How do I calculate concrete for an irregular-shaped slab?

Break the irregular shape into rectangles, triangles, or circles. Calculate each section separately, add the volumes together, then add 10–15% for waste. For complex L-shapes or curved patios, always add at least 15% waste to account for the additional waste when cutting or forming edges.

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Concrete Slab Calculator (2026) – Yards, Cost & Rebar

Q: How many yards of concrete for a 20×20 slab?

A 20×20 slab at 4 inches thick = 133.3 ft³ / 27 = 4.94 yd³ net. With 10% waste = 5.43 yd³ to order. At $160/yd³ (Midwest average), material cost is roughly $869.

Q: How much does a concrete slab cost?

A concrete slab costs $4–$8 per square foot installed in the Midwest and South, and $6–$12 per square foot in the Northeast and West Coast. A 10×10 patio slab typically costs $400–$1,200 total installed. Material alone is usually $1–$3 per sqft; labor and finishing account for the rest.

Q: How long does a concrete slab take to cure?

A concrete slab is typically walkable after 24–48 hours and reaches 70% strength at 7 days. Full design strength is achieved at 28 days. Avoid parking vehicles on a new slab for at least 7 days (14 days in cold weather).

Q: What PSI concrete should I use for a driveway slab?

Use a minimum of 4,000 PSI concrete for residential driveways. In cold climates with freeze-thaw cycles, specify 4,500 PSI with air entrainment (4–7% air). Air-entrained concrete resists surface scaling caused by de-icing salts far better than standard mixes.

Q: What is the difference between concrete and cement?

Cement is one ingredient in concrete. Concrete = cement + sand + aggregate (gravel) + water. Portland cement acts as the binder that holds the mix together. You cannot pour a slab from cement alone — it must be mixed into concrete first.

Q: How much does a 20×20 concrete slab cost installed?

A 20×20 concrete slab (400 sqft) at 4 inches thick costs $1,600–$3,200 installed in the Midwest and South, and $2,400–$4,800 in the Northeast and West Coast. The range depends on labor rates, site prep, rebar, and finishing requirements.

Calculate concrete for any slab, pad, patio, driveway, or garage floor. Get cubic yards, bag counts, 2026 cost by US region, and rebar quantity.

Length (ft)

Width (ft)

Thickness

Unit

Price ($/yd³)

US avg $140–$225 in 2026

US Region

Waste Factor (%)

Rebar Spacing (in)

Slab Results

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0.00 yd³ Cubic Yards (with waste)

—Square Feet

—Est. Material Cost

—60 lb Bags Needed

—80 lb Bags Needed

Full Breakdown

Net volume—

Waste addition—

Rebar needed—

Region adj. cost—

→ Order this amount—

*Material cost only. Labor adds $2–$5/sqft. Always get 3 contractor quotes.

Common Concrete Slab Sizes — Quick Reference

Project	Common Size	Thickness	Cubic Yards	Est. 80 lb Bags
Small patio	10×10 ft	4 in	1.23 yd³	56 bags
Standard patio	12×16 ft	4 in	2.37 yd³	107 bags
Large patio	20×20 ft	4 in	4.94 yd³	use ready-mix
1-car garage	12×20 ft	4 in	2.96 yd³	use ready-mix
2-car garage	20×20 ft	4 in	4.94 yd³	use ready-mix
Residential driveway	10×20 ft	6 in	3.70 yd³	use ready-mix
Sidewalk (50 ft)	50×4 ft	4 in	2.47 yd³	use ready-mix

All figures include 10% waste factor.

Concrete Slab Cost by US Region (2026)

Total installed slab cost = concrete material + labor + finishing. Labor and finishing typically add $2–$5 per square foot on top of material cost.

