SMS vs. SMS Credits: Character Limits & Billing Explained

SMS Credits versus SMS Messages

What is an SMS Message?

An SMS (Short Message Service) message is a single text message sent to a mobile device. Traditionally, SMS messages are limited to 160 characters when using the standard GSM-7 character set (basic Latin alphabet, numbers, and some special characters like @, $, etc.). This limit comes from the original SMS protocol designed for mobile networks.
If your message exceeds 160 characters or uses non-standard characters (e.g., emojis or non-Latin alphabets like Chinese), it may either:
  • Be split into multiple parts (called "segments"), or
  • Switch to a different encoding (like UCS-2), which reduces the character limit per segment.

What is an SMS Credit?

An SMS credit is a unit of billing used by SMS service providers (e.g., Twilio, Vonage, or your mobile carrier) to charge for sending messages. It doesn’t always correspond 1:1 with a single SMS message because of factors like message length, encoding, and destination. Essentially:
  • 1 SMS credit typically covers sending 1 standard SMS message (up to 160 characters using GSM-7 encoding).
  • If a message exceeds the standard limit or requires special handling, it might cost more than 1 credit.

How Does the 160-Character Limit Work?

Here’s how it calculates:
  1. Standard GSM-7 Encoding (160 characters):
    • If your message is 160 characters or fewer and uses only GSM-7 characters (e.g., A-Z, 0-9, and basic punctuation), it’s considered 1 segment and costs 1 SMS credit.
    • Example: "Hello, how are you today?" (25 characters) = 1 segment = 1 credit.
  2. Longer Messages (Concatenation):
    • If your message exceeds 160 characters, it’s split into segments. Each segment has a slightly reduced character limit (153 characters instead of 160) because 7 characters are reserved for a header (User Data Header, or UDH) that tells the phone how to reassemble the parts.
    • Example: A 300-character message:
      • Segment 1: 153 characters
      • Segment 2: 147 characters
      • Total: 2 segments = 2 SMS credits.
    • Providers charge per segment, so a 161-character message would still be 2 segments (and 2 credits).
  3. Non-GSM-7 Characters (UCS-2 Encoding):
    • If you use emojis, accented letters (e.g., é), or non-Latin scripts (e.g., Arabic, Chinese), the message switches to UCS-2 encoding, which has a limit of 70 characters per segment. Beyond 70 characters, it splits into segments of 67 characters each (again, due to the UDH).
    • Example: "Hello " (7 characters + emoji) = UCS-2 encoding = 1 segment (70-character limit) = 1 credit.
    • Example: A 150-character message with an emoji:
      • Segment 1: 67 characters
      • Segment 2: 83 characters
      • Total: 2 segments = 2 credits.

Idea
Emojis reduce the character limit from 160 to 70
Other Factors Affecting SMS Credits

  • International Messaging: Sending SMS to different countries might cost more credits (e.g., 1.5 or 2 credits per segment) depending on the provider’s rates.
  • Premium Services: Some providers offer features like delivery receipts or dedicated shortcodes, which could increase credit costs.
  • Carrier Rules: Some mobile carriers impose their own limits or fees, which the SMS provider might pass on.

Practical Example

Let’s say you send this message:
"Hi there! Just checking in to see how you’re doing today. Hope all is well with you and your family! "
  • Total characters: 97 (including spaces and emoji).
  • Encoding: UCS-2 (due to the emoji).
  • Segments: 1 (since 97 < 70 is within the UCS-2 limit).
  • Cost: 1 SMS credit.
Now, if you extend it to 150 characters with the emoji:
  • Segments: 2 (67 + 83 characters).
  • Cost: 2 SMS credits.

Warning
NOTE: these are guidelines only to help you understand the differences between credits and messages. Due to the possible inclusion of special characters and multi-media compoenents (MMS), your messaging credit consumption may vary.

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