Lensmaker's equation:
1/f = (n₁/n_m-1) * (1/r₁-1/r₂)
Where,
n₁ -Refractive Index of Lens Material
n_m -Refractive Index of Ambient Medium
r₁ -Radius of Curvature of the First Surface
r₂ - Radius of Curvature of the Second Surface

Basically, the Lensmaker Equation is used to make Lenses. This online calculator helps you dynamically to determine the focal length of the lens.