Saltpeter math

Nitrogen: The math

From Wikipedia: 

• Grams of urea in a liter of urine: 9.3
• Molar mass of urea: 60g
• Molar mass of potassium nitrate: 101g

Break out the Unit Factor Analysis manual...

So to get 1kg of Potassium Nitrate, you would need about 32L of feedstock.