Concrete Slab Calculator

A slab's length, width and thickness go in, and a concrete slab calculator returns the volume in cubic yards, a ready-mix order rounded up to the nearest quarter yard, and the bag count for 40, 60 or 80 lb bags. It converts thickness from inches to feet and adds a 5% waste allowance by default.

Last reviewed September 2026Method & sources Report an error

Not to scale.

Concrete Needed (with waste)

1.30 cu yd

Includes a 5% waste allowance.

Shopping list

Concrete mix59 80 lb bagsRounded up from 58.3 bagsOr 1.5 cu yd of ready-mix delivered.
Either

Bagged vs. Ready-Mix

Borderline range: bagged concrete works, but that's a lot of bags to mix by hand in one pour. Call a local ready-mix plant for a short-load quote before deciding - many will deliver this much, sometimes with a small short-load fee.

Breakdown
Concrete Needed (with waste)	1.30 cu yd
Total Volume (with waste)	35.0 cu ft
Bags Needed	59 bags (80 lb)
Ready-Mix Order Size	1.50 cu yd
Estimated Cost (Bags)
Estimated Cost (Ready-Mix)

How to Calculate Concrete Slab

The calculator converts the size of your pour into a volume. It then expresses that volume the two ways concrete is actually sold: cubic yards for a ready-mix truck, or a bag count for a DIY hand-mix. Volume = Length × Width × (Thickness ÷ 12), in cubic feet, since thickness is entered in inches. A waste allowance is then added to cover subgrade irregularity, spillage and placement loss. NRMCA's own guidance for ordering ready-mixed concrete is 4% to 10% more than the plan-dimension estimate. This calculator defaults to 5%, with a range up to 20% for irregular or heavily-formed pours. The result is divided by 27 (cubic feet per cubic yard) for the ready-mix figure, rounded up to the nearest quarter yard since that's how plants actually sell it. For the bag count, the same waste-inclusive volume is divided by the bag's published yield. That yield is 0.60 cu ft for an 80 lb bag, 0.45 cu ft for 60 lb and 0.30 cu ft for 40 lb, per the Quikrete and Sakrete concrete mix data sheets. The result is rounded up, since a partial bag isn't purchasable.

Example: 10 ft × 10 ft patio slab, 4 inches thick

A 10 ft × 10 ft patio slab, 4 inches thick, at the default 5% waste allowance, using 80 lb bags: 1. Thickness in feet: 4 ÷ 12 = 0.333 ft 2. Raw volume: 10 × 10 × 0.333 = 33.3 cubic feet 3. With 5% waste: 33.3 × 1.05 = 35.0 cubic feet 4. Cubic yards: 35.0 ÷ 27 = 1.30 cubic yards 5. Ready-mix order size, rounded up to the nearest ¼ yard: 1.50 cubic yards 6. Bags (80 lb, 0.60 cu ft each): 35.0 ÷ 0.60 = 58.3, rounded up to 59 bags At just over 1 cubic yard, this pour sits in the calculator's "borderline" range. 59 bags is a full day of mixing for one person, so it's worth calling a local ready-mix plant for a short-load price before committing to bags.

Concrete Slab: Compare Scenarios

Scenario	Concrete Needed (with waste)	Total Volume (with waste)	Bags Needed	Ready-Mix Order Size
Small (8×8 ft shed pad, 4 in.)	0.83 cu yd	22.4 cu ft	38 bags (80 lb)	1.00 cu yd
Medium (10×10 ft patio, 4 in.)	1.30 cu yd	35.0 cu ft	59 bags (80 lb)	1.50 cu yd
Large (20×24 ft driveway apron, 5 in.)	7.78 cu yd	210.0 cu ft	350 bags (80 lb)	8.00 cu yd

What Is Concrete Slab?

A concrete slab calculator turns the dimensions of a pour into two purchasable quantities. The first is cubic yards, which is what a ready-mix truck is loaded and priced by. The second is a bag count, which is what you'd buy off a shelf to mix by hand or with a rented mixer. It exists because the raw geometry (a volume in cubic feet) isn't the unit either supplier sells in. Getting the order wrong in either direction is expensive. Too little means a second trip or a visible cold joint where an old pour meets a new one. Too much means paying for concrete, and often a disposal fee, you didn't need.

Concrete Slab: Pros and Cons

Pros

Applies the NRMCA-recommended 4-10% overage automatically instead of leaving waste to guesswork
Shows both the bagged-mix and ready-mix path side by side, so you can compare cost against labor before you order
Rounds to what you can actually buy: whole bags, and the quarter-yard increments a ready-mix plant sells in, not a raw decimal

Cons

Assumes a flat, uniformly thick, rectangular pour on a solid, compacted subgrade. An uneven base, a sloped grade, or a thickened perimeter footing needs more concrete than the geometry alone suggests
Does not size reinforcement (rebar or wire mesh), control joints, or base/gravel depth - those are separate calculations
The cost estimate is only as good as the price you enter. Concrete pricing varies by region, mix strength (PSI) and delivery distance, and this calculator has no built-in pricing data

Concrete Slab: What a Pro Checks

Plan control joints before the pour, not after. NRMCA recommends spacing them 24 to 36 times the slab's thickness, so a standard 4 in. slab should get joints roughly every 10 ft and never more than 15 ft apart.
Cut contraction joints within about 4 to 12 hours of finishing (sooner for early-entry dry-cut blades). Waiting too long lets uncontrolled cracks form first, before the planned joint can control where they happen.
Keep each slab panel roughly square. NRMCA recommends panel length not exceed 1.5 times its width, since a long, narrow panel cracks in the middle far more often than a square one does.

Frequently Asked Questions

How much extra concrete (waste) should I order?

The National Ready Mixed Concrete Association recommends ordering 4% to 10% more than your plan-dimension estimate, to cover subgrade irregularity, spillage and placement loss. This calculator defaults to 5%; use closer to 10% for an irregular shape, a rough or unconfirmed subgrade, or a pumped placement.

How many 80 lb bags of concrete make a cubic yard?

About 45 bags, since an 80 lb bag yields roughly 0.60 cubic feet and a cubic yard is 27 cubic feet (27 ÷ 0.60 ≈ 45). A 60 lb bag (0.45 cu ft) takes about 60 bags per cubic yard, and a 40 lb bag (0.30 cu ft) takes about 90.

Should I use bagged concrete or order ready-mix delivery?

Under about 1 cubic yard, bagged concrete is usually more practical - no delivery minimum and no truck access needed. Above about 3 cubic yards, ready-mix is almost always cheaper and dramatically less labor. In between, it depends on how much hand-mixing labor you're willing to do versus a possible short-load delivery fee. Call a local plant and compare.

Does this calculator include a footing, or just the flat slab?

No. It assumes one uniform thickness across the whole area. If your slab has a thickened edge or a separate perimeter footing, calculate that volume on our Concrete Footing calculator and add it to this result.

What thickness should I use?

4 inches is standard for a patio, sidewalk, or shed slab under normal foot traffic. Driveways and anything carrying vehicle weight typically use 5-6 inches or more, often with reinforcement. Check your local building code, since minimum thickness requirements vary by jurisdiction.

Why does the ready-mix order size round up to a quarter yard?

Ready-mix plants measure and price in quarter-cubic-yard increments, so a raw figure like 1.30 cubic yards isn't an orderable amount - this calculator rounds it up to 1.50 so the number on screen is the number you can actually phone in.

Sources

Concrete & Masonry