How thick should a concrete slab be?

4 in (100 mm) for patios, walkways and shed bases on well-compacted ground; 5 in for single-car driveways; 6 in where pickups, RVs or machinery will park. Going from 4 to 6 inches adds 50% to the concrete volume, so match thickness to the actual load, not habit.

Do I need gravel under a concrete slab?

On free-draining sandy soil, you can pour on compacted native ground. Everywhere else, 4 in of compacted crusher-run gravel gives uniform bearing, drainage and a capillary break. The IRC (R506.2.2) requires a 4-inch base course except in Group I well-drained soils.

How much does a 20×20 slab cost?

A 20 × 20 ft slab at 4 in needs about 5.5 yd³ including waste. At $140–170/yd³ delivered, that's $770–940 for concrete alone; with base gravel, forms, mesh and finishing labor, installed prices typically land between $2,400 and $4,800. See our concrete cost guide for the full breakdown.

Should slab thickness include the wire mesh or rebar?

No — reinforcement sits inside the slab (mesh at mid-depth, rebar on chairs), not on top of it. Calculate volume from formwork height. What matters is cover: keep steel at least 1.5 in from the slab surface and 3 in from ground contact per ACI 318.

Can I pour a slab in sections?

Yes — large slabs are routinely poured in alternating strips with construction joints. Each section becomes its own volume calculation. Plan joint locations first: control joints should divide the slab into panels no larger than about 24–36 times the thickness in inches (roughly 8–12 ft squares for a 4-inch slab).

Concrete · Slabs

Concrete Slab Calculator

Pick a code-appropriate thickness, enter your slab dimensions, and get yards, bags and weight. The thickness dropdown tells you what each option is rated for.

Formula

V = Area × Thickness × (1 + waste)

Area: slab length × width (ft² or m²)
Thickness: uniform slab depth (ft or m)
waste: 0.05–0.15 depending on base quality

A 4-inch slab uses 1 yd³ per 81 ft² of area. That single ratio lets you sanity-check any slab quote in your head.

Concrete Slab Calculator

Sized for patios, garage floors, shed bases and driveways. Includes a waste allowance and bag counts.

Choosing the slab thickness

Thickness is the single biggest cost lever — and the most common overspecification.

Residential guidance consistent with IRC Ch. 5 and PCA recommendations. Local codes govern.
Application	Recommended thickness	Notes
Walkways, shed bases	4 in (100 mm)	Foot traffic and garden equipmentMesh optional on good base
Patios	4 in (100 mm)	Furniture, foot trafficSlope 2% away from house
Single-car driveway	5 in (125 mm)	Cars and light SUVs#3 rebar 24 in o.c. or fiber mix
Garage floor / RV pad	6 in (150 mm)	Trucks, lifts, machinery#4 rebar 18 in o.c. typical
Commercial / loading	8 in+ (200 mm+)	Forklifts, delivery trucksRequires engineered design

Formula and what it assumes

The calculator above assumes a uniform-thickness slab on grade. Three situations need manual adjustment: thickened edges (add a footing-shaped volume around the perimeter — see the example below), slopes poured to falls (use the average of the high and low thickness), and monolithic slab-and-footing pours (calculate the slab and the footing separately, then add them).

Worked example: garage slab with thickened edge

Worked example

A 24 × 24 ft garage slab, 6 in thick, with a 12 in wide × 12 in deep thickened edge around the full perimeter (monolithic pour).

1
Main slab volume
24 × 24 × 0.5 ft = 288 ft³
2
Perimeter length
4 × 24 = 96 ft
3
Thickened edge (extra 6 in below slab, 1 ft wide)
96 × 1.0 × 0.5 = 48 ft³
4
Total + 10% waste
(288 + 48) × 1.10 = 369.6 ft³
5
Convert to yards
369.6 ÷ 27 = 13.7 yd³

Result: Order 14 yd³ — the thickened edge added 2 full yards over a flat slab.

Mistakes that ruin slab estimates

Slab questions, answered

How thick should a concrete slab be?: 4 in (100 mm) for patios, walkways and shed bases on well-compacted ground; 5 in for single-car driveways; 6 in where pickups, RVs or machinery will park. Going from 4 to 6 inches adds 50% to the concrete volume, so match thickness to the actual load, not habit.
Do I need gravel under a concrete slab?: On free-draining sandy soil, you can pour on compacted native ground. Everywhere else, 4 in of compacted crusher-run gravel gives uniform bearing, drainage and a capillary break. The IRC (R506.2.2) requires a 4-inch base course except in Group I well-drained soils.
How much does a 20×20 slab cost?: A 20 × 20 ft slab at 4 in needs about 5.5 yd³ including waste. At $140–170/yd³ delivered, that's $770–940 for concrete alone; with base gravel, forms, mesh and finishing labor, installed prices typically land between $2,400 and $4,800. See our concrete cost guide for the full breakdown.
Should slab thickness include the wire mesh or rebar?: No — reinforcement sits inside the slab (mesh at mid-depth, rebar on chairs), not on top of it. Calculate volume from formwork height. What matters is cover: keep steel at least 1.5 in from the slab surface and 3 in from ground contact per ACI 318.
Can I pour a slab in sections?: Yes — large slabs are routinely poured in alternating strips with construction joints. Each section becomes its own volume calculation. Plan joint locations first: control joints should divide the slab into panels no larger than about 24–36 times the thickness in inches (roughly 8–12 ft squares for a 4-inch slab).

Pour isn't a simple rectangle?

The general concrete calculator handles multiple pours and custom depths in one order.

Use the Concrete Calculator

Keep planning your slab

Sources & references

[1]2021 International Residential Code, Ch. 4: Foundations — International Code Council, 2021
[2]ACI 318-19: Building Code Requirements for Structural Concrete — American Concrete Institute, 2019
[3]Design and Control of Concrete Mixtures, 17th ed. — Portland Cement Association, 2021