How do I calculate a concrete slab?

Measure length, width, and slab thickness, then multiply them to get volume. Convert cubic feet to cubic yards by dividing by 27, or let the calculator handle the conversion automatically.

How many yards do I need for a 10x10 slab?

A 10x10 slab at 4 inches thick needs about 1.23 cubic yards before extra waste is added. With a 10% waste allowance, the order-ready total is about 1.36 cubic yards.

How many square feet does 1 yard of concrete cover at 4 inches?

At 4 inches thick, 1 cubic yard of concrete covers about 81 square feet. That is a useful rule of thumb, but exact slab dimensions and waste percentage should still drive the final order.

How many square feet does 1 yard of concrete cover at 5 inches?

At 5 inches thick, 1 cubic yard covers about 64.8 square feet. The extra inch of thickness reduces coverage more than many people expect, which is why thickness deserves its own check.

How many 80 lb bags do I need for 100 square feet at 4 inches?

Using the same 0.6 cubic foot yield per 80-lb bag used in this calculator, 100 square feet at 4 inches thick needs about 56 bags before extra waste is added.

What is the difference between a cement calculator and a concrete calculator?

Most people searching cement calculator are really looking for finished concrete volume. This page estimates the concrete you need for the slab, not just one ingredient inside the mix.

When does ready-mix start making more sense than bags?

There is no single cutoff that fits every project. Ready-mix becomes more attractive when slab volume, labor, pour pace, and finish consistency all start pulling against on-site bag mixing.

How much extra concrete should I add for waste?

A waste allowance helps cover spillage, uneven subgrade, form movement, and cleanup. Many slab estimates use a modest extra margin, but the right percentage depends on site conditions and how tight your layout is.

What slab thickness is common for patios, walkways, and driveways?

Many patios start around 4 inches, walkways and utility slabs often land around 4 to 5 inches, and driveway slabs commonly push into the 5 to 6 inch range. Final thickness should still follow load, soil, reinforcement, and local requirements.

Does this calculator estimate slab weight too?

Yes. Total slab weight is estimated with a standard 150 lb per cubic foot assumption, which gives you useful handling context alongside cubic yards and bag counts.

Concrete Slab Calculator

Estimate cubic yards, slab weight, 40/60/80-lb bag counts, 4-inch coverage, and worked examples for patios, walkways, and driveways in one place.

Choose shape

Used to calculate volume & materials.

Editing Section 1

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Edited

Length

Width

Thickness

Quantity

Waste Control

Suggested 10 percent, adjust as needed.

10%

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Results Card

Results update when you press Calculate.

Press Calculate to see results.

Calculation Details

Why this slab calculator is more useful than a raw volume tool

Most slab searches boil down to the same practical questions: how many yards, how many bags, what does 1 yard cover at 4 or 5 inches, and what does a common slab size like 10×10 or 4×8 actually turn into once waste is added. This page keeps those checks next to the live calculator instead of scattering them across separate tabs.

Check cubic yards and bag counts together

The page answers the two quantity questions that show up most often in slab searches: total yardage and equivalent 40, 60, and 80-lb bag counts.

Use square-foot coverage as a fast sanity check

Coverage rows for 4-inch, 5-inch, and 6-inch slabs make it easier to verify the estimate before you call a supplier or stage bagged mix.

Compare query-backed slab examples before ordering

Worked examples focus on the slab sizes people actually search for, including 10×10, 4×8, and 24×24 pours.

Stay on the right geometry for the job

If the project is a round pier, post hole, or sonotube, the page points you to the matching calculator instead of forcing slab math to answer the wrong shape.

Reference Tables

Coverage, Bag Yield, and Common Slab Starting Points

Use these tables as quick planning checks for slab coverage, bag counts, and typical starting thickness ranges. They support the calculator; they do not replace the final project dimensions, waste allowance, or local requirements.

Concrete coverage by slab thickness

Quick checks for the slab thicknesses most often compared in residential planning.

