WhatsApp charges for using the Business API based on how many conversations you have with customers and who initiated the conversation. Note that WhatsApp does not charge by message. There are no whatsapp imposed limits to the number of messages you can send in a in a 24 hour session called the "conversation"
As mentioned earlier WhatsApp charges will depend on who initiated the conversation. Conversations can fall into two types and are charged differently:
All conversations are measured in fixed 24-hour sessions. A conversation starts when the first business message in a conversation is delivered, either initiated by the business or in reply to a user message. Businesses and users can exchange any number of messages, including template messages, within a 24 hour conversation session without incurring additional charges. Each 24 hour conversation session results in a single charge.
Charges for conversations are based on the user’s country code. A user here is defined as the customer that your business is communicating with. Rates for business-initiated and user-initiated conversations vary by country or region.
The first 1,000 conversations each month are free. This way your business can build experiences your customers will love before having to pay. Each WhatsApp Business account (WABA) will receive 1,000 free conversations per month. These conversations can be either user or business-initiated. Even if a WABA has multiple numbers attached, the free tier threshold is still 1000, as the free tier is provided at the WABA level. The free tier is refreshed monthly, based on the timezone associated with the WABA.
Conversations are not charged when users message businesses using call-to-actions buttons on Ads that Click to WhatsApp or a Facebook Page call-to-action buttons. Free entry point conversations can only be user-initiated. The first conversation that starts from the entry point is free of charge, then subsequent conversations with the user are charged.
Standard pricing applies for Ads that Click to WhatsApp. The conversation that initiates from the ad is free, but not the ad itself.
You can download the individual rate cards and find the country to region mapping for countries where regional rates apply.