Concrete Bags Calculator
Estimate how many bags of pre-mixed concrete a project needs. Enter the volume of concrete to place and a waste allowance, choose your bag size, and the calculator returns the whole number of bags to buy — always rounding up, because you cannot buy part of a bag. It also compares 40, 60 and 80 lb bags so you can pick the size that costs least for your job.
Calculate
Default result: 28
Result
Bags needed
- Exact (before rounding up)
- 27.5
- 80-lb bags
- 28
- 60-lb bags
- 37
- 40-lb bags
- 55
How to calculate
Enter the concrete volume (in cubic feet or cubic meters) and a waste percentage, then pick the bag size you plan to buy. The big number is the bags needed for that size. The comparison rows show how many 40, 60 and 80 lb bags the same job would take, and the exact figure shows the unrounded count so you can see how close you are to the next bag.
bags = ceil( volume_ft³ × (1 + waste / 100) ÷ yield ), where yield is the cubic feet of mixed concrete per bag: 0.60 ft³ for an 80-lb bag, 0.45 ft³ for a 60-lb bag, and 0.30 ft³ for a 40-lb bag. The result is always rounded up to a whole bag.
Example calculation
For 15 cubic feet of concrete with a 10% waste allowance you need 16.5 cubic feet of mix. At the 80-lb bag yield of 0.60 cubic feet per bag that is 16.5 ÷ 0.60 = 27.5 bags, which rounds up to 28 bags. The same job takes 37 sixty-pound bags or 55 forty-pound bags.
- bagsPrimary
- 28 bags
- exactBags
- 27.5
- bags80
- 28
- bags60
- 37
- bags40
- 55
Assumptions
- Bag yields are the manufacturer-standard nominal values: 0.60 ft³ (80 lb), 0.45 ft³ (60 lb), 0.30 ft³ (40 lb). Actual yield varies slightly with brand, mix, and how much water you add.
- The bag count is always rounded up — you cannot buy a fraction of a bag — so the exact (pre-round) figure is shown separately to explain the rounding.
- The waste allowance defaults to 10% to cover spillage and an uneven subgrade; 5–10% is common. Set it to 0 to size for the exact volume.
- The 40/60/80-lb comparison helps you choose a size: fewer large bags are usually cheaper per cubic foot but heavier to handle.
- This calculator takes a volume directly. To get the volume from slab dimensions, use the Concrete Slab calculator first.
Common mistakes
- Rounding the bag count down or to the nearest whole number. A job needing 27.5 bags needs 28 — running short means a second trip and a cold joint in the pour.
- Forgetting a waste allowance. Spillage and an uneven base mean the placed volume is a little more than the nominal volume, so most jobs add 5–10%.
- Assuming every bag size yields the same. An 80-lb bag yields twice the volume of a 40-lb bag, so the bag count changes with the size you buy.
Frequently asked questions
How many 80-lb bags of concrete are in a cubic yard?
A cubic yard is 27 cubic feet. At 0.60 cubic feet per 80-lb bag that is 27 ÷ 0.60 = 45 bags, rounded up. Enter 27 cubic feet (or 1 cubic yard) to confirm for your own waste allowance.
Why does the calculator always round up?
You cannot buy part of a bag, and running short mid-pour creates a weak cold joint. The calculator rounds the bag count up to the next whole bag and shows the exact pre-round figure so you can see how much extra that adds.
Should I use a waste allowance?
Yes, for most jobs. Spillage, an uneven subgrade, and over-excavated edges mean you place a little more concrete than the nominal volume. A 5–10% allowance is typical; set it to 0 if you want the exact volume only.
Is it cheaper to buy fewer large bags or more small bags?
Larger bags are usually cheaper per cubic foot, but they are heavier to lift and mix. The comparison rows show the bag count for all three sizes so you can weigh cost against handling for your project.