Concrete Slab Calculator

By Uzair Arshad , Senior Civil and Structural Engineer

Last updated: April 27, 2026

Use this free concrete slab calculator to estimate cubic yards, cubic feet, and bag counts. Enter your slab length, width, and thickness to get exact volume, waste-adjusted order quantity, and bag totals for 40 lb, 50 lb, 60 lb, and 80 lb mix before you call your supplier.

How to use this calculator

This concrete slab calculator turns your slab dimensions into cubic yards, bag counts, and an estimated order quantity. Enter length, width, thickness, and a waste factor, then review your results before ordering.

Enter the slab length in feet. Measure the long side of the area you plan to fill.
Enter the slab width in feet. Measure the short side.
Enter slab thickness in inches. Use 4 inches for patios and walkways, 6 to 8 inches for driveways and garage slabs. Use the thickness from your project plan, not a guess.
Set a waste factor. Use 5% for well-prepared, level subgrade. Use 10% for most residential jobs where the ground is uneven or forms are imperfect.
Choose a bag size or leave "Show all sizes" selected to compare 40 lb, 50 lb, 60 lb, and 80 lb options side by side.
Click "Calculate concrete" to see cubic yards, bag counts, estimated weight, and cost ranges.

Pro tips for accurate slab estimates

Measure inside the forms, not from the outside edges. Form boards add 1.5 inches per side on standard 2× lumber, and those extra inches inflate your volume if you measure from the wrong face.

Take three depth measurements across the area and average them. Uneven subgrade means actual thickness varies, and averaging prevents under-ordering.

Compare 60 lb and 80 lb bag counts before you buy. An 80 lb bag yields 0.60 cu ft versus 0.45 cu ft for a 60 lb bag. The 80 lb bag is a better value per cubic foot, but hand-mixing more than 30 to 40 bags is exhausting. For larger pours, consider ready-mix delivery. Run your numbers through this concrete slab calculator before making a trip to the store.

Quick reference: common slab sizes at 4 inches thick

Slab size	Area (sq ft)	Cubic yards	80 lb bags
10 × 10 ft	100	1.23	56
10 × 12 ft	120	1.48	67
12 × 16 ft	192	2.37	107
20 × 20 ft	400	4.94	223
20 × 30 ft	600	7.41	334
30 × 40 ft	1,200	14.81	667

Values before waste factor. Add 5% to 10% for your order quantity.

How the calculation works

Volume:
Thickness (ft) = Thickness (in) / 12
Slab Area (sq ft) = Length (ft) × Width (ft)
Volume (cu ft) = Length × Width × Thickness (ft)
Cubic Yards = Volume (cu ft) / 27

Waste:
Order Volume = Cubic Yards × (1 + Waste % / 100)

Bag Count:
40 lb Bags = Order Volume (cu ft) / 0.30 (rounded up)
50 lb Bags = Order Volume (cu ft) / 0.375 (rounded up)
60 lb Bags = Order Volume (cu ft) / 0.45 (rounded up)
80 lb Bags = Order Volume (cu ft) / 0.60 (rounded up)

Weight:
Estimated Weight (lb) = Order Volume (cu ft) × 150

Length: Long side of the slab in feet
Width: Short side of the slab in feet
Thickness: Planned slab depth in inches, converted to feet for volume math
Waste %: Extra percentage for uneven grade, spills, and ordering safety

This concrete slab calculator multiplies your slab's three dimensions to get total volume, converts to cubic yards, then applies a waste factor so your order covers real jobsite conditions.

Main formula

Volume (cubic feet) = Length (ft) × Width (ft) × Thickness (ft)

Cubic yards = Volume (cubic feet) / 27

Order volume = Cubic yards × (1 + Waste % / 100)

Convert thickness from inches to feet first by dividing by 12. So 4 inches = 0.333 feet, and 6 inches = 0.5 feet.

