These are only a rule of thumb as the ultimate size of Water lilies and whether they reach their potential size will depend on many factors which will determine how happy they are in situ. These factors include amount of sun, whether the water is calm and they are not subject to continual splashing water, depth of planting, growing medium, pH level of the pond, availability of nutrients and fertilisation etc.
Also remember you are aiming to cover approx. 50% of the surface area of the pond, not the whole pond, when you are determining numbers and you should also take account of other deep water and floating plants you are considering introducing or already have in your pond.
See Our Water lilies by size for guidance on which lilies fall into the categories below.
Dwarf Water lilies: 1 per 0.25 sq.m (2.7 sq. ft)
Small Water lilies: 1 per 1.0 sq.m (10.7 sq. ft)
Medium Water lilies: 1 per 1.5 sq. m (16 sq. ft)
Large Water lilies: 1 per 2-3 sq. m (21 -32 sq. ft)
To calculate the area of a circular pond simply multiply the radius squared by 3.142.