Drag an orange dot to resize; a dashed line shows when it snaps to a common size. Click a dot, then use the arrow keys to nudge 1/8″ (hold Shift for 1″). Every size can also be typed in the panel.
Checks
- USPS and road departments ask for a post that bends or falls away if a car hits it: a 4×4 wood post or 2″ steel pipe, buried no more than 24″. Some towns don't allow concrete at the road; ask.
- Enter your town's frost depth: posts in concrete that stop above the frost line get pushed up a little every winter.
Shopping list
| Qty | Item | Est. |
|---|---|---|
| 1 | 4×4 pressure-treated post, 8′ (board) | $12.80 |
| 1 | 1×4 pine board, 8′ (board) | $8.80 |
| 2 | Concrete mix, 80 lb bag (0.6 cu ft) (bag) | $13.00 |
| 1 | Crushed gravel (0.5 cu ft bag) | $6.00 |
| 1 | Wood stakes, 18″ (bundle of 12) | $9.00 |
| 1 | Mason's line (nylon string), 250′ (roll) | $6.00 |
| 1 | Plastic sheeting, 10′ × 25′ (for curing concrete) (roll) | $12.00 |
| 1 | 2 1/2″ wood screws (box of 100) | $10.00 |
About $78 · rough estimates, check the store
Parts
| Part | Qty | Length | Width | Material | |
|---|---|---|---|---|---|
| A | Post Ground-contact rated. 2″ extra on top, trimmed after the concrete sets. | 1 | 68″ | 3 1/2″ | 4×4 pressure-treated post |
| B | Temporary brace Reused scrap is fine. Removed after 2 days. | 2 | 44″ | 3 1/2″ | 1×4 pine board |
Cut list
4×4 pressure-treated post, 96″ → A 68″
1×4 pine board, 96″ → B 44″ · B 44″
Kerf 1/8″ included. Every cut runs edge to edge.
Steps
- Tape measure
- Speed square
- Pencil
- Drill/driver
- Clamps
- Safety glasses
- Hearing protection
- Dust mask
- Circular saw or miter saw
- Miter saw, or circular saw in two passes, for 4×4 posts
- Post-hole digger (rent a powered auger for rocky soil)
- Digging bar for rocks and roots
- Post level or a 2′ level
- Wheelbarrow and hoe
- Small sledgehammer
- Square shovel
- Sander, 120 and 180 grit
1Before you start
Read every step once, start to finish, before you buy or cut anything. Each step names the parts it uses by letter; the parts list gives their sizes.
Set up somewhere flat with good light. A pair of sawhorses with a sheet of plywood on top makes a fine work table.
Wear safety glasses whenever a tool is running, hearing protection for saws, and a dust mask for sanding and for cutting treated wood or sheet goods. Tie back long hair and loose sleeves. Keep children and pets out of the work area.
- Tip At the store, pick straight boards: hold each one up to your eye and look down its length. Skip boards that bow, twist or have big knots near the ends.
- Tip Measure twice, cut once. Most mistakes are a misread tape, not a bad cut.
2Cut and label the parts
Cut every part on the cut list. Cut the longest parts first: if one goes wrong, it can still become a shorter part.
To mark a cut, hook your tape on the end of the board, make a small V with its point exactly at the measurement, and draw a square line through the point with a speed square. Put an X on the waste side.
Cut on the waste side of the line, so the blade's kerf (about 1/8″) comes out of the scrap, not your part.
Write each part's letter in pencil on a face that won't show, and stack the parts by letter.
Sand the faces before assembly while they're flat and easy to reach: 120 grit, then 180.
✓ Lay the parts out and count them against the parts list. Measure a few: if one is off by more than 1/16″, recut it now.
STOP. Let the saw reach full speed before it touches the wood, keep both hands clear of the blade's path, and never reach under a board you're cutting. Support long boards on both sides of the cut so the blade doesn't pinch.3Call 811 and check the spot
At least 3 working days before you dig, call 811 (or use call811.com). It's free. They send people to paint or flag buried gas, electric, water and cable lines near where the mailbox will go.
Check your property line on your survey or plot plan, and ask your town about setbacks, easements and whether this needs a permit.
Pick ground that drains: not a low spot where puddles sit after rain, and not over a septic field.
STOP. Don't dig until the marks are down. Hitting a gas or power line can kill you and is expensive to repair.4Mark the post spots
Stand at the road. The front of the mailbox door should end up 6–8″ back from the face of the curb or the edge of the road. Mark the post centre so that's true once the box is on it, usually about 12–18″ back from the edge.
Ask your post office if they want the box on a particular side of the driveway or a set distance from it, and check your town or HOA rules for mailbox posts.
Each hole is 10″ wide, so keep the marks at least 10″ from fences, walls and paving.
✓ Step back and look from the road or the house. The mark is where you want the post to stand.
5Dig the post holes
Dig a hole 10″ wide and 30″ deep with a post-hole digger.
That depth is 24″ for the post plus 6″ of gravel underneath so water drains away from the post end.
Make the holes slightly wider at the bottom than the top, like a bell. Frost then can't grip the concrete and lift it.
Pour 6″ of gravel in each hole and tamp it with the end of a 2×4 or the post itself.
