Concrete planning estimate

Concrete Slab Calculator

Estimate concrete volume, bag count, gravel base, form boards, reinforcement, and rough material cost for any flat concrete slab. Enter dimensions, choose a waste factor, and get a transparent breakdown.

Open slab calculator How it works

What the concrete slab calculator estimates

Enter your slab dimensions and the calculator produces a full planning breakdown using the same formulas the concrete industry uses. It does not replace engineering or code review.

Concrete volume in cubic yards (imperial) or cubic meters (metric).
40 lb, 60 lb, and 80 lb bag counts rounded up to whole bags.
Ready-mix recommendation for pours over 1 yd³.
Compacted gravel base volume and approximate weight.
Form board count from slab perimeter.
Wire mesh or rebar planning quantities.
Rough material cost range for concrete, gravel, forms, and rebar.

Imperial formula

length (ft) × width (ft) × thickness (in) ÷ 12 = ft³

ft³ ÷ 27 = yd³

Quick rule: sq ft ÷ 81 ≈ yd³ for a 4 in slab.

Metric formula

length (m) × width (m) × thickness (cm) ÷ 100 = m³

Waste factor

Add 5% for simple rectangular slabs. Use 10% for complex shapes, sloped grades, or uneven subgrade. Multiply net volume by (1 + waste%) before ordering.

Slab size	Net yd³	Ordered yd³	80 lb bags	Gravel base
8 × 8 × 4 in	0.79	0.83	38	0.79 yd³
10 × 10 × 4 in	1.23	1.30	59	1.23 yd³
12 × 12 × 4 in	1.78	1.87	85	1.78 yd³
16 × 16 × 4 in	3.16	3.32	151	3.16 yd³
20 × 20 × 4 in	4.94	5.19	236	4.94 yd³
20 × 20 × 5 in	6.17	6.48	295	6.17 yd³

Ordering checklist

Use these estimates as a starting point before calling suppliers or ordering material.

Concrete: ready-mix in cubic yards for pours over 1 yd³; bags for smaller projects.
Compacted gravel or road base at the selected depth.
Form lumber, stakes, duplex nails, and layout string.
Reinforcement: wire mesh or rebar depending on load and code.
Finishing supplies: float, edger, jointing tool, curing compound.
Not included: labor, delivery, permits, excavation, vapor barrier, chairs, or ties.

How much concrete does a 10×10 slab need?

A 10 ft × 10 ft × 4 in slab needs about 1.23 yd³ before waste and about 1.30 yd³ with 5% waste — roughly 59 bags of 80 lb concrete.

How do I calculate concrete volume for a slab?

Multiply length × width × (thickness ÷ 12) to get cubic feet, then divide by 27 to get cubic yards. Add 5–10% for waste before ordering.

How many 80 lb bags of concrete are in a cubic yard?

Each 80 lb bag yields about 0.022 yd³ of mixed concrete, so one cubic yard takes roughly 46 bags. Check the yield on the bag you buy.

When should I use ready-mix instead of bags?

Ready-mix is generally more practical once a project exceeds 1 yd³ (about 46 bags). It saves time and produces more consistent results for driveways, garage slabs, and large patios.

What thickness should a concrete slab be?

Patios, walkways, and shed pads are typically 4 in thick. Driveways are usually 5 in. Heavy vehicle areas may need 6 in. Verify with local code and your soil conditions.

Do I need a gravel base under a slab?

A compacted gravel or road-base layer is standard practice for drainage, leveling, and support. Most residential slabs use 4–6 in of compacted base.

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Planning estimates only

These are rough material estimates for planning purposes. They are not purchasing guarantees, engineering designs, structural recommendations, or code compliance guidance. Verify slab thickness, subbase design, reinforcement, drainage, control joints, curing, and local requirements with a qualified professional before starting any structural or load-bearing work.