What waste percent should I use?

Many DIY slabs use about 5–10%. Increase waste for irregular forms, rocky subgrade, or first-time pours.

Does bag size matter if I enter yield?

Only the cubic-foot yield matters. An 80 lb bag is useful only if its label matches the yield you enter (often near 0.60 cu ft).

Can I use this for footings or steps?

Yes for rough volume if you enter equivalent rectangular dimensions. Complex shapes need multiple rectangles summed by hand.

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Concrete Calculator

Turn slab dimensions into cubic yards—and a realistic bag count with waste.

Yards + bags

Waste allowance

Bag-yield driven

Converts slab length, width, and thickness into cubic feet and yards, applies an optional waste percent, then divides by bag yield to count bags. Educational estimating only.

Limitation: Educational material estimate only.

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Length (ft)Slab length in feet.Width (ft)Slab width in feet.Thickness (in)Slab thickness in inches (for example 4 for a patio slab).Waste percentExtra allowance for spillage and uneven subgrade (0–100).Bag yield (cu ft)Cubic feet of mixed concrete per bag. Common yields: ~0.30 (40 lb), ~0.45 (60 lb), ~0.60 (80 lb)—confirm on the bag.

How to Estimate Concrete

Measure the rectangle, enter thickness in inches, set waste, and use the yield printed on your bags.

Enter slab length and width in feet.

Prefer ready-mix for large pours

Bag math is for small pads and repairs. Large slabs are usually ordered by the cubic yard from a ready-mix plant.

What Cubic Yards and Bags Mean

The primary result is cubic yards after waste. Bag count is that volume divided by the yield you entered, rounded up.

Worked Concrete Examples

10×10 ft patio, 4 in thick

Inputs

lengthFt:10

widthFt:10

thicknessIn:4

wastePercent:10

bagYieldCuFt:0.6

Yards with waste / bags

1.358 cu yd with waste; 62 bags

100 sq ft × 4/12 ft = 33.333 cu ft ≈ 1.235 cu yd; +10% waste ≈ 1.358 cu yd; ÷ 0.60 cu ft/bag → 62 bags.

Small pad with 60 lb bags

Inputs

lengthFt:4

widthFt:4

thicknessIn:4

wastePercent:5

bagYieldCuFt:0.45

Yards with waste / bags

0.207 cu yd with waste; 13 bags

16 × 4/12 = 5.333 cu ft; +5% waste; ÷ 0.45 → 13 bags.

Check the bag label

Yields differ by product. If the label lists coverage for a 4-inch slab, convert that coverage into cubic feet before using bag yield.

Frequently Asked Questions

27. The calculator divides cubic feet by 27 to report yards.

Still have questions about this calculation?

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Concrete Volume Formulas

Uses rectangular prism volume only. Odd shapes should be split into rectangles first.

Formula

L × W × (T ÷ 12)

Cubic feet

L × W × (T ÷ 12)

Cubic yards

cu ft ÷ 27

With waste

yards × (1 + waste%/100)

Bags

ceil(waste cu ft ÷ bag yield)

Scientific Background

Assumptions: rectangular slab, uniform thickness, user-supplied bag yield. Limitations: no reinforcement, slump, temperature, or pump loss modeling.

Sources & review

Concrete Calculator documents the references behind its formulas and assumptions. Always treat results as educational estimates, not personalized professional advice.

Last reviewed

July 22, 2026

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