Worked example

A 10 ft × 12 ft patio slab at 4 inches thick with 10% waste:

Thickness in feet = 4 / 12 = 0.333 ft
Slab area = 10 × 12 = 120 sq ft
Volume = 10 × 12 × 0.333 = 40 cu ft
Cubic yards = 40 / 27 = 1.48 cu yd
With 10% waste = 1.48 × 1.10 = 1.63 cu yd (44 cu ft)
80 lb bags = 44 / 0.60 = 74 bags (rounded up)
60 lb bags = 44 / 0.45 = 98 bags (rounded up)

At that volume, hand-mixing 74 bags of 80 lb concrete is a full day of work for two people. For anything over 2 cubic yards, most contractors call for a ready-mix truck.

Bag yield reference

Bag size	Yield per bag	Typical 2026 price
40 lb concrete mix	~0.30 cu ft	$3.50 to $5.00
50 lb concrete mix	~0.375 cu ft	$4.50 to $6.50
60 lb concrete mix	~0.45 cu ft	$5.00 to $7.50
80 lb concrete mix	~0.60 cu ft	$6.50 to $9.00
Ready-mix delivery	Sold by cubic yard	$170 to $220 per yard

Why bag counts round up

You can't buy a fraction of a bag. If the math says 73.2 bags, you need 74. Rounding down leaves you short, and a second trip to the store costs more in time and fuel than one extra bag of mix.

Assumptions and limitations

This concrete slab calculator assumes a flat, rectangular slab with uniform thickness. Slabs with thickened edges, turndowns, or irregular shapes need separate volume calculations for each section.

Bag yields (0.30, 0.375, 0.45, and 0.60 cu ft) are manufacturer specs at standard water ratios. Adding too much water reduces yield and weakens the finished slab.

The estimated weight uses 150 lb per cubic foot, a standard density for normal-weight concrete. Lightweight mixes weigh less, and actual weight varies with aggregate type and water content.

This concrete slab calculator estimates material quantity only. It does not calculate structural thickness, reinforcement requirements, load capacity, or code compliance. Check with your local building department for those specifications.

Common concrete slab calculator mistakes

Measuring from outside the forms instead of inside inflates volume by a few percent. Always measure the space the concrete will fill, not the lumber around it.

Ordering the exact calculated volume with no waste factor is the most common reason for running short. Uneven subgrade, small spills, and over-excavation at edges all consume extra material. A 5% to 10% buffer costs far less than a short-load delivery fee or emergency bag run.

Skipping subgrade compaction changes effective slab thickness. Loose soil settles under the weight of wet concrete, and your 4 inch slab can become 3.5 inches in spots. Compact and level the base before you pour.

Frequently Asked Questions

How do you calculate concrete for a slab?

Multiply length × width × thickness to get volume in cubic feet, then divide by 27 for cubic yards. For example, a 10 ft × 12 ft slab at 4 inches thick is 40 cubic feet, or 1.48 cubic yards before waste. Add 5% to 10% extra for uneven subgrade.

How do you calculate materials for a concrete slab?

Calculate concrete volume first, then convert to cubic yards and bag counts. A 120 sq ft slab at 4 inches thick equals 1.48 cubic yards. With 10% waste, you'd order about 1.63 cubic yards or 74 bags of 80 lb mix.

How do you calculate concrete slab yardage?

Convert slab thickness to feet by dividing inches by 12. Multiply length × width × thickness in feet, then divide by 27. A 20 ft × 20 ft slab at 4 inches thick equals 133.33 cubic feet, or 4.94 cubic yards before waste.

How do you calculate cubic yards for a concrete slab?

Find cubic feet first by multiplying length × width × thickness in feet. Divide by 27 because one cubic yard contains 27 cubic feet. A 12 ft × 16 ft slab at 6 inches thick equals 96 cubic feet, which is 3.56 cubic yards.

How do you calculate concrete for a slab in cubic meters?

Use meters for every dimension and multiply length × width × thickness. A 3 m × 4 m slab at 0.10 m thick equals 1.2 cubic meters. This calculator uses feet and inches, so apply the formula manually for metric projects.

How much concrete do I need for a 10×10 slab?

A 10 ft × 10 ft slab at 4 inches thick needs about 1.23 cubic yards before waste. With a 10% waste factor, plan for 1.36 cubic yards or 62 bags of 80 lb mix. Round up for bag counts because partial bags are not practical.