Concrete Bag Calculator
How many 40-, 60-, or 80-lb bags for your project
What is concrete bag calculator?
A concrete bag calculator translates project dimensions into the number of bags you need to buy. The math is volume ÷ bag yield, where bag yields come straight from the manufacturer’s data sheet: 0.60 ft³ per 80-lb bag, 0.45 ft³ per 60-lb bag, and 0.30 ft³ per 40-lb bag of standard concrete mix.
Why bag yields aren’t 80/150 lb/ft³: cured concrete weighs about 150 lb/ft³, so a naive read says one 80-lb bag should yield 0.53 ft³. The actual yield is 0.60 ft³ because wet mix incorporates water and air voids that compact during cure. The published yields are empirical — measured from actual pours, not derived. Quikrete and Sakrete both publish identical numbers.
Bag size economics. 80-lb bags are the cheapest per cubic yard at every retailer. 60-lb bags cost about 30% more per yard. 40-lb bags cost about double. The trade-off is lift weight: 80 lb is at the top of what most people can lift comfortably for 50 reps in a row. If you have a bad back, 60-lb is a reasonable upgrade — and the cost difference on a 1-bag job is negligible.
The 1-cubic-yard rule. Past about 1 cu yd (~50 of 80-lb bags), ready-mix delivery beats bags on every metric: price, time, consistency, and the absence of cold joints between batches. Most ready-mix companies have a 1-yard minimum and a short-load fee for under 3 yards, but it’s still cheaper than buying and mixing bags by hand.
Always round up by 1-2 bags. Stores accept unopened returns; they don’t deliver mid-pour. The 10% waste factor in this calculator covers most spillage, but rough subgrade or rebar interference can push real waste to 15%.
When to use a concrete bag calculator
- Hardware store run — You know your slab dimensions but not how many bags fit on the cart. Pick your bag size; the calculator returns the count, including the 10% waste factor.
- Bag size comparison — Switch between 40, 60, and 80 lb bags to see the count change. 80-lb bags are the cheapest per yard but the heaviest to lift.
- Ready-mix decision — Past about 50 of 80-lb bags (~1 cu yd), ready-mix delivery beats bags on price and time. The output flags this threshold.
How to use the Concrete Bag Calculator
- Pick the shape — Slab/Pad for flat pours, Footing for rectangular footings, Column for sonotubes, Wall for vertical pours.
- Pick the bag size — 80-lb bags = 0.6 ft³ each. 60-lb = 0.45 ft³. 40-lb = 0.3 ft³. Yields are identical between Quikrete, Sakrete, and store brands.
- Read the bag count — The output shows total cubic yards plus the bag count, including waste. If the count is over 50 of 80-lb, consider ready-mix instead.
- Buy 1-2 extra bags — Stores will take back unopened bags, but they won't deliver mid-pour. Round up — running 2 bags short on a slab pour is a disaster.
Worked examples
Small repair: 1 ft × 1 ft × 4 in patch
Input: Slab tab: 1 ft × 1 ft × 4 in
Output: 0.01 cu yd / 1 of 80-lb bagPatio: 10 × 10 ft × 4 in
Input: Slab tab: 10 ft × 10 ft × 4 in
Output: 1.23 cu yd / 56 of 80-lb bags or 75 of 60-lbFooting: 12 × 12 × 12 in
Input: Footing tab: 1 ft × 12 in × 12 in
Output: 0.04 cu yd / 2 of 80-lb bags