Rectangular flatwork

Concrete slab calculator

Enter the slab dimensions to estimate concrete volume, ready-mix yards, 40/60/80 lb bags, waste, and material cost.

Enter project dimensions

Use the finished dimensions inside your forms.

Measurements

Imperial feet + inches Metric meters + centimeters

Length (ft or m)

Width (ft or m)

Thickness (in or cm)

Identical sections

Order assumptions

Waste allowance

Ready-mix price (optional)

Bag size for cost

Price per bag (optional)

Prices are your inputs, not local market estimates. Bag yields are approximate and vary by product.

How it works

Concrete slab formula

volume = length × width × thickness × number of sections

The calculator converts every dimension to feet before multiplying. It then adds the selected waste percentage and divides cubic feet by 27 to report cubic yards.

For bagged mix, the waste-adjusted volume is divided by the approximate yield per bag and rounded up to a whole bag.

Measure well

How to measure a slab

Measure the inside length and width of the forms, not the outside edges.
Use the planned finished thickness. Check several points if the subbase varies.
Split an irregular slab into rectangles and calculate each section separately.
Use identical sections only when their length, width, and thickness truly match.

Worked example

12 ft × 10 ft slab at 4 in thick

Measured volume12 × 10 × (4 ÷ 12) = 40.00 ft³

Add 10% waste40.00 × 1.10 = 44.00 ft³

Convert to yards44.00 ÷ 27 = 1.63 yd³

80 lb bags44.00 ÷ 0.60 = 73.34, rounded up to 74 bags

Assumptions and limitations

The slab is treated as a uniform rectangular prism.
Waste covers ordinary spillage, uneven grade, and minor measurement variation; it does not replace a site assessment.
Bag yields are approximate. Product, water content, mixing, and consolidation affect actual yield.
Ready-mix and bag prices include material only unless your entered quote says otherwise.

Before ordering

Confirm finished grade, subbase compaction, form dimensions, reinforcement, joint plan, concrete strength, access, and supplier minimums. For structural slabs, follow the plans and local requirements.

Bag yield reference

Approximate bags per cubic foot

Planning values used by this calculator for general-purpose concrete mix
Bag size	Approximate yield	Bags for at least 1 ft³
40 lb	0.30 ft³	4 bags
60 lb	0.45 ft³	3 bags
80 lb	0.60 ft³	2 bags

Approximate yields follow published data for QUIKRETE Concrete Mix 1101. The selected product label controls, and every order rounds up to whole bags.

Common questions

Concrete slab calculator FAQ

How much extra concrete should I order?

Ten percent is a common planning allowance for modest site and measurement variation, but the right amount depends on subgrade accuracy, formwork, placement method, and supplier rules. Use the editable waste field.

Should I round cubic yards?

The calculator keeps two decimals so you can see the math. Your ready-mix supplier may sell in specific increments or impose a minimum load, so confirm the final order quantity with the supplier.

When should I use bags instead of ready-mix?

Bagged mix can suit small, accessible placements. Larger pours often require too many bags to mix and place consistently. Consider crew size, mixer capacity, working time, and delivery access—not cost alone.