Concrete & Masonry

Concrete Calculator: Cubic Yards and Premix Bags

Estimate concrete for a rectangular slab or pad from length, width and thickness.

Project measurements

Change the defaults to match your project and the product you plan to buy.

Project result

Your concrete volume plan

Calculate slab volume before and after extra allowance, then translate it into cubic yards or cubic meters and common premix bag counts.

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Planning estimate only. Verify product coverage, package size, supplier specifications and project requirements before purchasing.

Methodology

Formula & assumptions

Base volume

length × width × thickness

Thickness is converted from inches to feet before multiplying.

Planned volume

base volume × (1 + extra allowance %)

Extra material helps cover subgrade variation, spillage and ordering tolerance.

Cubic yards

planned cubic feet ÷ 27

Ready-mix concrete in the U.S. is commonly ordered by cubic yard.

Premix bags

planned cubic feet ÷ approximate bag yield, rounded up

Always replace approximate yield with the product label when purchasing bagged mix.

How to measure this project

Measure slab length and width at the formwork and use the planned finished thickness. If the excavation or subgrade is uneven, average dimensions can understate the true volume; separate irregular sections or increase the allowance where appropriate.

Before you buy materials

For larger pours, ready-mix delivery is usually planned by volume rather than bag count. Bag yields vary by product and moisture conditions, so the 80 lb and 60 lb counts are planning references only. Structural concrete work should follow project specifications, reinforcement details and local requirements.

Reference

Common slab thickness conversions

Thickness	Feet used in formula	Volume for 100 sq ft
3 in	0.25 ft	25.0 cu ft
4 in	0.333 ft	33.3 cu ft
5 in	0.417 ft	41.7 cu ft
6 in	0.50 ft	50.0 cu ft

Questions

Frequently asked questions

How much extra concrete should I order?

The calculator lets you choose the allowance. Ten percent is a conservative planning default for many small projects, but form accuracy, subgrade variation and supplier practices should guide the final order.

Are the premix bag yields exact?

No. They are planning approximations. Use the yield printed on the specific bag because mix design and package labeling vary.

Can this replace an engineer or concrete contractor?

No. It estimates material volume only and does not address structural design, reinforcement, joints, subgrade, curing or code requirements.

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