Why is a movie's runtime determined by the screenplay's page count?

A movie's runtime tracks its screenplay's page count because standardized screenplay formatting was designed so that one page equals roughly one minute of screen time. That makes page count the industry's first runtime estimate — a 120-page script signals a two-hour film — which is why budgets, schedules, and shooting plans are all built from it.

The rule works because screenplay formatting is standardized: 12-point Courier, fixed margins, and strict conventions for scene headings, action lines, and dialogue. When every writer formats the same way, a page holds a roughly consistent amount of screen action, and page count becomes a unit of time. Write 90 pages and you've written a ~90-minute film; write 120 and you've written a two-hour one.

That conversion is why the industry keeps the convention. Long before a frame is shot, producers need to estimate runtime to budget the production, schedule shooting days, and judge whether a script fits its genre's expected length — and page count is the only runtime signal a script on paper can give. Readers and studios use it the same way: a 160-page draft reads as an overlong film before anyone times a scene.

It is an approximation, not a law. Dense dialogue pages tend to play longer than a minute, while sparse action pages — a car chase described in three lines — can play far shorter or far longer than their page space suggests. Screenwriting communities regularly document finished films running 15–20 minutes off their script's page count for exactly this reason, so treat the rule as a planning estimate that individual pages deviate from in both directions.

The same logic applies when you generate a film with AI: duration should be derived from the script, not set as an independent parameter. If you load a script into the invideo agent and ask it to generate the first five pages, you are effectively asking for the first five minutes of the film — the script determines the length, and a tool that instead asks you to pick a duration as a standalone input is ignoring how films are actually planned. One filmmaker testing AI film agents on a full script put it directly: "Generally films are written in a way that each page of script is about equivalent to 1 minute of screen time. So, by asking to generate the first five pages, I'm effectively asking to generate the first five minutes of the film. I don't decide the duration independently."

Watch some of these to see what works for you:

Generally films are written in a way that each page of script is about equivalent to 1 minute of screen time. So, by asking to generate the first five pages, I'm effectively asking to generate the first five minutes of the film. I don't decide the duration independently.

— a filmmaker who tested AI film agents on a full screenplay