CONSTRUCTION · FREE ONLINE CALCULATOR

Concrete Slab Calculator

Figure how much concrete you need for a slab, patio, driveway or shed base. Get cubic yards, cubic feet and cubic meters with a waste allowance, the number of premixed bags, and cement, sand and aggregate for mixing on site.

Detailed optionsStep-by-step guideWorked examples

01 / SLAB VOLUME

How much concrete does your slab need?

Enter the slab size and thickness. The calculator gives cubic yards, cubic meters, premix bags and site-mix materials.

Slab shapeLength (ft)Width (ft)Thickness (in)Number of slabsExtra for waste (%)Site-mix ratio (cement : sand : aggregate)Price per cubic yard (USD, optional)

Common slabs:

Concrete needed (incl. 10% extra)1.36 yd³

10 × 10 ft × 4 in

Cubic feet: 36.67 ft³ · cubic meters: 1.038 m³Without waste: 1.23 yd³Premix bags: 123 × 40 lb or 82 × 60 lb or 62 × 80 lb

02 / BAGS & MATERIALS

Bags of premix, or mix it on site

Bag counts are rounded up. Site-mix quantities use the dry-volume method with 50 kg cement bags.

Premixed concrete bags (approximate yield)
Bag size	Yield per bag	Bags needed
40 lb	0.3 ft³	123
60 lb	0.45 ft³	82
80 lb	0.6 ft³	62

Site mix 1 : 2 : 4 (about M15)
Material	Quantity	Also equals
Cement (50 kg bags)	7 bags	6.58 bags exact
Sand	0.46 m³	16.1 ft³
Aggregate (crush/gravel)	0.91 m³	32.3 ft³

Bag yields vary by brand: check the yield printed on the bag. Structural slabs should follow your engineer’s or local code’s specification.

YOUR ANIMATED GUIDE

How to figure concrete for a slab, step by step.

These steps use your current inputs. Choose a step or play the walkthrough.

STEP 1 / 4

100 ft²

1234

Measure the slab area

Multiply the length by the width.

10 ft × 10 ft = 100 ft²

Explore at your own pace

How to use this concrete slab calculator

A concrete slab calculator figures how much concrete you need for a patio, shed base, driveway, garage floor or footpath. Enter the length, width and thickness in feet and inches or in meters and centimeters, add a waste allowance, and get the volume in cubic yards, cubic feet and cubic meters, the number of premixed bags, and the cement, sand and aggregate needed to mix it on site.

How to figure concrete for a slab

Multiply length × width × thickness, with all three in the same unit. In feet and inches, divide the thickness in inches by 12 first: a 10 ft × 10 ft slab 4 inches thick is 10 × 10 × 0.333 = 33.3 cubic feet. Divide by 27 to convert to cubic yards: 1.23 yd³. Then add 5–10% for waste: about 1.36 yd³.

Ready-mix, bags or site mix?

For slabs over about 1 cubic yard (0.75 m³), ready-mix delivered by truck is usually easier and more consistent. For small jobs, premixed bags are convenient: an 80 lb bag yields about 0.60 cubic feet. On many sites, especially in South Asia, concrete is mixed from cement, sand and aggregate; the calculator estimates 50 kg cement bags and the sand and crushed stone for common nominal mixes.

Typical slab thickness

Walkways and patios are often 4 inches (10 cm). Driveways and garage floors are often 5 to 6 inches (12 to 15 cm), especially for heavier vehicles. Roof slabs and structural slabs need an engineer’s design and reinforcement detailing. Thickness has the largest effect on volume, so measure it carefully, including any low spots in the sub-base.

Concrete Slab formulas, explained

Rectangular slab

Volume = length × width × thickness

Imperial: thickness in inches ÷ 12. Metric: thickness in cm ÷ 100.

Round slab

Volume = π × (diameter ÷ 2)² × thickness

Unit conversion

1 yd³ = 27 ft³ · 1 m³ = 35.31 ft³ = 1.308 yd³

Site mix (dry volume method)

Dry volume = wet volume × 1.54; cement bags = dry × cement share × 1,440 ÷ 50

For 1 : 2 : 4, the cement share is 1/7. 1 m³ of concrete needs about 6.3 bags of cement.

EVERY OPTION, EXPLAINED

Know what goes into your result.

Use this reference to check units, assumptions and advanced settings.