- Tip Your town knows the frost depth: it's how deep footings must go so winter frost can't push them up. Enter it in the options and the holes will be dug deep enough.
✓ The hole is 30″ deep before the gravel and 24″ deep after it.
6Set the posts plumb
Stand the post in the hole.
Hold a level against one face of a post until the bubble is centred: that face is plumb. Then check the face around the corner. Both faces must read plumb.
Brace each post with two 1×4 or 2×4 boards: screw one end to the post and the other to a stake hammered into the ground, one brace on each of two sides at right angles.
Cut the two braces (B) for each post, 44″ long. Screw one end to the post about 25″ up with one 2 1/2″ screw, the other to a stake hammered into the ground 25″ away. Leave both screws loose enough to adjust.
✓ The post is plumb on two faces and the braces hold it there when you let go.
7Mix the concrete
You'll use 2 bags of 80 lb concrete mix for the post hole. Mix one or two bags at a time in a wheelbarrow: concrete starts to stiffen in about 30–45 minutes (faster on a hot day), so only mix what you can place right away.
Pour about 2 quarts of clean water into the wheelbarrow first, then empty one bag on top. (Each 80 lb bag needs about 3 quarts in total.)
Mix with a hoe or square shovel: pull the dry powder from the corners and bottom into the wet middle and chop through it. Keep going until there are no dry pockets.
Add the last of the water a cup at a time, mixing between each.
Get the thickness right. It should look and feel like thick oatmeal or stiff peanut butter. A hoe dragged through leaves ridges that stand up, and a handful squeezed in a gloved hand holds its shape. Too dry: it crumbles and won't stick together, so add a little water. Too wet: it flows flat, water pools on top, and the concrete will be weak and crack. Then add some dry mix from a spare bag.
- Tip More water makes concrete easier to push around and much weaker. Resist the urge.
- Tip Rinse the wheelbarrow, hoe and shovel before the concrete sets on them. Don't wash concrete into drains or onto the lawn: let leftovers harden in a scrap box and throw them out.
- Tip Buy one more bag than you need. Running out halfway through a pour is the most common mistake.
✓ Even grey colour throughout, no dry powder, no puddles of water on top; a ridge made with the hoe stands up.
STOP. Wet concrete is caustic and burns skin. Wear waterproof gloves, safety glasses, long sleeves and long pants. Open bags slowly and wear a dust mask: the dust has silica. Rinse any concrete off your skin right away.8Fill the holes
Shovel concrete into each hole around the post. Every 6″ or so, poke it hard with a stick or a piece of rebar to work out air pockets.
Fill to about 1″ above the ground and shape the top into a little mound with a trowel, sloping away from the post so rain runs off.
Check plumb again on both faces while it's still wet, and adjust the braces if needed.
Concrete stays workable for about 30–45 minutes. Do the final plumb check in the first 10 minutes; after 20 the post won't move without cracking it.
✓ The post is still plumb after the concrete is in.
STOP. Don't pour in rain or when it will drop below 40°F (5°C) in the next 2 days. Keep wet concrete off your skin.9Let the concrete cure
Concrete hardens by reacting with water, not by drying out, so keep the concrete around the mailbox posts damp for the first 7 days: mist it with a hose morning and evening, or cover it with plastic sheeting held down at the edges.
Keep it from freezing for the first week. Don't pour when it's below 40°F (5°C) or frost is forecast in the next two days. On hot days, pour in the early morning and shade it.
You can walk on it or remove braces after 1–2 days. Wait 3 days before putting real weight on it. It reaches full strength in about 4 weeks.
✓ The surface is hard and grey-white when dry, with no powder when you rub it.
10Remove the braces and trim the top
After 2 days, unscrew the braces and pull the stakes. Fill the stake holes with soil.
Measure 42″ up from the ground (or from the road surface, for a mailbox) and mark all four faces with a square. Cut the top off along the lines; the 2″ extra is there so the height comes out exact even if the post settled a little.
Cut the top at a slight angle, or fit a post cap, so rain runs off instead of soaking into the end grain. Brush end-cut preservative on the fresh cut.
Wait 3 days, then fix the mailbox support board and box as the mailbox maker shows.
✓ The post is plumb on two faces, the top is at 42″, and the concrete mound sheds water away from the post.
STOP. Wear gloves and a dust mask when cutting pressure-treated wood, and never burn the offcuts.11Finish
Ease every sharp edge and corner with 120-grit sandpaper so nobody gets a splinter.
Sand all over with 180 grit, always along the grain (the direction the wood fibres run). Scratches across the grain show through stain.
Wipe off the dust with a barely damp rag and let it dry.
Outdoors, use an exterior finish: a penetrating deck oil or exterior stain, or primer and exterior paint. Leave cedar bare to weather silver if you like. If it holds soil for food plants, leave the inside unfinished.
Let it dry for the time on the can before using it.
- Tip Try your finish on a scrap of the same wood first: stain looks different on every board.
Conceptual layout, material estimate and assembly guide from the sizes you entered. Not engineering, not a building plan, not stamped or approved. Confirm measurements on site; follow local codes and product instructions; anchor tall pieces before use. Using this plan means you accept the Terms of Use.