18x18 pickup and ready-mix checkpoint

For most 18x18 slabs, use bag counts as a reality check before pricing ready-mix. A slab this size is usually a delivery job unless access, delivery minimums, or project staging make bagged concrete the only practical route.

If you plan to haul bagged concrete, use the pallet count guide before you load a pickup. Then turn the result into a material shopping list or compare it against a delivered truck quote in the Ready-Mix vs Bags Calculator.

Use the Ready-Mix vs Bags Calculator to compare delivery against bagged concrete. Use the Concrete Bag Calculator when you want to adjust thickness, bag size, waste factor, and bag price.

Sources and checks

This page uses common bag yields for planning, not a universal product rule. Always confirm the yield printed on the bag before buying.

18x18 slab decision in one screen

At 18x18, the useful answer is no longer just "how many bags." It is whether you should stop thinking like a bag buyer and start thinking like a delivery and quote-review customer.

Turn the 18x18 result into the next decision

If an 18x18 slab still looks like a bagged-concrete job after this check, make that decision with transport, mixer time, cold-joint risk, and written scope in front of you.

Answer first

• About 198 80-lb bags with 10% waste.
• About 264 60-lb bags with 10% waste.
• About 396 40-lb bags with 10% waste.
• About 4.40 yd3 ready-mix.
• Use ready-mix vs bags before buying pallets.

18x18 slab bag calculator quick answer

Use this table when the slab is 18 ft by 18 ft and you need the 4 inch answer fast.

Citation-ready answer: an 18x18 concrete slab at 4 inches thick needs about 4.00 yd3 before waste or 4.40 yd3 with 10% waste. That is about 198 80 lb bags, 264 60 lb bags, or 396 40 lb bags when using common bag yields.

For a continuous 18x18 slab, get a ready-mix price before buying bags. The bag count is useful as a cost and labor comparison, but the job is large enough that mixing speed, finish timing, and material staging become major risks.

Quick answer for an 18x18 slab

These estimates use common bag yields:

Approximate whole bags for an 18 ft by 18 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 an 18x18 slab

Start with slab volume:

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

For an 18 ft by 18 ft slab at 4 inches:

18 x 18 x 4 / 12 = 108 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)
  )

For a slab this size, 10% waste is a practical planning number. Use more if the base is uneven, the form depth varies, the slab has thickened edges, or the shape is not a simple square.

Example: 18x18 slab at 4 inches

Volume:

18 x 18 x 4 / 12 = 108 ft3

Using 40 lb bags:

108 / 0.30 = 360 bags
360 x 1.10 = 396
round up to 396 bags

Using 60 lb bags:

108 / 0.45 = 240 bags
240 x 1.10 = 264
round up to 264 bags

Using 80 lb bags:

108 / 0.60 = 180 bags
180 x 1.10 = 198
round up to 198 bags

For one-bag square-foot coverage by thickness, use the 40, 60, and 80 lb concrete bag coverage chart. For a smaller large-slab comparison, see the Concrete Bags for a 16x16 Slab Guide. For a larger ready-mix comparison, see the Concrete Bags for a 20x20 Slab Guide.

Cost and delivery check for an 18x18 slab

An 18x18 slab is large enough that bagged concrete should be priced against ready-mix before anything is purchased. Using 80 lb bags at 4 inches thick with 10% waste:

198 bags x $6.50 = $1,287.00

Using the same 4.40 yd3 planning volume with a $165 per yd3 ready-mix price and a $125 delivery or short-load fee:

4.40 x $165 = $726.00
$726.00 + $125 = $851.00

Local prices, delivery minimums, truck access, and weekend fees can change the answer, but this example shows why an 18x18 slab should get a ready-mix quote before you commit to bagged concrete.

That is usually pallet-scale material. Plan unloading, staging, water, mixer capacity, placement help, and finishing time before choosing bags.

How many cubic yards is an 18x18 slab?

An 18x18 slab at 4 inches is exactly 4.00 cubic yards before waste:

108 / 27 = 4.00 yd3

With 10% waste, that is about 4.40 cubic yards. That is why an 18x18 slab should usually be priced as ready-mix before you commit to bagged concrete.

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 an 18x18 slab?

Usually no for a continuous slab. Even with 80 lb bags and a 10% waste factor, an 18x18 slab at 4 inches takes about 198 bags. That is roughly 15,840 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 remote job unusually expensive. For a continuous patio, garage pad, driveway section, or shed base, 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?.

Adjust for thickened edges and uneven base

The table assumes a flat 18 ft by 18 ft slab with the same thickness across the whole area. Site conditions can add material quickly at this size.

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 an 18x18 slab, the bag size mainly changes how many bags you must lift, move, open, and mix. 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 many mixing cycles for an 18x18 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 18 ft by 18 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 an 18x18 slab?

At 4 inches thick, an 18x18 slab needs 108 cubic feet, or exactly 4.00 cubic yards before waste. With a 10% waste factor, plan around 4.40 cubic yards.

How many bags of concrete do I need for an 18x18 slab?

At 4 inches thick with 10% waste, an 18x18 slab needs about 396 40 lb bags, 264 60 lb bags, or 198 80 lb bags. Without waste, the same slab is about 360 40 lb bags, 240 60 lb bags, or 180 80 lb bags.

How many 80 lb bags for an 18x18 slab at 4 inches?

An 18x18 slab at 4 inches needs about 180 bags of 80 lb concrete before waste or about 198 bags with a 10% waste factor.

How many 60 lb bags for an 18x18 slab at 4 inches?

An 18x18 slab at 4 inches needs about 240 bags of 60 lb concrete before waste or about 264 bags with a 10% waste factor.

How many 40 lb bags for an 18x18 slab at 4 inches?

An 18x18 slab at 4 inches needs about 360 bags of 40 lb concrete before waste or about 396 bags with a 10% waste factor.

How many cubic yards is an 18x18 slab at 4 inches?

An 18x18 slab at 4 inches is exactly 4.00 cubic yards before waste. With 10% waste, it is about 4.40 cubic yards.

Should I use bags or ready-mix for an 18x18 slab?

For an 18x18 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 an 18x18 slab with bagged concrete?

You can, but it is rarely the easiest path for a continuous slab. At 4 inches thick with 10% waste, even the 80 lb option is about 198 bags, so mixing speed, crew size, cold joints, and finish timing become major risks.

How much does an 18x18 slab weigh in dry bags?

At 4 inches thick with 10% waste, the dry bagged mix is about 15,840 lb whether you use 396 40 lb bags, 264 60 lb bags, or 198 80 lb bags.

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.