Milk is a valuable resource which unlocks the Kitten series of upgrades that greatly increase cookie output rate in direct correlation to the amount of Milk the player has. Milk is gained by unlocking normal (non-Shadow) Achievements at a rate of 4% milk per Achievement. As of version 1.0465, the total number of Achievements that count towards milk is 157 and the maximum attainable Milk amount is 628%.
The Milk is represented by a wave-animated overlay at the bottom of the Big Cookie partition. The overlay gradually gains height as the Milk percentage increases. The milk changes colour and name when the Milk percentage reaches a new hundred, although this has no effect on cookie production. New Milk types and Kitten Upgrades have been introduced with new Achievements and the consequential higher possible Milk levels.
Below 100%, the Milk is plain white. There is also very little Milk at the start of the game. When the Milk reaches 100%, it turns into chocolate Milk, with the color turning brown. As it reaches 200%, it becomes Raspberry Milk, giving the Milk a pink color. At 300%, it is orange juice and is colored orange. As it reaches 400%, it becomes Caramel Milk.
Below are five images the game uses to overlay the lower background.
At 52% Milk, you may unlock the Kitten helpers upgrade for 9 million cookies. At 100% Milk, you may unlock the Kitten workers upgrade for 9 billion cookies. At 200% Milk, you may unlock the Kitten engineers upgrade for 90 trillion cookies. At 300% Milk, you may unlock the Kitten overseers upgrade for 90 quadrillion cookies. At 400% Milk, you may unlock the Kitten managers upgrade for 900 quintillion cookies.
With Kitten helpers, you gain 5% of your Milk percentage as more cookies, stacked multiplicatively with cookie upgrades. If you had 100% Milk, Kitten helpers, and 2 cookie upgrades 5% each, your multiplier would be 115.5%
With Kitten workers, another 10% of your Milk is added to your multiplier. This is stacked multiplicatively with Kitten helpers as well as cookie upgrades.
Kitten engineers adds another 20% of your Milk into your multiplier. This is stacked multiplicatively with cookie upgrades as well as previous Kitten upgrades.
Kitten overseers adds yet another 20% of your Milk into your multiplier. Same rules as before.
Finally, Kitten managers adds yet another 20% of your Milk into your multiplier. Same rules as before. See CpS for details of the calculation.
Let's add several cookie upgrades 150% total, 300% milk and all five kittens: (665%+150%)*(100%+300%*5%) *(100%+300%*10%)*(100%+300%*20%)*(100%+300%*20%)=(665%+150%)*1.15*1.30*1.60*1.60*1.60=3743.0016%
Santa's Milk and CookiesEdit
The Santa's Milk and Cookies upgrade makes "Milk 5% more powerful". This is a final (additional) 5% bonus. It does NOT affect the individual kitten multipliers.
Dunking the Cookie Edit
The Cookie-dunker Achievement is gained by accumulating enough Milk and adjusting the browser window in a way that the Big Cookie is overlayed by the Milk graphic on a sizable area. This can be done by zooming in a lot or making the window very small.