Concrete Bags Required Calculator

Work out concrete bags required instantly with clear inputs, formula shown and shareable results.

About Concrete Bags Required Calculator

Slab volume is length times width times thickness, with thickness converted from inches to feet. Bagged concrete states a yield in cubic feet per bag, so dividing the volume by that yield gives the bag count. A five percent allowance covers spillage, uneven subgrade and over-excavation.

Formulas

Slab volume

ft³ = length x width x thickness in / 12

Bag count

Bags = volume x 1.05 / yield per bag

yield: 0.60 ft³ for an 80 lb bag, 0.45 ft³ for 60 lb, 0.375 ft³ for 50 lb

Worked example

With the default values below, Concrete Bags Required Calculator returns Bags required of 59. Change any input above and every figure here is recalculated in your browser.

Inputs used

Slab lengthminimum 0.1 ft: 10 ft
Slab widthminimum 0.1 ft: 10 ft
Slab thicknessminimum 0.5 in: 4 in
Yield per bagminimum 0.05 ft³: 0.6 ft³

Results

Bags required: 59 bags
Concrete volume: 33.33 ft³
Volume in cubic yards: 1.235 yd³
Volume including 5 percent waste: 35.00 ft³

Step by step

1Values used
2Slab volume
3Bag count
4Bags required= 59 bags
5Concrete volume= 33.33 ft³
6Volume in cubic yards= 1.235 yd³
7Volume including 5 percent waste= 35.00 ft³

Assumptions and limits

The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

How is the Concrete Bags Required figure worked out?

ft³ = length x width x thickness in / 12. Slab volume is length times width times thickness, with thickness converted from inches to feet.

When should I switch to ready mix?

Above about one cubic yard, roughly 45 eighty-pound bags, mixing on site becomes slower and less consistent than ordering a delivery.

What values do I need to enter?

This calculator takes 4 inputs: Slab length, Slab width, Slab thickness, Yield per bag. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.