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
- Box bottom 42″ above the road and its front 6″ back from the curb: inside the USPS 41–45″ and 6–8″ ranges. Ask your post office before you move or install a box; some routes have their own rules.
- Breakaway setting: a 4×4 wood post, 24″ deep in tamped gravel with no concrete, so it bends or snaps if a car hits it. Confirm what your road authority requires.
Shopping list
| Qty | Item | Est. |
|---|---|---|
| 1 | 4×4 pressure-treated post, 10′ (board) | $16.00 |
| 1 | 2×6 pressure-treated, 8′ (board) | $8.00 |
| 3 | Crushed gravel (0.5 cu ft bag) | $18.00 |
| 1 | Wood stakes, 18″ (bundle of 12) | $9.00 |
| 2 | Galvanized 6″ L-strap (corner plate) with screws (each) | $8.00 |
| 1 | 5/16″ × 6″ structural wood screws (box of 12) | $24.00 |
| 1 | 3″ exterior screws (box of 75) | $12.00 |
| 1 | 1 1/4″ exterior screws (box of 100) | $8.00 |
| 1 | Large rural mailbox, post-mount (T2 size) (each) | $42.00 |
| 1 | Reflective mailbox numbers, 3″ (pack) | $6.00 |
About $151 · rough estimates, check the store
Parts
| Part | Qty | Length | Width | Material | |
|---|---|---|---|---|---|
| A | Post Ground-contact rated. 24″ goes in the ground. | 1 | 61″ | 3 1/2″ | 4×4 pressure-treated post |
| B | Arm | 1 | 18 3/4″ | 3 1/2″ | 4×4 pressure-treated post |
| C | Arm brace Cut both ends at 45°, parallel, long point to long point. | 1 | 17″ | 3 1/2″ | 4×4 pressure-treated post |
| D | Mounting board | 1 | 21 3/4″ | 5 1/2″ | 2×6 pressure-treated |
Cut list
4×4 pressure-treated post, 120″ → A 61″ · B 18 3/4″ · C 17″
2×6 pressure-treated, 96″ → D 21 3/4″
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
- Level (2 ft or longer)
- Socket or impact driver for the 6″ structural screws
- 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 post 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.4Find the spot
Ask your post office before you start: they confirm which side of the driveway the box goes on and any local rules. Most boxes on a route sit on the same side of the road so the carrier can reach them from the driver's window.
USPS curbside rules: the bottom of the box 41″–45″ above the road surface, and the front of the box 6″–8″ back from the curb (or the edge of the road if there is no curb). This plan puts the bottom at 42″ and the front 6″ back.
Measure 24″ back from the curb face, square to the road, and drive a stake: that is the post centre. The arm reaches from the post toward the road so the box front lands 6″ from the curb.
The ground at the post is level with the road. Lay a straight board from the road edge to the stake with a level on it to check, and measure the gap.
✓ The stake is 24″ from the curb face and the spot is clear of the driveway edge, sidewalk and drainage ditch.
5Dig the post hole
Dig one hole 10″ wide and 30″ deep with a post-hole digger: 24″ for the post plus 6″ of gravel underneath so water drains away from the post end.
Don't go deeper than that. A breakaway post works because only 24″ of a 4×4 is held by packed gravel: hit by a car, it leans or snaps instead of stopping the car like a pole.
Pour 6″ of crushed gravel in the bottom and tamp it hard with the end of a 2×4.
✓ The hole is 24″ deep after the gravel goes in.
6Set and tamp the post
Stand the post in the hole, its best face toward the road. Measure: its top should be 37″ above the ground. Add or scrape gravel under it to adjust.
Hold a level on one face, then on the face around the corner, until both read plumb. Brace it with two scrap boards screwed to the post and to stakes.
Shovel crushed gravel in around the post 4″ at a time. Tamp each layer hard with the end of a 2×4, all the way round, before adding the next. Finish with the dug soil, tamped, mounded slightly so rain runs away from the post.
- Tip Packed in thin layers, gravel holds a post almost as firmly as concrete, and the post is easy to replace when it rots or gets hit.
✓ The post is plumb on two faces, the top is at the right height, and it doesn't move when you push it hard.
7Fit the arm and brace
Check the post top is level both ways. If not, mark a line all round with a square and saw it flat.
Lay the arm (B) on top of the post, its back end flush with the post's back face and pointing straight at the road. Screw a galvanized L-strap across the joint on each side face, using every hole.
Fit the brace (C) under the arm on the road side of the post at 45°: its long points touch the post 12″ below the arm and the arm 12″ out. Predrill and drive one 6″ structural screw through each end.
- Tip Sight down the arm from behind the post: it should point straight across the road, not at an angle.
✓ The arm is level and square to the road, and its end is 7″ from the curb face.
8Fix the mounting board
Centre the mounting board (D) on top of the arm, flat side down, with its front end 1/2″ back from where the box front will be (6 1/2″ from the curb face).
Predrill and drive four 3″ screws down through the board into the arm.
✓ The top of the board is 42″ above the road surface.
9Mount the mailbox and number it
Set the mailbox on the board with its door 6″ back from the curb face and centred side to side. Most rural boxes have flanges with holes along the bottom edges: drive the six 1 1/4″ screws through them into the board.
Measure from the road surface to the bottom of the box: 42″.
Put your house number on both sides of the box (or on the door, and on the post facing the direction the carrier comes from) in reflective numbers at least 1″ tall. If the box is on a different street from your house, add the street name.
- Tip Fit the flag on the side the carrier approaches from, so it is visible from the car.
✓ Box bottom 42″ above the road, front 6″ back from the curb, door opens and closes fully.
10Finish
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.