Two different datums, one post
Everything about a roadside mailbox is measured from the road: how high the box sits, how far back its face is from the pavement edge. The post, meanwhile, stands in the ground, and at almost every rural and suburban entrance that ground is lower than the road. Roads are crowned so water runs off them, and where it runs to is a shoulder or a shallow ditch a few feet out. Six inches of fall over that distance is ordinary and a foot is not unusual.
So the post is not the height figure long. It is the height figure plus the drop, plus whatever goes in the hole. Miss the drop and the box comes out low by exactly that amount, and the fix is pulling a post out of concrete. Measuring it takes a straightedge or a string line from the pavement edge out to where the post will stand, and two minutes.
Where the post lands depends on how the box is carried
The setback is measured to the front of the box, not to the post, so the mounting decides where the post goes. A box sitting on a platform on top of a post has the post roughly under the middle of the box, which pulls it forward toward the road. A box on an arm has the post behind it, which pushes it back by the whole box depth plus the gap. On a nineteen inch box that is a difference of well over a foot in where you dig, and it changes whether the post is in the ditch, on the bank, or in the middle of the shoulder.
| Mounting | Post centre, back from the road edge | Practical effect |
|---|---|---|
| Platform on top | Setback plus about half the box depth | Post close to the road, box overhangs behind it |
| Arm behind the box | Setback plus the whole box depth plus the gap | Post well back, arm carries the load out |
Both key numbers belong to somebody else
The height and the setback are not values this page has or should have. Your postal carrier and the local post office set what they need to serve the route from the vehicle, and they are the ones to ask — the carrier who actually drives it, because what works on that route is the practical test. A separate question, to a separate body, is whether anything may stand in that strip at all: the verge beside a road is usually public right of way and what may be built there is decided by the road authority, not by the postal service.
That second question is worth taking seriously if masonry is involved. A brick or stone pillar beside a road is a fixed, unyielding object at exactly the height of a vehicle, and road authorities have views about that which are stricter than most people expect. Ask before laying anything.
A run of boxes on one structure
Shared entrances and private roads often end up with several boxes on a header board. The board length is simply the boxes plus the gaps between them, and how many posts hold it up and how far apart they may be is a decision for whoever is building it — span tables for a mailbox header are not a thing that exists, and the honest approach is to put a post where you would want one. The calculator takes the post count from you rather than inventing one, and reports the resulting spacing so you can see whether it looks sensible.
The concrete arithmetic here is one hole, multiplied. It is the hole volume less the post in it, less the gravel at the bottom, converted to bags. For a whole fence line rather than one or two holes, the fence post calculator does counts, rails and concrete together.
The rest of the entrance
Before anything is dug at an entrance, have the underground utilities located. Call 811 or use the online locate request; it is free everywhere in the United States and the locate is the whole point of the service. Entrances are the single worst place to guess, because water, gas, electric, telecoms, irrigation and the neighbour's unrecorded conduit all converge in that strip and shallow cover is common. Overhead lines matter too: a post, a pillar or a length of pipe swung up on end near a service drop is a fatal accident that happens every year. Working at a driveway mouth also means working beside moving traffic, which is its own hazard and is not managed by anything on this page.
If the entrance is also getting a gate, the gate and the mailbox compete for the same few feet of frontage and it is worth laying both out at once. The swing gate clearance calculator and the slide gate backspace calculator give the ground each type needs, and the gate setback and stacking calculator gives the depth. Address numerals sized to be read from a moving vehicle are a viewing distance problem, handled on the sign letter height calculator — and the fire district often has views on address legibility at an entrance, which is a conversation worth having with them.
Questions people ask
How tall should a mailbox post be?
Long enough that the bottom of the box lands at the height your carrier asks for, measured above the road surface, plus the drop from the road down to the ground where the post stands, plus whatever is going into the hole. The drop is the part that gets left out. The height figure itself comes from your carrier or local post office and this page does not supply one.
Why does my new mailbox sit lower than the old one?
Usually because the post was cut to the height figure measured from the ground at the post rather than from the road surface. If the shoulder falls six inches away from the pavement, that is exactly how much low the box ends up. Run a string line from the road edge out to the post position and measure down to get the real number.
Where exactly does the post go relative to the box?
It depends on the mounting. A box on a platform sits over the post, so the post centre is roughly the setback plus half the box depth back from the road edge. A box on an arm has the post behind it, so the post face is the setback plus the whole box depth plus the gap you leave. On a typical box that is more than a foot of difference in where you dig the hole.
How deep does the hole need to be?
That is a frost depth question and it is local. Your building department gives the frost depth for your area, and how much post is standing above the ground matters too, since a taller post levers harder on whatever holds it. The calculator takes the depth from you and works out the concrete for it; it does not choose the depth and will not.
Can I build a brick pillar around the mailbox?
That is a question for the road authority before it is a question of materials. A masonry pillar beside a road is a fixed object a vehicle can strike, and what may be built in the verge, how far back it must be and who carries the liability are decided locally and vary a lot. If you get the answer you want, the brick or block count is on the brick column calculator and the pad under it on the concrete footing calculator.