Concrete Bag Calculator

Calculate how many bags of concrete you need. Enter dimensions, choose bag weight (40, 60, or 80 lb), and optionally enter your cost per bag for a budget estimate.

ft

Length of the slab, footing, or area to fill.

ft

Width of the slab, footing, or area to fill.

in

Thickness in inches. Common: 4" slab, 6" driveway, 8-12" footing.

lb

Bag size to purchase. 80 lb is most common for large projects.

$

Optional: enter the price per bag for a cost estimate. No fabricated prices.

%

Extra for spillage, overpour, uneven subgrade. Default 10%.

How this calculator works

This concrete bag calculator focuses on bag-count estimation:

  1. Volume: length × width × (depth / 12) = cubic feet.
  2. Bag yield: each bag yields (bag weight / 133.4) cubic feet of cured concrete. 80 lb → 0.599 cu ft, 60 lb → 0.449 cu ft, 40 lb → 0.300 cu ft.
  3. Bags needed: ceil(volume with waste / bag yield).
  4. Cost estimate: bags × your input price per bag (no fabricated prices).

The 133.4 lb/cu ft constant is the density of cured concrete — derived from the weight of cement, sand, aggregate, and water in a standard mix.

Formula

volume_cuft = length × width × (depth / 12)
volume_with_waste = volume_cuft × (1 + waste_pct / 100)
bag_yield = bag_weight / 133.4  (lb per cu ft of cured concrete)
bags = ceil(volume_with_waste / bag_yield)
total_cost = bags × cost_per_bag  [if cost_per_bag > 0]

Example

For a 10 × 10 × 4" slab with 80 lb bags and 10% waste:

  • volume_cuft = 10 × 10 × (4/12) = 33.33 cu ft
  • volume_with_waste = 33.33 × 1.10 = 36.67 cu ft
  • bag_yield (80 lb) = 80 / 133.4 = 0.599 cu ft
  • bags = ceil(36.67 / 0.599) = 62 bags
  • Cost at $5/bag: 62 × $5 = $310

Important assumptions

  • Concrete density: 133.4 lb per cubic foot (standard mix). This is a derived constant — bag yields match manufacturer specs.
  • 80 lb bag yields ~0.60 cu ft. 60 lb bag yields ~0.45 cu ft. 40 lb bag yields ~0.30 cu ft.
  • 10% waste factor covers spillage, overpour, and uneven subgrade.
  • Cost estimate uses YOUR input price — no fabricated market averages. Check local prices at Home Depot, Lowes, or your building supply store.
  • For pours over 1 cubic yard (27 cu ft), consider ready-mix delivery — often cheaper and saves labor.

FAQ

How many 80 lb bags of concrete do I need for a 10×10 slab?
A 10×10 slab at 4" thick needs ~33.33 cu ft. With 10% waste: 36.67 cu ft. Each 80 lb bag yields 0.599 cu ft. Bags needed: ceil(36.67/0.599) = 62 bags.
How many 60 lb bags equal an 80 lb bag?
An 80 lb bag yields 0.599 cu ft; a 60 lb bag yields 0.449 cu ft. So 1.33 sixty-pound bags ≈ 1 eighty-pound bag in coverage.
Should I use 40 lb, 60 lb, or 80 lb bags?
80 lb bags are most cost-effective for large projects (fewer bags, less packaging). 60 lb bags are a good middle ground. 40 lb bags are easiest to carry but you'll need more of them — best for small repairs.
How much does a bag of concrete cost?
Prices vary by location and supplier. This calculator lets YOU enter the price per bag — it does not use fabricated averages. Check Home Depot, Lowes, or your local building supply store for current pricing.