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Concrete Bag Calculator
Choose a slab or fence posts, enter the size and pick a bag size to see how many bags you need and how much they weigh. Six 10 in × 24 in post holes around 4×4 posts need about 11 bags of 80 lb mix with 10% extra.
Updated · Sources re-checked by
11 bags of 80 lb
880 lb (399 kg) to carry · 6.08 cubic feet · with 10% waste
About 1.7 bags per hole.
| Bag size | Bags | Weight |
|---|---|---|
| 40 lb | 21 | 840 lb |
| 50 lb | 17 | 850 lb |
| 60 lb | 14 | 840 lb |
| 80 lb | 11 | 880 lb |
You can type 10, 10.5, 4 1/2 or 10 ft 6 in. Nothing you type is saved or sent anywhere. This is an estimate; order a little extra.
How the bag calculator works
Bags needed = cubic feet of concrete ÷ the yield of one bag, rounded up. For a slab, cubic feet is length × width × thickness (in) ÷ 12. For a post hole, cubic feet is π × (hole width ÷ 2)² × depth, minus the post itself (post width × post width × depth), all ÷ 1,728 when the sizes are in inches. Total weight is the number of bags × the weight on the bag. The waste percentage is added before the bags are counted.
| Hole (width × depth) | 80 lb bags per hole | 50 lb bags per hole |
|---|---|---|
| 8 in × 24 in | 1 | 1.5 |
| 8 in × 36 in | 1.5 | 2.3 |
| 10 in × 24 in | 1.7 | 2.7 |
| 10 in × 36 in | 2.5 | 4.1 |
| 12 in × 24 in | 2.6 | 4.1 |
| 12 in × 36 in | 3.9 | 6.2 |
| Slab size | 80 lb bags | Weight of those bags |
|---|---|---|
| 2 ft × 2 ft | 3 | 240 lb |
| 3 ft × 3 ft | 6 | 480 lb |
| 4 ft × 4 ft | 10 | 800 lb |
| 5 ft × 5 ft | 16 | 1,280 lb |
| 6 ft × 6 ft | 22 | 1,760 lb |
| Bag size | Yield (cubic feet) | Bags per cubic foot |
|---|---|---|
| 40 lb | 0.3 | 3.33 |
| 50 lb | 0.375 | 2.67 |
| 60 lb | 0.45 | 2.22 |
| 80 lb | 0.6 | 1.67 |
Worked example: six 10 in × 24 in post holes, 4×4 posts
- One hole: π × (10 ÷ 2)² × 24 = 1,885 cubic inches.
- The post takes up room: 3.5 × 3.5 × 24 = 294 cubic inches.
- Concrete in one hole: 1,885 − 294 = 1,591 cubic inches = 0.92 cubic feet.
- Six holes: 0.92 × 6 = 5.52 cubic feet. With 10% extra: 6.08 cubic feet.
- 80 lb bags: 6.08 ÷ 0.6 = 10.1, rounded up to 11 bags. That is 880 lb to carry.
Common questions
How many bags of concrete do I need?
Work out the cubic feet of concrete you need, add a little extra for waste, and divide by the yield printed on the bag. The yield is about 0.6 cubic feet for an 80 lb bag, 0.45 for 60 lb, 0.375 for 50 lb and 0.3 for 40 lb. Round up to a whole bag.
How many bags of concrete for a fence post?
It depends on the hole and the post. A hole 10 in wide and 24 in deep around a 4×4 post needs about 1.7 bags of 80 lb mix per hole with 10% extra. That is 2 bags for one hole, or 11 bags for six holes, because the bags are shared between holes. The table above shows other hole sizes. Post sizes vary, so measure yours.
Should I use 40, 50, 60 or 80 lb bags?
All four sizes make the same amount of concrete per pound, so the choice is about lifting and mixing. Bigger bags mean fewer bags to open but each one is heavy to lift. Smaller bags are easier to carry and to mix in a wheelbarrow. This calculator shows the count for every size.
How much will my bags weigh?
Multiply the number of bags by the weight on the bag. The six post holes in the example need 11 bags of 80 lb mix, about 880 lb. Check what your vehicle, trailer and ramp can safely carry before you load up.
How many bags of concrete make a cubic yard?
A cubic yard is 27 cubic feet, so it takes 45 bags of 80 lb mix, 60 bags of 60 lb, 72 bags of 50 lb or 90 bags of 40 lb.
When should I order ready-mix instead of bags?
Bags suit small jobs you can mix and place yourself. As the bag count grows, mixing gets slow and heavy, and it is hard to keep a large pour consistent. For bigger pours, ask a ready-mix supplier for a quote. Suppliers often have a minimum load.
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Sources
- QUIKRETE Concrete Mix 1101 data sheet (bag yields: 40 lb 0.30, 50 lb 0.375, 60 lb 0.45, 80 lb 0.60 cubic feet) (checked October 26, 2026)
This calculator gives an estimate for planning. Bag yield varies by product, so check the label on your bags. Post and hole sizes depend on your plans and local rules. Order a little extra.