What it means
The window decides what you are allowed to send. When a customer messages your business, a 24-hour window opens in which you can reply with anything: free-form text, images, documents, lists and buttons. Each new customer message resets the clock. Outside a window, only a pre-approved template can be sent.
Until 30 September 2026 those replies are free. From 1 October 2026 Meta bills them as service messages: every business phone number gets 1,000 free a month, and from the 1,001st each reply costs the utility rate for the customer's country. A utility template sent inside the window is charged at the utility rate with no free allowance.
Click-to-WhatsApp ads open a longer 72-hour free entry point window, which is unchanged: every message you send inside it is free to deliver. Outside any window, a broadcast, reminder or re-engagement is a template at its category rate: Marketing, Utility or Authentication.
Why it matters
Most cost surprises on WhatsApp come from misunderstanding the window. Teams who reply within 24 hours stay inside the free 1,000 on a typical line; teams who let messages pile up and send a template the next day pay the template rate on every one.
From October the second lever is reply density: an AI employee that answers in one complete message instead of three fragments uses a third of the allowance. The cost calculator on this site runs on Meta's 1 October 2026 rate card, service messages included.
Example
A spa owner sees 200 enquiries a month. An AI employee answers 180 of them within 24 hours, about 700 replies in total, all inside the 1,000 free service messages for the number. The remaining 20 need a reminder the next day, which is a utility template at a small per-message fee. The Meta bill for the month is a few dollars, not a few hundred.