PracticalCalcs

Home improvement calculator

Concrete Bag Calculator

Estimate bagged concrete for small rectangular projects using length, width, thickness, bag size, waste allowance, and price per bag.

Estimate concrete bags

Buy estimate

10bags

Base bags
9 bags
Extra bags from waste
1 bags
Concrete volume
5.33 ft³
Volume in cubic yards
0.198 yd³
Estimated bag cost
$65

Formula and methodology

  1. Thickness in feet = thickness in inches / 12.
  2. Cubic feet = length in feet x width in feet x thickness in feet.
  3. Cubic yards = cubic feet / 27.
  4. Base bags = cubic feet / bag yield, rounded up to a whole bag.
  5. Total bags = cubic feet x (1 + waste percentage) / bag yield, rounded up.
  6. Estimated cost = total bags x price per bag.

Worked example

4 ft x 4 ft pad at 4 in thick

  • Length: 4 ft
  • Width: 4 ft
  • Thickness: 4 in
  • Bag size: 80 lb
  • Waste: 10%
  • Bag price: $6.50

Base volume is 5.33 ft³. With 10% waste and 80 lb bags, buy about 10 bags, estimated at $65.

Assumptions

  • The project is rectangular and has a consistent thickness.
  • Default bag yields are 40 lb = 0.30 ft³, 50 lb = 0.375 ft³, 60 lb = 0.45 ft³, and 80 lb = 0.60 ft³.
  • Waste allowance defaults to 10 percent.
  • Price defaults to $6.50 per 80 lb bag and should be replaced with local store pricing.
  • Bag yields vary by brand, mix type, and water amount. Check the bag label before buying.
  • The estimate covers bagged concrete material only; labor, forms, reinforcement, gravel base, tools, and delivery are not included.

Last updated: 2026-06-07

FAQ

Why does the calculator round up bags?

Concrete bags are sold as whole bags, so the calculator rounds up to avoid under-buying.

Are all 80 lb bags exactly 0.60 cubic feet?

No. 0.60 ft³ is a common planning yield, but bag yield can vary by brand and mix type. Check the label before purchasing.

When should I use ready-mix instead of bags?

For larger slabs, ready-mix delivery can be more practical. Compare this bag estimate with the concrete slab calculator's cubic yard result.

This estimate is for planning only. Confirm bag yield, project depth, and site conditions before buying materials.