Slant Height of a Cone l = √(r²+h²). Radius Height Slant 9.487 Step-by-step with your numbers: 1. Values used: 2. Radius = 3 3. Height = 9 4. Slant = 9.487 Did we solve your problem today? 👍 Yes 👎 No Thanks for the feedback! Pythagoras on cone. FAQ Use? Surface area. Related calculators Cone Calculator Find a cone's volume, slant height and surface area. Slant Height of a Cone Calculator l = √(r² + h²). Pythagorean Theorem Calculator Find the hypotenuse of a right triangle from the two legs.