I am specifically referring to the red flag on many curbside mailboxes, generally made out of plastic, which is toggled when mail is delivered.

I'm quite certain, regardless, that any recognizable flag would be understood and used by a mailman; but what about the symmetries in traditional ones? Are there any constraints on their height, width, or depth? Is this a simple fluke of manufacturing, or did someone lay the dimensions down in law at some point?

(As has been, rather dramatically, highlighted in the comments, I am from New Mexico, United States; I am, understandably, totally unaware of the postal standards in other parts of the globe, and they do not relate to the origin of my question. Additionally note the United States tag.)

  • Never seen a mailbox with a flag. Where is this strange construction?
    – Dale M
    Commented Aug 20, 2021 at 23:49
  • @DaleM westernbay.govt.nz/property-rates-and-building/…
    – Greendrake
    Commented Aug 20, 2021 at 23:59
  • @DaleM every "rural" mailbox (a mailbox you place on a pole at the street in front of your house) has a flag in the US.
    – Tiger Guy
    Commented Aug 21, 2021 at 19:01

1 Answer 1


Because you indicate that you're from New Mexico, I'm assuming that you're asking about the rules governing curbside mailboxes used by the United States Postal Service.

If that's the case, there are constraints on the design of signal flags, but they are far more focused on function and ergonomics than on aesthetics, and they can be found at USPS STD 7C, which says:

The flag must be mounted on the right side when facing the mailbox from the front. The flag must not require a lift of more than 2 pounds of force to retract. Additionally, when actuated (signaling outgoing mail), the flag must remain in position until retracted by the carrier. The color of the flag must be in accordance with the requirements described in 3.9. The operating mechanism of the flag must not require lubrication and must continue to operate properly and positively (without binding or excessive free play) after being subjected to the test described in Section 4. Optionally, the flag may incorporate a self-lowering feature that causes it to automatically retract when the carrier service door is opened provided no additional effort is required of the carrier. The self-lowering feature cannot present protrusions or attachments and must not interfere with delivery operations in any manner or present hazardous features as specified in 3.1.

The regulations provide "preferred" designs -- generally the rectangle you're likely used to -- but say that other designs may be permitted if they meet the other requirements.

As I read it, this means USPS doesn't care if you use a circle or a square or a triangle or a swallowtail, as long as at isn't too heavy to lift, too flimsy to stand up on its own, etc.

  • 1
    I would add that these rules don't have the force of law, they just govern whether or not your mailbox is acceptable for use for mail delivery. If unacceptable, your mail may not be delivered.
    – Tiger Guy
    Commented Aug 21, 2021 at 19:03

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