Thickness

4 in

Coverage per yd³: 81.0 sq ft / yd³
Yards per 100 sq ft: 1.23 yd³
80-lb bags per 100 sq ft: Approx. 56 bags

Thickness

5 in

Coverage per yd³: 64.8 sq ft / yd³
Yards per 100 sq ft: 1.54 yd³
80-lb bags per 100 sq ft: Approx. 70 bags

Thickness

6 in

Coverage per yd³: 54.0 sq ft / yd³
Yards per 100 sq ft: 1.85 yd³
80-lb bags per 100 sq ft: Approx. 84 bags

Thickness	Coverage per yd³	Yards per 100 sq ft	80-lb bags per 100 sq ft
4 in	81.0 sq ft / yd³	1.23 yd³	Approx. 56 bags
5 in	64.8 sq ft / yd³	1.54 yd³	Approx. 70 bags
6 in	54.0 sq ft / yd³	1.85 yd³	Approx. 84 bags

These rows assume the exact slab thickness shown and no extra waste. Use the calculator when the slab has multiple sections, thickened edges, or site waste.

Approximate bag counts from this calculator's default yields

These counts follow the same live-calculator assumptions used on the page: 40-lb = 0.30 ft³, 60-lb = 0.45 ft³, and 80-lb = 0.60 ft³ of concrete per bag.

Bag size

40 lb

Bags per 1/2 yd³: 45 bags
Bags per 1 yd³: 90 bags
Planning note: Smaller bags drive handling count up quickly on slab work.

Bag size

60 lb

Bags per 1/2 yd³: 30 bags
Bags per 1 yd³: 60 bags
Planning note: Useful when suppliers stock 60-lb mix, but still labor-heavy on larger pads.

Bag size

80 lb

Bags per 1/2 yd³: 23 bags
Bags per 1 yd³: 45 bags
Planning note: Best single reference point because the page treats 80-lb bags as the standard comparison size.

Bag size	Bags per 1/2 yd³	Bags per 1 yd³	Planning note
40 lb	45 bags	90 bags	Smaller bags drive handling count up quickly on slab work.
60 lb	30 bags	60 bags	Useful when suppliers stock 60-lb mix, but still labor-heavy on larger pads.
80 lb	23 bags	45 bags	Best single reference point because the page treats 80-lb bags as the standard comparison size.

Rounded to whole bags. Add waste after the base quantity is known.

Typical residential slab starting points

Conservative planning guidance for common slab types. Final thickness, reinforcement, and base prep should follow site conditions and local requirements.

Use case

Patio / walkway

Often starts around: 4 in
Why the order changes: Pedestrian slabs are often thinner, but finish quality, joints, and base prep still matter.

Use case

Walkway / landing slab

Often starts around: 4 to 5 in
Why the order changes: Utility slabs and landings often land a little thicker than a simple patio once edge support and concentrated foot traffic are considered.

Use case

Driveway slab

Often starts around: 5 to 6 in
Why the order changes: Vehicle loads, turning forces, and edge support usually push the order thicker than a patio slab.

Use case	Often starts around	Why the order changes
Patio / walkway	4 in	Pedestrian slabs are often thinner, but finish quality, joints, and base prep still matter.
Walkway / landing slab	4 to 5 in	Utility slabs and landings often land a little thicker than a simple patio once edge support and concentrated foot traffic are considered.
Driveway slab	5 to 6 in	Vehicle loads, turning forces, and edge support usually push the order thicker than a patio slab.

This is a rule-of-thumb reference only. Confirm final slab design against local code, reinforcement plans, soil conditions, and expected load.

Need a smaller shed, AC, or equipment base instead? Try the pad calculator. Need isolated support footing quantity instead of a broad slab? Use the footing calculator. Need a round pier or sonotube workflow instead? Switch to the column calculator so the geometry matches the job.

