If you are looking for an answer to your question of "How to calculate horizontal velocity of a projectile?", then here is the projectile motion horizontal velocity formula for you to calculate the velocity on your own. It is a constant and can be calculated based on the distance travelled by the object and time taken to reach the ground. Time taken to reach value to be substituted in the formula should be in 'seconds' units, but the distance travelled value can be in any distance units.
The motion of an object in horizontal is independent of its vertical motion. Refer the above horizontal velocity formula to calculate the velocity of a projectile with the basic division operation.