Example:

Enter the amount of spacing required in the calculator.
Required plant spacing = 14"
The multiplier for 14" plant spacing for rectangular pattern planting = 0.73
The multiplier for 14" plant spacing for triangular pattern planting =  0.85

An area of 20 feet by 30 feet for planting.
Square footage of area to be planted = 20' x 30' = 600 sq feet

Multiply your square footage by the multiplier.
Estimated amount of plants needed for rectangular pattern planting 600 x 0.73 =  438 plants.
Estimated amount of plants needed for triangular pattern planting 600 x 0.85 =   520 plants.