Region	Material ($/yd³)	Total Installed ($/sqft)	10×10 Slab Total
Northeast	$175–$225	$7–$12	$700–$1,200
West Coast	$165–$215	$6–$11	$600–$1,100
Mountain West	$150–$190	$5–$9	$500–$900
Midwest	$140–$175	$4–$8	$400–$800
South	$130–$165	$4–$7	$400–$700

Slab Thickness by Application

Sidewalks & walkways: 4 inches — ACI minimum for exterior flatwork
Patios & pool decks: 4 inches — increase to 5 in if heavy furniture
Garage floors: 4–6 inches — 6 in recommended for trucks/heavy vehicles
Residential driveways: 4–6 inches — 6 in standard in cold climates
Commercial driveways: 6–8 inches — for delivery vehicles and forklifts

Rebar & Wire Mesh Guide for Slabs

Rebar and wire mesh add tensile strength — concrete is strong in compression but weak in tension. For residential slabs, the choice is between rebar grids and fiber mesh additives:

Method	Application	Spacing	Notes
#3 Rebar grid	Patios, sidewalks, garage floors	12–18 in on center	Place at mid-depth (2 in up from bottom)
#4 Rebar grid	Driveways, commercial slabs	12–24 in on center	Required where vehicle loads are expected
6×6 Wire mesh	Patios, light-duty slabs	6-in grid	Less effective than rebar but easier to install
Fiber mesh	All slabs	Mixed into concrete	Reduces plastic shrinkage cracks; doesn't replace structural rebar

Concrete Slab FAQ

How many yards of concrete for a 20×20 slab?

A 20×20 slab at 4 inches thick = 400 sqft × 0.333 ft = 133.3 ft³ / 27 = 4.94 yd³ net. With 10% waste = 5.43 yd³ to order. At $160/yd³ (Midwest average), material cost is ~$869. Use the calculator above for your exact thickness and region.

How thick should a concrete patio slab be?

4 inches is the ACI-recommended minimum for residential exterior flatwork including patios. Increase to 5–6 inches if the patio will support a hot tub (which can weigh 3,000–5,000 lbs when full), heavy planters, or a pergola with anchor bolts. Thicker slabs also reduce cracking risk in freeze-thaw climates.

How much does a concrete slab cost?

In 2026, a concrete slab costs $4–$8 per square foot installed in the Midwest/South, and $6–$12 per square foot in the Northeast and West Coast. A 10×10 patio slab typically costs $400–$1,200 total installed. Material alone (concrete + rebar) is usually $1–$3/sqft; labor and finishing account for the rest.

Do I need rebar in a 4-inch concrete slab?

For most residential patios and sidewalks, #3 rebar at 18-inch spacing or 6×6 wire mesh is recommended but not always code-required. For garage floors and driveways (where vehicle loads apply), rebar is strongly recommended. In freeze-thaw climates, rebar significantly reduces cracking from ground heave. Check your local building code — some municipalities require rebar for any slab over a certain size.

How long does a concrete slab take to cure before use?

A concrete slab is typically walkable after 24–48 hours and reaches approximately 70% design strength at 7 days. Full design strength (100%) is achieved at 28 days. Avoid parking vehicles on a new garage floor or driveway for at least 7 days (14 days in cold weather). Keep the surface moist — covered with plastic sheeting or burlap — for the first 3–7 days to prevent surface cracking from rapid drying.

What PSI concrete should I use for a driveway slab?

Use a minimum of 4,000 PSI concrete for residential driveways. In cold climates with significant freeze-thaw cycles, specify 4,500 PSI with air entrainment (4–7% entrained air). Air-entrained concrete resists surface scaling caused by de-icing salts far better than standard mixes. For standard patios and walkways with no vehicle traffic, 3,500 PSI is typically sufficient.

How do I calculate concrete for an L-shaped or irregular slab?

Break the shape into simpler rectangles. Calculate each rectangle separately, add all volumes together, then add 10–15% for waste. Example: An L-shaped patio that is 20×10 ft plus a 10×8 ft extension = (200 + 80) × (4 / 12) = 93.3 ft³ = 3.46 yd³ net. With 12% waste = 3.87 yd³ to order. For complex curves or round sections, add at least 15% waste.

What is the difference between concrete and cement?

Cement is just one ingredient in concrete. Concrete = cement + sand + aggregate (gravel) + water. Portland cement acts as the binder — when mixed with water it forms a paste that coats the aggregate and hardens. You cannot pour a slab from cement alone. When people say "cement slab" or "cement driveway," they almost always mean concrete. The confusion is common but important to understand when ordering materials.

How much does a 20×20 concrete slab cost installed?