Concrete Slab calculator input reference
Input or option	What it means
Units	Feet & inches: length and width in feet, thickness in inches. Meters & centimeters: length and width in meters, thickness in centimeters.
Slab shape	Rectangle or square, or a round slab measured by its diameter.
Length, width, diameter	The outside dimensions of the slab. For L-shaped slabs, split into rectangles and add the results.
Thickness	The finished slab depth. Measure at several points if the sub-base is uneven.
Number of slabs	How many identical slabs, footings or pads to pour.
Extra for waste	A percentage added for spillage, over-excavation and form movement. 5–10% is common.
Site-mix ratio	Cement : sand : aggregate by volume, used for the hand-mixed material estimate.
Price per cubic yard / m³	Optional. Your local ready-mix price, used for a rough concrete cost.

PUT THE FORMULA TO WORK

Worked examples you can try.

Load an example into the calculator, then change one value to explore.

EXAMPLE 01

10 × 10 ft patio, 4 inches thick

33.3 ft³ = 1.23 yd³. With 10% waste: 1.36 yd³, or 62 bags of 80 lb premix.

EXAMPLE 02

20 × 20 ft garage slab, 4 inches

133.3 ft³ = 4.94 yd³; with 10% waste about 5.43 yd³. Ready-mix is the practical choice.

EXAMPLE 03

3 × 3 m slab, 10 cm, mixed on site

0.9 m³, or 0.945 m³ with 5% waste. A 1 : 2 : 4 mix needs about 6 bags of cement (50 kg), 0.42 m³ of sand and 0.83 m³ of aggregate.

MAKE THE NUMBERS MAKE SENSE

Common mistakes to avoid

Entering the thickness in feet or meters when the field asks for inches or centimeters.
Ordering the exact volume with no allowance for waste, spillage and uneven ground.
Forgetting that dry materials shrink when mixed with water; that is why the site-mix method uses a 1.54 dry-volume factor.

A LITTLE MORE CLARITY

Frequently asked questions

What does the concrete slab calculator include?

Slab volume in cubic yards, cubic feet and cubic meters, a waste allowance, premixed bag counts, site-mix cement, sand and aggregate, and an optional cost estimate.

Can I use metric measurements?

Yes. Switch to meters and centimeters. Results are shown in both metric and imperial units.

How do you figure concrete for a slab?

Multiply the length by the width by the thickness in the same unit. In feet, convert inches to feet by dividing by 12. Divide cubic feet by 27 to get cubic yards, then add 5–10% for waste.

How many yards of concrete do I need for a 10×10 slab?

A 10 × 10 ft slab 4 inches thick needs 1.23 cubic yards, or about 1.36 yd³ with 10% extra. At 6 inches thick it needs 1.85 yd³ before waste.

How many 80 lb bags of concrete do I need for a 10×10 slab?

About 56 bags of 80 lb premix for a 4-inch slab with no waste, or 62 bags with 10% extra. Each 80 lb bag yields roughly 0.60 cubic feet.

How many bags of cement are needed for 1 cubic meter of concrete?

For a 1 : 2 : 4 mix, about 6.3 bags of 50 kg cement per cubic meter. For 1 : 1.5 : 3 it is about 8.1 bags, and for 1 : 3 : 6 about 4.4 bags.

How thick should a concrete slab be?

4 inches (10 cm) is typical for patios, sheds and walkways; 5–6 inches (12–15 cm) for driveways and garages. Structural and roof slabs must follow an engineer’s design and local building codes.

How much extra concrete should I order?

Most people add 5–10%. Use more if the ground is uneven or the forms are not perfectly level. Running short partway through a pour causes cold joints.

How do I convert cubic feet to cubic yards or cubic meters?

Divide cubic feet by 27 for cubic yards, or by 35.31 for cubic meters. The calculator shows all three.

How much does a concrete slab cost?

Enter your local ready-mix price per cubic yard or cubic meter to estimate the concrete cost. Labor, forms, reinforcement, delivery fees and finishing are extra.

Methodology & assumptions

Calculations run in your browser using the formulas and assumptions explained on this page. Intermediate values retain full floating-point precision; displayed amounts are rounded. Volume = area × thickness, multiplied by the number of slabs and the waste allowance. Bag counts assume about 0.30, 0.45 and 0.60 ft³ for 40, 60 and 80 lb bags (about 2,135 kg/m³ for metric bags) and are rounded up. Site-mix quantities use the common dry-volume method (dry = wet × 1.54) with cement at 1,440 kg/m³ and 50 kg bags; water, admixtures and reinforcement are not included. Estimates only: structural work should follow an engineer’s specification. Sharing an input link places those inputs in the URL.