Does my coffee hydrate or dehydrate me?

coffee_hydration.rb

# Do you hydrate or dehydrate your body when having a coffee?

# Constants based on references below
MG_OF_CAFFEINE_PER_SHOT  = 77  # mg
SHOT_VOLUME              = 44  # ml
WATER_PERCENT_IN_MILK    = 87  # %
WATER_LOSS_FROM_CAFFEINE = 1.17 # ml/mg

# number_of_shots : standard single shot
# size_of_cup     : in ml
# milk_based      : true if the body of the cup is mainly milk
def calculate_water_change(number_of_shots, size_of_cup, milk_based)
  # How much water gained?
  shot_volume = number_of_shots * SHOT_VOLUME
  other_volume = size_of_cup - shot_volume
  
  water_gained = if milk_based
    other_volume / 100 * WATER_PERCENT_IN_MILK
  else
    other_volume
  end
  
  # How much water lost?
  caffeine = MG_OF_CAFFEINE_PER_SHOT * number_of_shots
  water_loss_from_caffeine = caffeine * WATER_LOSS_FROM_CAFFEINE

  # Conclusion
  net_water = water_gained - water_loss_from_caffeine

  word = net_water > 0 ? "gain" : "lose"
  puts "For each coffee you #{word} #{net_water.abs.to_i}ml of water"
end

# double shot 8oz cappuccino:
calculate_water_change(2, 237, true) #=> lose 93ml

# double shot 12oz black coffee:
calculate_water_change(2, 355, false) #=> gain 86ml

# double shot 12oz latte:
calculate_water_change(2, 355, true) #=> lose 6ml


# Facts:
# 
# * Water loss due to caffeine is 0.00117 L/mg of caffeine [1]
# * 77mg of caffeine per 1.5 ounce shot [2]
# * Cow milk is about 87% water [3]
# * 8 US fluid ounces = 236.588237 ml
# * 1.5 US fluid ounces = 44.3602943 ml
#
# References:
# 
# 1 "The diuretic effects of alcohol and caffeine and total water intake misclassification." - http://www.ncbi.nlm.nih.gov/pubmed/10204649
# 2 "Espresso" - http://www.energyfiend.com/caffeine-content/espresso
# 3 "Milk Composition Water" - http://classes.ansci.illinois.edu/ansc438/milkcompsynth/milkcomp_water.html

Note:

• It assumes your cup is full to the brim
• For non-milk based drinks it assumes you're having black coffee and don't add any milk.