Two Different Channels for Two Different Jobs
SMS and WhatsApp look similar on the surface, but they serve completely different purposes in your marketing stack. SMS is an interruption-based channel built for pure urgency. In my books, you should only really be sending three types of SMS messages: major sales, product releases, and restock alerts.
WhatsApp covers those exact same commercial use cases, but it excels at conversational marketing. You can initiate a two-way dialogue, answer questions, and guide a customer toward checkout far more naturally than over SMS. That fundamental difference dictates how each channel performs, how much it costs, and where you should use it.
Benchmarks: Open, Click, and Conversion Rates
Both channels deliver impressive engagement metrics compared to standard email, but the interaction patterns differ. SMS delivers open rates between 90% and 98%, with messages read within minutes of delivery. Click rates typically land between 6% and 15% (at In-box, we aim higher and regularly hit around 22%), while conversion rates sit between 1% and 5%.
WhatsApp delivers slightly lower open rates of roughly 80% to 95%, but its click-through rates are noticeably higher at 10% to 30%. Conversion rates follow suit, hovering between 2% and 8%. More importantly, the response rate on WhatsApp blows SMS out of the water.
SMS is a direct revenue injection. WhatsApp is a conversation that leads to a conversion.
The Cost Model: Per Message vs 24-Hour Windows
Here is where the economics flip entirely. In Klaviyo, SMS operates on a straightforward pay-per-message structure. You pay roughly 1 cent to 5 cents per send depending on the recipient country, and any message running over 160 characters will cost you multiple credits. If you send 10 SMS messages to one subscriber over a 24-hour period, you pay for 10 separate sends.
WhatsApp does not charge per message. Instead, Meta charges per 24-hour conversation window. Once a conversation window is opened, you can exchange as many back-and-forth messages with that customer as needed within 24 hours for a single flat fee.
WhatsApp does require pre-approved message templates and carries slightly more setup friction inside Klaviyo. But if your strategy relies on back-and-forth engagement or customer support assisted selling, WhatsApp becomes substantially cheaper at scale. SMS gets expensive fast.
Consent and Opt-In Rates
Collecting phone numbers for SMS carries notable customer friction because shoppers know they are signing up for direct promotional pings. WhatsApp often achieves higher opt-in rates on sign-up forms simply because international shoppers use the app daily for personal communication and feel more comfortable sharing their handle.
If you are already collecting SMS consent on your sign-up forms, adding WhatsApp is painless. It usually requires only a minor adjustment to the legal wording in your terms and conditions to capture permission for both channels simultaneously.
Geography Dictates the Winning Channel
Your subscriber geography should be the ultimate deciding factor here. WhatsApp dominates consumer communication across Europe and Latin America (especially markets like Mexico and Brazil), making it an absolute must-have if you sell into those territories.
In the United States, Australia, and New Zealand, SMS remains the default standard for mobile marketing. However, sending SMS in Australia and New Zealand is notoriously expensive on a per-carrier basis. If your brand has an international footprint spanning both Western and European or LATAM markets, running a blended strategy inside Klaviyo gives you the best return on spend.
Final Thoughts
SMS is ideal for fast, high-urgency revenue spikes in traditional markets like the US and Australasia. WhatsApp is the better, more cost-effective tool for conversational sales and international audiences in Europe and Latin America. Choosing the right channel comes down to where your customers live and whether you want a quick broadcast or a two-way conversation.
Ready to Scale Your Mobile Marketing?
If you want to build high-converting SMS and WhatsApp flows without burning your marketing budget, explore our SMS campaign services to see how we manage both channels for growing ecommerce stores.
