Example:

Enter the amount of spacing required in the calculator.
Required plant spacing = 20 centimetres
The multiplier for 20cm plant spacing for rectangular pattern planting = 2.5
The multiplier for 20 cm plant spacing for triangular pattern planting =  2.89

An area of 10  metres by 3 metres for planting.
Square metres of area to be planted = 10m x 3m  =  30 sq metre

Multiply your square metres by the multiplier.
Estimated amount of plants needed for rectangular pattern planting 30 x 2.5 =  75 plants.
Estimated amount of plants needed for triangular pattern planting 30 x 2.89 =   87 plants.