United States · +1
US phone number format
A US number is ten digits: a three-digit area code, a three-digit exchange
code, and a four-digit line number. Written for software it is
+1 followed by those ten digits and nothing else —
+12125550123. Everything below is why that form exists, and what breaks
when a list is not in it.
The rule, in one line
+1 NPA NXX XXXX → +1 212 555 0123 → +12125550123
Eleven digits in total including the country code. No spaces, no brackets, no dashes, no leading zero, no trunk prefix. That is the E.164 form, and it is what every SMS gateway, dialler and CRM import expects.
What NPA and NXX mean
The North American Numbering Plan splits those ten digits into three parts:
- NPA — the area code, three digits. There are 403 of them carrying allocations in the data we hold.
- NXX — the exchange code, the next three digits. Together NPA-NXX identifies a block of 10,000 consecutive numbers.
- XXXX — the line number inside that block.
Since number pooling, most blocks are subdivided again: the seventh digit splits an NPA-NXX into ten thousands-blocks of 1,000 numbers, each of which can belong to a different carrier. That is why two numbers sharing an area code and exchange can sit on different networks.
The scale of it, in the table this site runs on: 226,841 six-digit NPA-NXX blocks and 1,311,108 seven-digit thousands-blocks for +1 alone, across 5,236 distinct carrier names. The full structure is written up here.
The five things that break a US number
1 (212) 555-0123
Human formatting. Brackets, spaces and dashes are not part of the number. Strip them all; keep the digits.
12125550123
The long-distance 1 with no plus. Inside the US this dials fine, but software cannot
tell a US trunk prefix from a country code without the +.
212-555-0123
Ten digits, no country code. Unambiguous in the US and ambiguous everywhere else — an importer with a non-US default will reject it or guess wrong.
2.1256E+09
Excel. A spreadsheet reads a long numeric string as a number and converts it to scientific notation. Import the column as text, or export to CSV and treat it as text from the start.
+1 212 555 0123 x204
An extension. It belongs in its own column; left attached it makes the number invalid.
Which US numbers are not real
Format is only half of it. A perfectly formatted US number can still be one that cannot ring, because its block was never allocated to a carrier. Those are the numbers worth removing before you pay anyone per lookup, and the only way to find them is to check the number against the numbering plan.
The 555-01xx range is the clearest case: +1 212 555 0123 is
reserved for fiction and will never connect. Most unallocated numbers are not that
obvious — they look entirely ordinary.
Check a single US number here, or run a whole list through the browser cleaner, which normalises to E.164 and flags what cannot exist without uploading the file anywhere.
Mobile or landline, in the US specifically
This is harder in North America than almost anywhere else, and it is worth knowing why. In most countries mobile and fixed numbers come from separate ranges, so the number itself tells you which it is. The NANP does not work that way: mobile and landline share the same NPA-NXX space, so the only route to an answer is the carrier the block was allocated to.
In the +1 data we hold that splits as 609,875 blocks recorded mobile, 596,655 landline and 199,741 VoIP. The caveat that applies to all of it: those are original allocations. A ported number keeps its digits and its block while moving to a different network, so the carrier shown is where the block started, not necessarily where the number lives today. No offline data can close that gap. How line type is actually derived.
Converting a US list to E.164
- Strip everything that is not a digit.
- If eleven digits remain and the first is
1, drop it. - If ten digits remain, prefix
+1. - Anything else is not a US number — do not pad it to make it fit.
That last step is the one people get wrong. Repairing transport damage is not the same as repairing a wrong number, and a validator that invents digits to make a row pass is worse than one that marks it invalid.
Common questions
How many digits is a US phone number?
Ten nationally, eleven with the country code. In E.164 it is written +1
followed by the ten digits, so +12125550123 — twelve characters
including the plus.
Do I need the 1 in front of a US number?
For software, yes, but as +1 rather than a bare 1. The bare
leading 1 is a trunk prefix for dialling long distance within the country; the
+1 is the country code. They look identical and mean different things, which
is exactly why E.164 requires the plus.
What does a +1 number look like in E.164 format?
+12125550123. Plus sign, country code, national number, no separators of any
kind. If your export has brackets, spaces or dashes in it, it is not in E.164 yet.
Why does my CRM reject valid-looking US numbers?
Almost always formatting rather than the number. Klaviyo, HubSpot, Attentive and Twilio all expect E.164 and reject anything else — sometimes silently, with a row count that does not match what you uploaded. The Klaviyo case in detail.
Is a US number with 555 always fake?
No. Only 555-0100 to 555-0199 are reserved for fiction. Other
555 numbers are ordinary assignable numbers, and some are in service.