Why segments matter more than characters
An SMS carries 140 bytes of payload. With the 7-bit GSM alphabet that is 160 characters; with 16-bit Unicode it is 70. Anything longer is split into concatenated parts, and each part gives up 6 bytes to a header that tells the receiving phone how to reassemble them. That is why multipart segments hold 153 GSM-7 characters or 67 Unicode characters instead of 160 and 70.
Messaging platforms bill per segment, not per message. A campaign to 20,000 people that drifts from one segment to two does not cost a little more, it costs twice as much. The segment count is the number to watch while you write, which is why this calculator shows it as a fuel gauge that fills and changes colour as you type.
GSM-7 versus UCS-2, and the characters that flip the switch
GSM-7 covers the Latin alphabet, digits, common punctuation, a handful of accented letters (é, è, ü, ö, ä, ñ, à, ì, ò, ù, ø, å, æ) and a few Greek capitals. Nine characters sit in an extension table and cost two units: the caret, curly braces, backslash, square brackets, tilde, pipe and the euro sign.
The moment a message contains one character outside that set, the whole message is encoded as UCS-2. The usual offenders are invisible in a word processor: curly quotes and apostrophes, en and em dashes, the ellipsis character, non-breaking spaces and emoji. Lowercase accented letters such as á, í, ó, ú and ç are also outside GSM-7, even though é and ü are inside it. The calculator lists each offending character with its Unicode code point so you can find it in a template or a CRM field.
What "Make GSM-safe" changes and what it leaves alone
The button replaces characters that have an obvious plain equivalent: curly single and double quotes become straight quotes, en and em dashes become hyphens, the ellipsis becomes three dots, non-breaking and zero-width spaces become ordinary spaces or are removed, and symbols such as the bullet, multiplication sign and trademark sign become ASCII stand-ins. Nothing about the meaning of the message changes.
It does not touch emoji or letters that have no GSM-7 equivalent, because guessing a replacement could change a name or a word. If the badge still says UCS-2 after you click, the remaining hazard marks show exactly what needs a manual rewrite. Often the fix is to drop a single emoji from a signature.
Reading the split preview
When a message needs more than one segment, the preview shows the text of each part exactly as the carrier would cut it. Phones reassemble the parts in almost every case, but there are two reasons to look: a link that straddles a boundary can be broken if a part arrives late or out of order, and a part that ends mid-word reads badly on the handful of devices that show segments separately. Moving a sentence or shortening a link usually fixes both.
Writing campaign messages that stay in one segment
- Lead with the action and the brand: "Acme Dental: Reply C to confirm your 2:30 PM appointment Tuesday." People read the first line.
- Use a short link domain. A tracked link with UTM parameters can take 80 characters; a branded short link takes about 22.
- Test personalisation with your longest real values. "Christopher-Alexander" is 21 characters where "Sam" is 3.
- Keep opt-out text standard and short. "Reply STOP to opt out" is 21 characters and satisfies carrier guidance.
- Leave 10 to 15 characters of headroom so a long name or date does not push the message into a second segment.
- If you register the campaign for A2P 10DLC, the call-to-action generator produces sample messages that already include the brand name and STOP wording.
What the cost estimate includes
The estimate multiplies segments by your provider price per segment, the number of recipients and the number of sends per month. The default price is a typical US list price for application-to-person SMS; replace it with the rate on your own invoice. It does not include carrier pass-through fees (a fraction of a cent per segment on each major US carrier), monthly number rental or A2P 10DLC registration and campaign fees. The 10DLC cost calculator adds those in and shows the first-year total.
For a quick count without the campaign maths, the SMS character counter uses the same engine with a simpler layout focused on the 160 and 70 character limits.