Garage Foundation Concrete Estimator

Concrete Garage Slab Calculator

Estimate garage slab concrete from length, width, thickness, waste factor, and bag yield. Get cubic yards, cubic feet, 40/60/80 lb bag pressure, and material-only cost before you compare ready-mix or contractor quotes.

Garage Size

Start with the finished formed floor dimensions.

Thickness

Compare 4 in and 5 in before ordering concrete.

Ready-Mix Check

Use high bag counts as a signal to compare delivery.

Quick answer

A 20 ft x 20 ft garage slab at 4 in thick with 10% waste needs about 5.43 yd3. That is roughly 245 common 80 lb bags, so a full garage slab is usually planned as a ready-mix pour.

Enter Garage Slab Size

Results

Live estimate

Concrete Volume

5.4321 yd³

146.6667 ft³ / 4.1531 m³

Bags Needed

245 bags

Rounded up to whole bags

Estimated Cost

$1,225.00

Based on bag price

Waste Factor

10%

Included in total volume

Garage slab formula

Slab area

Area = garage slab length x garage slab widthArea = 20 x 20 = 400.00 sq ft

Concrete volume

Volume = length x width x thicknessVolume = 20 x 20 x 0.333

Waste and bags

Final volume = 133.3333 x (1 + 10%/100)Bags = ceil(final volume / 0.6)

Quick Garage Slab Yardage and Bag Counts

Garage slab	Thickness	Concrete with 10% waste	40 lb bags	60 lb bags	80 lb bags
20 x 20 ft	4 in	5.43 yd3	489	326	245
24 x 24 ft	4 in	7.82 yd3	704	470	352
24 x 24 ft	5 in	9.78 yd3	881	587	441
30 x 30 ft	4 in	12.22 yd3	1100	734	550

These bag counts use common bag yields and are meant to show why full garage slabs usually need ready-mix planning. Use the calculator above for your exact slab size, thickness, and waste.

Example Garage Slab Estimates

20 ft x 20 ft garage slab

One-car or compact detached garage slab at 4 inches thick.

Result: 5.4321 yd3 / 146.6667 ft3 / 245 bags

24 ft x 24 ft garage slab

Two-car garage slab at 4 inches thick with 10% waste.

Result: 7.8222 yd3 / 211.2 ft3 / 353 bags

24 ft x 24 ft at 5 inches

Same footprint with a thicker planning pass.

Result: 9.7778 yd3 / 264 ft3 / 440 bags

Garage Scope Items to Keep Separate

Item	Estimate check	Why
Main garage floor	Length x width x slab thickness.	Core cubic yards and material-only cost.
Thickened edge	Perimeter length x added width x added depth.	Adds concrete, forms, and reinforcement scope.
Garage apron	Separate slab area outside the garage door.	Often has different slope, finish, and joint details.
Footings or stem wall	Use footing dimensions instead of slab thickness.	Structural and frost-depth rules may apply.

Before You Compare Quotes

Confirm finished formed dimensions before ordering.
Run both 4 in and 5 in thickness if the design is not final.
Keep apron, thickened edges, and footings as separate sections.
Ask whether vapor barrier, reinforcement, joints, permits, and inspection are included.

Garage Slab Planning Notes

A garage slab calculator estimates material quantity. It does not approve the slab design, vapor barrier detail, reinforcement, frost-depth requirement, drainage slope, or inspection scope.

For the full written guide, see the concrete garage slab calculator guide. For thickness assumptions, use the concrete slab thickness guide. If the garage includes a footing or stem wall, estimate that separately with the concrete footing calculator.

Frequently Asked Questions

How do I calculate concrete for a garage slab?

Multiply garage slab length by width by thickness, convert the result to cubic yards, then add a waste factor before ordering ready-mix or checking bag counts.

How many yards are needed for a 20x20 garage slab?

A 20 ft by 20 ft garage slab at 4 inches thick is about 4.94 cubic yards before waste. With 10% waste, plan about 5.43 cubic yards.

How thick should a garage slab be?

Four inches is a common planning starting point for some residential garage slabs, but vehicle loads, subbase, reinforcement, vapor barrier, edge detail, frost, and local code can change the design.

Can I pour a full garage slab with bags?

For a full garage slab, bag counts are usually very high. A 20x20 slab at 4 inches thick with 10% waste is about 245 common 80 lb bags, so ready-mix is usually the practical planning assumption.

Does this include vapor barrier, forms, or labor?

No. This calculator estimates concrete material only. Excavation, gravel base, compaction, vapor barrier, reinforcement, joints, forms, finishing, permits, inspections, and labor are separate.

Should I calculate the garage apron separately?

Yes. A garage apron or approach slab may have different thickness, slope, finish, and joint details. Estimate it as a separate slab section before adding totals.

Move to the calculator that matches your next estimating step.

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Disclaimer: This calculator is for concrete material planning only. Site conditions, base preparation, reinforcement, drainage, joints, form accuracy, and local building rules can change actual requirements. Consult a qualified contractor or engineer for structural work.