The effective cost per delivered message is the price you actually pay for each message that arrives, calculated as tariff per send divided by the first-attempt delivery rate (plus the cost of retries).
Effective cost per delivered message is the price of a message that actually reaches the user, as opposed to the tariff you are billed for every send. The basic form is simple: tariff divided by first-attempt delivery rate. A $0.05 tariff on a route that delivers 92% of the time has an effective cost of about $0.054 per delivered message, because the sends that failed were still billed. The all-in figure is higher once retries are counted: total spend, including repeat sends, divided by the number of codes that actually worked.
For OTP and payment flows this is the number that matters, because an undelivered one-time password is not five lost cents — it is an authentication that failed and, often, a transaction that never completed. Comparing vendors on tariff alone hides this: a slightly higher tariff with stronger first-attempt delivery routinely produces a lower effective cost per delivered message, and a lower cost per delivered OTP, than the cheapest quote on the table.
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You pay per SMS sent, but only delivered OTPs work. A worked example shows why the cheapest vendor costs more.
Bulk SMS alone leaks budget. Verify the list, then run WhatsApp-first with SMS fallback.
