How to Set a Fence or Mailbox Post in Concrete

Hole size and depth

Two manufacturer sources give slightly different depth rules, and we show both. The data sheet for a general-purpose mix says to dig the hole about three times the diameter of the post, with a depth of one third of the overall post height. The manufacturer's post-setting instructions say the hole should be three times the width of the post and as deep as one third to one half of the above-ground length plus 6 in. If you are unsure, use the deeper figure, since the extra depth adds stiffness.

Example: a 4 in post that stands 6 ft (72 in) above ground. The hole is 3 x 4 = 12 in wide. One third of 72 in is 24 in, and one half is 36 in, so the depth is 30 in to 42 in with the extra 6 in. The tall end suits a gate post or loose soil.

How many bags of concrete

Work out the volume of concrete in the hole, not the whole hole, because the gravel takes the bottom 6 in. Using a 12 in hole 36 in deep: the concrete section is 30 in (2.5 ft) deep and has a radius of 0.5 ft. Volume is 3.14 x 0.5 x 0.5 x 2.5 = 1.96 cubic feet. At about 0.60 cubic feet per 80 lb bag that is 3.3, so buy 4 bags. The post displaces some volume, so you may end up with a partial bag, which is the safer error.

Check the label on the product you buy, because yields differ by product and bag size.

Step by step

• 1. Mark the post positions and check for buried utilities before you dig.
• 2. Dig each hole to the width and depth above. Keep the sides straight and the bottom level.
• 3. Add 6 in of gravel or crushed stone, then tamp it. The gravel keeps water from sitting under the post, which helps protect against rot.
• 4. Set the post and check that it is plumb on two adjacent sides with a level.
• 5. Brace the post. The manufacturer suggests attaching angled 2 x 4 braces to two adjacent sides, with a stake driven near the lower end of each brace, and checking plumb again before fastening.
• 6. Place the concrete. The general-purpose method is to mix the concrete with water before placing it in the hole. Fast-setting concrete is poured dry and water is added to the hole.
• 7. Fill to 3 in to 4 in below ground level if you plan to backfill with soil or sod, or fill to grade and smooth the top once standing water has gone. Taper the top away from the post so rain flows off.
• 8. Leave the braces in place until the concrete has set. Fast-setting concrete sets in 20 to 40 minutes, with heavy loads allowed after 4 hours under normal curing conditions. General-purpose mix needs to wait 24 hours before the post is under any strain.

What to buy

• Post material suited to burial: rot-resistant wood, rust-resistant metal or a synthetic. The manufacturer says to use one of these.
• Concrete in one bag size, with a spare bag. Fast-setting mix suits fence and mailbox posts and is poured dry.
• Gravel or crushed stone for the 6 in base.
• A post-hole digger or auger, level, two stakes and 2 x 4 bracing per post.
• A tube form if the soil is loose or sandy. The manufacturer recommends a form in loose or sandy soil.

When this is not enough

The manufacturer states that the simple steps do not cover structural or load-bearing uses such as concrete footings for deck posts, and that those need forms and the separate load-bearing steps. The data sheet tells you to follow local building codes for load-bearing footings. Residential codes set a minimum footing of 12 in wide and 6 in thick, with depth below frost where applicable. If the post holds a deck, pergola, gate with a heavy load or anything people can fall from, talk to your building department first.

Common mistakes

• Setting the post directly in soil-level concrete with no gravel under it, which traps water at the base.
• Concrete left flat or sloped toward the post, which holds water against the wood.
• Not bracing, so the post drifts out of plumb while the concrete sets.
• Loading the post too soon: 24 hours for general-purpose mix, 4 hours for fast-setting under normal conditions.
• Using soft, rot-prone wood. Post material must be rot-resistant or protected.

Safety

Wear nitrile gloves and eye protection, and keep wet concrete off skin. NRMCA says fresh concrete can cause severe chemical burns to skin and eyes. Wash promptly with clean water, and flush the eyes immediately and repeatedly with water if concrete gets in them.

Frequently asked questions

Sources: Manufacturer data sheet: general-purpose concrete mix (yield, water, curing, slabs, posts), Manufacturer instructions: setting posts in concrete, Washington State residential code, WAC 51-51-0403 (footings), NRMCA CIP 31: Ordering Ready Mixed Concrete. Read 7 November 2026.