Fast Setting Concrete for Posts: How to Use It Right
Fast setting concrete for posts is a bagged mix designed to be poured dry into a post hole and then soaked with water, with no mixing needed. It typically sets hard in about 20 to 40 minutes, so you can move on to the next post quickly, and fencing or light loads can often be added after a few hours. It works well for fence posts, mailbox posts, and sign posts. It is not intended for slabs or large footings.
Concrete calculator →How to use it
- Dig the hole about 3 times the post width and deep enough for the post, often one third of its length or below frost depth.
- Add 3 to 4 inches of gravel in the bottom for drainage.
- Set and brace the post plumb.
- Add water first, about a quarter to a third of the hole, per bag instructions.
- Pour the dry mix into the water until the hole is filled to a few inches below grade.
- Let the water soak in; do not mix unless the bag says to.
- Wait about 20 to 40 minutes before releasing braces; about 4 hours before heavy loads.
Some brands reverse the order or ask you to add water after pouring dry mix. Follow the bag.
How many bags per post
| Post | Hole | Approx. 50-lb fast-set bags |
|---|---|---|
| 4x4 fence | 10 in × 24 in | 2 to 3 |
| 4x4 fence | 12 in × 36 in | 4 to 5 |
| 6x6 | 12 in × 36 in | 4 to 5 |
| 2-3/8 in steel | 8 in × 24 in | 1 to 2 |
A 50-pound bag yields about 0.375 cubic feet. See how many bags of concrete per fence post.
Fast set vs regular concrete
| Feature | Fast setting | Regular bagged |
|---|---|---|
| Mixing | Dry pour plus water | Mix in tub or mixer |
| Set time | 20 to 40 min | Hours |
| Yield per bag | Lower | Higher |
| Best for | Posts | Pads, slabs, footings |
Tips for good results
- Use enough water; dry pockets weaken the footing.
- Crown the top so rain sheds away from the post.
- On wood posts, stop concrete just below grade and top with soil or gravel.
- In hot weather, work faster; in cold weather, setting slows.
When to use regular concrete instead
For large footings, deck posts that need inspection, basketball poles, or anything over a few cubic feet, mix regular concrete or order ready-mix. Regular mixes are designed for uniform strength in larger volumes.
Bottom line
Fast setting concrete is ideal for fence and mailbox posts: water in the hole, dry mix poured in, and a set in 20 to 40 minutes. Plan 2 to 5 bags per typical post, follow the bag’s water instructions, and use regular concrete for bigger footings.
Questions people ask
Do you mix fast setting concrete for posts?
Usually not. Most are poured dry into water in the hole. Follow the bag directions.
How long does fast setting concrete take to set?
About 20 to 40 minutes for most products.
How many bags of fast setting concrete per post?
Often 2 to 5 bags of 50-pound mix, depending on hole size.
When can I hang a fence on posts set in fast concrete?
Many brands say about 4 hours before heavy loads.
Can I use fast setting concrete for a slab?
It is not designed for slabs. Use regular concrete or ready-mix.
Related guides
- How Many Bags of Concrete per Fence Post?
- How Much Concrete for a Mailbox Post? Bags and Depth
- Footing Depth and the Frost Line: How Deep to Dig
- Ready-Mix vs Bagged Concrete: Which Is Cheaper?
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