How long does a mobility ramp have to be?

You need to calculate the rise. Here is an online calculator https://www.discountramps.com/wheelchair-ramp-length/a/B20/

you need to call your town to make this properly. There is a building code for this type ramp

Five feet would be long enough if there is only one step. ADA requires handicap ramps to be no more than 15% grade. You can use a plastic protractor from the school supplies aisle—the ruler that look like a half circle.

Here are the ADA Rules and Regulations to be compliant.

Looks like a half circle, if I have my names wrong. Make it less if you can. Please consider putting a railing on at least one side. And add a cement landing at the bottom and a spacious landing at the top so the user doesn’t lose traction or balance using the ramp. Good for you for asking! ☺️

The ADA recommends a 1:12 slope, which means that every 1" of vertical rise requires at least 1' (12") of ramp length (5 degrees of incline). Example: A 24" rise requires a minimum ramp length of 24' (288") (24 divided by 1).

The ADA rule is one ft or 12 inches for every inch of elevation! Thirty inch elevation needs a 360 inches or a 30 ft ramp for wheelchair or scooter!

Hope this helps