How many segments is
your SMS really?

Because multipart messages need a header on every part telling the phone how to reassemble them. That header eats 7 characters, so a single SMS holds 160 but each segment of a multi-part message holds only 153 (or 67 instead of 70 in Unicode mode).

One single character outside the GSM-7 alphabet — most often a smart quote (’) pasted from a doc, an em dash, an emoji, or accented characters like á/í/ó/ú (which, unlike é or ñ, are NOT in GSM-7 — the classic Spanish-language gotcha). The calculator lists each offending character with a suggested replacement.

Carriers and SMS APIs bill per segment, not per message. A 320-character message in GSM-7 is 3 segments — three times the price. Going Unicode cuts capacity from 160 to 70, so the same text can silently double or triple in cost. At campaign volume, this is real money.

Modern phones, yes — they display one continuous message. But delivery of each part is independent, so on a flaky route a recipient can get part 2 without part 1. Mission-critical texts (codes, alerts) should always fit one segment.

One segment, always — and ideally well under it. SMS is a glance medium: name, reason, one question. If you need 300 characters, the message wants to be an email instead.