The short answer
Use fast-setting post mix for ordinary line posts when you want to keep working the same day: it goes in without a mixer and firms up quickly. Use regular concrete, mixed in a barrow or mixer, for gate posts, corner and end posts, very large holes and anything that will carry a heavy or swinging load. Many fences use both: fast-setting for the run, regular concrete where the strain is.
How they differ
| Fast-setting post mix | Regular concrete mix | |
|---|---|---|
| How it goes in | Usually placed dry in the hole and soaked, or poured into water in the hole, as the label directs | Mixed with a measured amount of water first, then shoveled in |
| Firm enough to stop bracing | Often within an hour; many labels say roughly 20 to 40 minutes | Usually several hours; leave braces on overnight |
| Before hanging rails or panels | Commonly a few hours, per the label | Commonly a day or more, per the label |
| Working time | Short, so you must plumb the post fast | Longer, so you can adjust the post |
| Best for | Line posts, repairs, small jobs | Gate, corner and end posts, large or deep holes |
| Common bag size | 50 lb (about 0.375 cu ft) | 60 or 80 lb (about 0.45 or 0.60 cu ft) |
These times are typical ranges, not promises. Each product's label gives its own setting and loading times, and cold weather stretches all of them. Below about 50°F (10°C) both kinds set slowly; never place concrete in frozen ground.
Why gate and corner posts get regular concrete
A gate post takes a twisting, swinging load every day, and corner and end posts carry the pull of the whole fence line. Regular mix gives you time to tamp it thoroughly around the post, check plumb in two directions and fine-tune the height before it stiffens. Mixing it to an even consistency also avoids dry pockets, which are more likely when a large hole is filled with dry powder. If the label of a fast-setting product says it is suitable for gate posts, follow its directions, and consider a wider, deeper hole for the extra load.
Setting a post with fast-setting mix
- Dig the hole and add 4 to 6 in of gravel. Tamp it firm.
- Stand the post on the gravel, plumb it and brace it in two directions before any mix goes in. You will not have time later.
- Add water and mix in the order the label gives. Some brands want water in the hole first, others want the powder first and water poured over it.
- Do not stir unless the label says to. Check plumb right away, then leave it alone.
- Wait the label time before you remove braces and the longer label time before hanging rails, panels or a gate.
Setting a post with regular concrete
- Prepare the hole and gravel base the same way, and brace the post.
- Mix the concrete with the amount of water on the bag. Too much water makes it easier to pour but weaker once cured.
- Shovel it in, working a stick or bar up and down to remove air pockets.
- Check plumb and height again, and adjust while the mix is still soft.
- Leave the braces on overnight, and wait the label time before loading the post. Concrete keeps gaining strength for weeks, so avoid hanging a heavy gate on day one.
The gravel base
A layer of compacted gravel under a wood post lets water drain away from the end grain instead of collecting in a concrete cup around it. It also gives the post a firm, level seat. Use the same base under either type of mix. Steel posts do not rot, but a gravel base still helps drainage and makes it easy to set the post height.
Crowning the top
Whichever mix you use, finish the top so water runs away from the post. Either build the concrete slightly above the ground and trowel it into a slope that falls away from the post, or stop it a few inches below grade and cover it with soil or sod. Do not leave a dished top that holds water against the wood. A bead of exterior sealant where the post meets the concrete can help once everything has cured.
Example: planning a mixed order
A 6 ft privacy fence has 14 line posts (4 × 4) and a gate with two 6 × 6 posts. The line posts go in 10 in holes, 30 in deep, on 4 in of gravel: about 1 cu ft of concrete each, 13.96 cu ft in all. In 50 lb bags of fast-setting mix at 0.375 cu ft each, that is 38 bags. The gate posts go in 12 in holes, 42 in deep, on 6 in of gravel: about 1.73 cu ft each, or 3.45 cu ft for the pair, which is 6 bags of regular 80 lb mix. Run your own holes through the post hole concrete calculator and check the yield printed on the bags you buy.
To compare cost fairly, divide each bag's price by its yield to get a price per cubic foot. Fast-setting mix is usually the more expensive way to buy a cubic foot of concrete; you are paying for time and convenience.
Safety
- Call your utility-locating service before you dig.
- Cement is caustic. Wear gloves, long sleeves and eye protection, and wash splashes off skin quickly. A dust mask helps when pouring dry powder.
- Lift heavy bags with your legs, or use a cart.
- Deck and porch posts are structural footings with their own code rules. Check with your building department.
Frequently asked questions
How long before I can hang a fence on fast-setting concrete?
Many fast-setting post mixes say to wait a few hours before hanging rails or panels, but times vary by brand and slow down in cold weather. Follow the label.
Can I use fast-setting concrete for gate posts?
Some labels allow it, but many builders use regular concrete for gate, corner and end posts because it gives more time to tamp, plumb and adjust a heavily loaded post.
Do you mix fast-setting concrete before pouring it?
Usually not. Most are placed dry in the hole with water added in the order the label specifies. Read the bag, because brands differ.
Do the math for your own project.
Open the post hole concrete calculator