SMS character and segment counter

An SMS carries 140 bytes. In GSM-7, the default alphabet of the GSM 03.38 standard, each character takes 7 bits, so one message holds 160 characters. One character outside that alphabet switches the whole message to UCS-2, 16 bits a character, and the limit falls to 70. Past the single limit the message goes out in segments, each carrying a 6-byte header that tells the phone how to stitch them back, which leaves 153 GSM-7 or 67 UCS-2 characters a part. Twilio notes one exception: toll-free multi-segment texts to the US or Canada carry 152 and 66.

The alphabet is smaller than it looks. It has 128 codes (one is the escape into an extension table), and the 10 extension characters, such as €, [ and {, cost two septets each. The counter shows characters as a reader counts them and the encoded length the limits apply to, and like Twilio's own calculator it never splits an emoji or an escape pair across two segments. Twilio's example, 152 GSM-7 characters and a single Unicode character, comes out at 3 segments here as it does there.

Segments matter twice in a compliance program: they set the bill, and the opt-out footer the carriers expect has to fit inside them. The rules for what a marketing text must carry are in TCPA rules for text messages; how 10DLC registration reviews your samples is in what is 10DLC.

The footer is the part of a marketing text nobody wants to pay for and nobody can drop. CTIA's Messaging Principles (§ 5.1.3) say: “Message Senders should state in the message how and what words effect an opt-out. Standardized “STOP” wording should be used for opt-out instructions”. The opt-in confirmation that starts a recurring program must carry “(1) the program name or product description” and “(3) how to opt-out” among its five parts (§ 5.1.2.1). On the carrier-registration side, Twilio's 10DLC campaign form says of the sample messages: “The Brand must be identified by name and/or website in each message.” The counter checks both: a STOP instruction, and the brand name you type.

The legal side is wider than the keyword. Under 47 CFR § 64.1200(a)(10) a person may revoke consent “by using any reasonable method”, and replying “stop,” “quit,” “end,” “revoke,” “opt out,” “cancel,” or “unsubscribe” counts on its own. The FCC's newer order, FCC 26-67, is adopted but not yet in effect: it would let a sender “designate an exclusive revocation method”, disclosed clearly in the text itself. Either way, the footer is where the instruction lives. The keyword mechanics are in SMS opt-out keywords, and the full carrier rulebook in CTIA messaging guidelines.

What it costs: “Reply STOP to opt out.” on its own line is 23 septets. In a GSM-7 message that leaves 137 for the body before a second segment, and once a message splits, every part drops to 153. In a UCS-2 message the same line takes 23 of the 70 units, a third of the room. That is why a smart quote in the body hurts twice: it shrinks the space and the footer still has to fit.

SMS is billed by the segment, not by the message: Twilio's docs say plainly that “Twilio bills SMS per message segment sent”. At a per-segment price, a three-segment promo to a list costs three times the one-segment version. Put your own per-segment price in the optional field and the record shows the cost of one message, of the whole send, and what the fix saves. We publish no prices here; the way a US bill is built is in Twilio SMS pricing explained, and the registration side in the 10DLC registration cost calculator.

Long is allowed, but Twilio caps a message at 1,600 characters and “recommends keeping SMS messages under 320 characters”. Toll-free numbers split slightly smaller parts than 10DLC (see 10DLC vs toll-free); switch the sending number in the counter to see it.

The character set comes from the Unicode Consortium's mapping of the ETSI GSM 03.38 alphabet; the segment sizes, the toll-free exception and the encoder's edge cases from Twilio's own documentation and source code; the opt-out and brand wording from CTIA and Twilio's 10DLC form. All read on 2026-10-22. Comparing providers? The SMS API provider comparison reads each one's compliance features. This is a counting aid, not legal advice.