How to calculate coffee amount for finished serving size?

Question

When you brew coffee, the grounds retain some of the water (roughly 2g of water for every 1g of coffee). If you prepare your coffee by weighing everything with a gram scale, the water:coffee ratio is based on all of the coffee and all of the water that goes into the brew.

But, for instance, suppose you want to brew a single serving, and want a specific amount of brewed coffee in your cup when you're done (and don't want to just brew a random larger amount that might be short of the desired serving, or a little more that you will need to drink after it's cold or toss).

You need to start with the desired serving size of water plus the amount of water that will remain in the grounds. And the amount of ground coffee required will be based on the amount of water in the finished serving plus the amount of water that will be retained in those grounds, which will be based on the amount of grounds you're trying to determine.

How do you figure out the required weight of coffee to produce the desired finished serving size? I had to figure this out for some testing, so I'll share it.

Answer 1

The water:coffee ratio can be expressed as:

Where:
R = water:coffee ratio
W_s = weight of the water in the serving
W_c = weight of the water retained in the coffee grounds
C = weight of the coffee grounds

Since 2g of water are retained for every 1g of grounds, we can substitute 2C for W_c :

With a little algebra rearranging, you get:

Here's an example of how you use the formula:

• Suppose you want a 12 fl oz serving brewed at a 16:1 ratio.
• You can approximate the water at 30 g/fl oz for easy math, which would give you 360 g of water in the serving. But if you're weighing grams, you're probably a stickler for precision, so the accurate conversion would be 355 g.
• The ratio minus 2 would be 14.
• The required coffee would be 355 / 14 = 25.4 g.

The required water would use the desired ratio (16) with the calculated coffee:

• 16 * 25.4 = 406 g