Worked Examples

Query-backed slab examples people actually search for

These examples reflect recurring slab-size queries from current search data and use the same 10% waste allowance shown in the calculator workflow.

Example 1

10×10 slab at 4 inches

This is the clearest search-backed slab example and a useful baseline for comparing yardage with bag counts.

Base volume

1.23 yd³

With 10% waste

1.36 yd³

80-lb bags

Approx. 62 bags

Query match

10×10 slab

1Area = 10 × 10 = 100 sq ft.
2Thickness = 4 inches = 0.333 ft.
3Base volume = 100 × 0.333 = 33.33 ft³ = 1.23 yd³.
4Add 10% waste to land at about 1.36 yd³, or roughly 62 80-lb bags.

Takeaway: A 10×10 slab is still manageable for planning with bags, but the labor climbs faster than the square footage suggests once waste and cleanup are included.

Example 2

4×8 slab at 4 inches

This size shows up in bag-count search phrasing and is a better fit for small-entry, landing, or utility-slab planning than a generic placeholder example.

Base volume

0.40 yd³

With 10% waste

0.44 yd³

80-lb bags

Approx. 20 bags

Query match

4×8 slab

1Area = 4 × 8 = 32 sq ft.
2Thickness = 4 inches = 0.333 ft.
3Base volume = 32 × 0.333 = 10.67 ft³ = 0.40 yd³.
4Add 10% waste to land at about 0.44 yd³, or roughly 20 80-lb bags.

Takeaway: This is a realistic small-slab bag job, but it still helps to see the full bag count before the first trip to the store.

Example 3

24×24 slab at 4 inches

This example answers the large-yardage query pattern directly and makes it obvious when a slab stops feeling like a bag-mix project.

Base volume

7.11 yd³

With 10% waste

7.82 yd³

80-lb bags

Approx. 352 bags

Query match

24×24×4 slab

1Area = 24 × 24 = 576 sq ft.
2Thickness = 4 inches = 0.333 ft.
3Base volume = 576 × 0.333 = 192.00 ft³ = 7.11 yd³.
4Add 10% waste to land at about 7.82 yd³, or roughly 352 80-lb bags.

Takeaway: At this scale, ready-mix is usually the more realistic conversation because volume, placement pace, and finish consistency all move together.

Common Mistakes

Where slab estimates usually go wrong

The formula is simple. Most slab estimate problems come from unit confusion, bag-yield confusion, or using the wrong geometry before the concrete is ordered.

Mistake 1

Using square footage without checking slab thickness carefully

People often know the slab area but still treat thickness as an afterthought, which quietly pushes the order low.

Always convert inches to feet before doing manual volume math.
A 4-inch slab and a 5-inch slab can have the same footprint but very different yardage.

This is why the 4-inch, 5-inch, and 6-inch coverage table belongs next to the calculator.

Mistake 2

Switching between yards and bag counts without a yield assumption

Bag totals are only trustworthy when the bag-yield assumption is clear. Otherwise the same slab can look smaller or larger than it really is.

Use one consistent yield assumption when moving from cubic feet to bag counts.
Check whether the supplier label matches the calculator defaults before buying.

The bag-yield reference table is there to keep the live calculator math and the planning math aligned.

Mistake 3

Adding waste only after the bag count feels final

A clean base volume is not the same thing as an order-ready quantity once spillage, uneven subgrade, and cleanup are involved.

Get the base volume first, then add the waste allowance deliberately.
Small slabs can absorb waste too, not just large pours.

The worked examples separate base volume from order-ready volume so the extra material is visible instead of hidden.

Mistake 4

Using the slab page for round holes, piers, or sonotubes

A rectangular slab formula is the wrong fit for cylindrical work, even when the total volume feels similar.

Use slab math for broader flat pours such as patios, walkways, and driveways.
Use the column calculator when the job is a post hole, pier, or sonotube.

Matching the calculator to the geometry is one of the fastest ways to avoid a bad estimate.