What Is a VSL? How to Make a Video Sales Letter With AI
By Steven Van ·
Synthesys explains VSL script order, length ranges and how to build one with its AI video agent, using a 3-minute test run.
A VSL, or video sales letter, is a scripted video that walks one viewer from a problem to a single offer. Synthesys has published a guide to the format and to making one with its AI video agent.
How long a VSL should be
The guide says a VSL should run as long as the offer needs to be believed. The more a viewer is asked to pay or commit, the more proof and time the video needs. Published advice it cites varies widely:
- Sendspark: 60 to 120 seconds for cold outreach, 3 to 7 minutes on a landing page, and 15 to 20 minutes for high-ticket offers.
- Versely: 6 to 9 minutes for cold traffic and 4 to 6 minutes for warm traffic.
- Rob Palmer: 5 to 15 minutes for low-cost offers and 20 to 45 minutes for higher-priced ones.
For script length, it uses about 150 words a minute for conversational US speech, so a 3-minute VSL is roughly 450 words and a 10-minute one roughly 1,500. A slower read of 120 words a minute turns 750 words into about 6 minutes 15 seconds. In the Synthesys app, the Sales Video template runs 1 to 10 minutes and presentation videos run 1 to 30 minutes.
What goes into the script
The guide lists the usual order: hook, problem, solution and mechanism, proof, offer, and one call to action. It notes that proof has to come from the user, because an agent should not invent it.
Making one in Synthesys
- Write a brief naming the offer, the viewer, the length, the aspect ratio and the action wanted. Pick the Sales Video template and one of its 31 styles, and attach existing sales copy, a landing page link or product photos.
- Choose a presenter from the character library, describe a custom one, or film a short clip for a digital twin. Voices come from a library of 400+ or a clone of the user's own. The user must hold the rights to any face or voice they upload.
- Review the storyboard and script before rendering. Scripts, storyboards, voiceover, music and chat messages are free.
- Check the credit cost on the approve button. In confirm mode the agent stops at each step for approval, while auto-run goes through to the finished cut. A failed generation costs nothing.
- Edit the cut by chat, for example to trim, reorder, re-voice or rewrite a line. Asking for another language changes the voice, captions and on-screen details together.
- Export for each placement: 16:9 for most landing pages, with shorter 9:16 or 1:1 cuts for the ads that send traffic.
The test run
Synthesys ran the Sales Video template on 2 October 2026 in confirm mode, using a brief for a 3-minute VSL selling an online bookkeeping course for freelancers, in the Warm Documentary style. The agent returned 18 scenes of 10 seconds each, but only 313 words of script, about 105 words a minute and well under the 450 words the table suggests. The brief gave the agent no results to work with, so the script went from the problem straight to the course and the call to action.