A 20×20 slab (400 sqft) at 4 inches thick costs approximately $1,600–$3,200 installed in the Midwest and South, and $2,400–$4,800 in the Northeast and West Coast. This range includes excavation, forming, concrete material, rebar, pour, finishing, and basic sealing. Site access, soil conditions, and decorative finishes (stamped, stained, or exposed aggregate) will push costs toward the higher end.

How to Prepare a Site for a Concrete Slab

The quality of a concrete slab depends as much on what happens before the pour as during it. Poor sub-base preparation is the leading cause of cracked and settled slabs — no amount of good concrete can compensate for a weak base.

Step 1 — Excavate and Grade

Remove all topsoil, organic material, and vegetation from the slab area. Topsoil compresses and decomposes over time, causing settlement. Excavate to the required depth (typically 6–8 inches below finished slab grade to allow for sub-base plus slab thickness). Grade the area to drain water away from structures — minimum 1% slope (⅛ inch per foot) away from buildings.

Step 2 — Compact the Sub-Base

Place 4–6 inches of compacted crushed gravel (¾-inch clean gravel or road base) over the excavated area. Compact in 2-inch lifts using a plate compactor — never compact the full 4 inches in one pass. Proper compaction prevents differential settlement that cracks slabs. In cold climates, a well-drained gravel base also helps prevent frost heave by preventing water from accumulating under the slab.

Step 3 — Install Forms

Set 2×4 or 2×6 lumber forms along the perimeter of the slab. Use a level and stakes to ensure forms are perfectly flat — any variation will show in the finished surface. Brace forms every 2–3 feet to prevent blowout when concrete is placed. Oil the inside of forms lightly to allow removal without damage.

Step 4 — Place Vapor Barrier (if on grade)

For interior slabs (garage floors, basement floors, shop floors), place 6-mil polyethylene sheeting over the gravel base before pouring. This prevents ground moisture from wicking up through the slab, which causes efflorescence, paint failure, and moisture problems in finished spaces.

Step 5 — Set Rebar or Wire Mesh

Place rebar on chairs or dobies at the correct height — typically 1.5–2 inches from the bottom of the slab for a 4-inch slab. Rebar that rests on the ground provides no tensile benefit. Overlap rebar splices by at least 18 bar diameters (18 × 0.375 in = 6.75 inches for #3 rebar).

Step 6 — Order and Pour Concrete

Confirm your cubic yardage (use this calculator) with the ready-mix plant and schedule delivery. Have all tools staged: screed board, bull float, hand float, edger, jointing tool, and knee boards. Pour concrete starting from the far corner and work back toward the truck. Strike off with a screed, float, then finish as the bleed water disappears. Cut control joints at 1/4 the slab thickness every 8–12 feet to control where cracking occurs.

Slab Prep Cost Summary

Prep Item	Typical Cost	DIY vs. Hire
Excavation (10×10 ft)	$150–$400	DIY possible with shovel; hire for large areas
Gravel sub-base (4 in)	$80–$200	DIY — order from a landscape supply yard
2×6 form lumber	$40–$120	DIY
Vapor barrier (6 mil)	$25–$60	DIY
Rebar (#3, 12-inch grid)	$80–$200	DIY with rebar cutter/bender or bolt cutters
Concrete finishing tools	$50–$150 (rental)	Rent bull float, edger, jointer

Common Concrete Slab Mistakes to Avoid

Adding water to loosen the mix. This is the single most damaging mistake. Adding ½ gallon of water per yd³ reduces concrete strength by 200 PSI. Never add water after the truck leaves the plant.
Finishing too early. Working the surface while bleed water is still present traps water beneath the surface layer, causing scaling and delamination. Wait until bleed water disappears before floating.
Skipping control joints. Concrete always cracks — the question is where. Control joints (saw cuts or hand-tooled grooves) at ¼ slab depth every 8–12 feet direct cracks to a straight, inconspicuous line.
Pouring in direct sun or high wind. Both cause rapid surface drying before the bulk of the concrete sets, leading to plastic shrinkage cracks. Erect windbreaks and sun shades for large exterior pours in summer.
Not testing soil compaction. Soft spots under the slab will settle. Walk the area after compacting — if it feels soft or spongy anywhere, add more gravel and re-compact.
Ordering exactly the calculated amount. Always add 5–10% to your calculation. Running short during a pour is a serious problem — a cold joint (where fresh meets partially set concrete) is a permanent structural weakness.