Concrete & Masonry
Concrete Bag Calculator
Enter the size and thickness of a small pour and pick a bag size. You get the bag count for every common size and the total weight to load and carry.
Worked example
- Volume = 6 x 4 x 4/12 = 8 cu ft; with 10% waste 8.8 cu ft.
- A 80 lb bag makes about 0.6 cu ft, so 8.8 / 0.6 = 14.7, rounded up to 15 bags.
Common sizes at a glance
| Size | 80 lb bags | Total weight |
|---|---|---|
| 3 x 3 ft pad, 4 in | 6 | 480 lb |
| 4 x 4 ft pad, 4 in | 10 | 800 lb |
| 4 x 8 ft walk, 4 in | 20 | 1,600 lb |
| 6 x 6 ft pad, 4 in | 22 | 1,760 lb |
| 8 x 10 ft shed base, 4 in | 49 | 3,920 lb |
How bag sizes compare
Premixed concrete is sold by weight, but what matters on site is how much volume each bag makes. An 80 lb bag yields about 0.6 cu ft, a 60 lb bag about 0.45 cu ft, a 50 lb bag about 0.375 cu ft and a 40 lb bag about 0.3 cu ft.
That is why the worked example, a 6 x 4 ft pad at 4 in, needs 15 bags of 80 lb but 30 bags of 40 lb. The concrete is the same; only the packaging changes.
Picking the right bag for your back
Bigger bags mean fewer to open, but each one is heavier to lift into a wheelbarrow or mixer. If you are working alone or the pour is far from where you park, 50 or 60 lb bags are easier to handle and the count difference is small on a little pad.
Keep one bag size for the whole job. Switching halfway makes it easy to lose count of how much water and mix are going into each batch.
Plan the haul
The total weight is the number to check against your vehicle. The 6 x 4 ft example comes to 1,200 lb of dry mix, and an 8 x 10 ft shed base at 4 in is 3,920 lb. Check the payload rating of your truck or trailer before loading it all in one trip.
Store bags off the ground and covered. A bag that has soaked up moisture and gone lumpy will not mix properly.
When bags stop making sense
Bags work well for post holes, small pads, step repairs and short walks. Past a cubic yard or so, the mixing time and the risk of a cold joint start to add up.
If the count climbs high, compare with a truck order using the concrete yard calculator. For fence posts, use the fence post concrete calculator, which subtracts the post from each hole.
Questions
How many 80 lb bags are in a yard of concrete?
About 45, since each bag makes roughly 0.6 cu ft and a yard is 27 cu ft.
How many bags for a 4 x 4 pad?
At 4 in thick with waste, about 10 bags of 80 lb, weighing 800 lb in total.
Why round up instead of to the nearest bag?
A partial bag left in the store does not help you finish the pour. We always round up to whole bags.
Can I mix bags in a wheelbarrow?
Yes, one bag at a time works for small jobs. A rented mixer speeds things up once you are past a dozen bags or so.
How this works: formulas and typical values are listed on our method page; every data source and its license is on sources.
Last reviewed November 2026.