For years, I was a tablespooner: You know, I tablespooned out what I thought was the correct amount of (preground) coffee every morning, dumped it into a paper filter, and flipped the coffee maker on. Sometimes, I didn't even bother to pull out the scoop, instead relying on what I guess I thought was some kind of sixth sense "eyeballing" skill.
It's not without humility that I admit this to you, nor is it without regret that I existed this way for so long. I know that there are still tons of people out there doing the tablespooning thing, and I want to help them drink better coffee. How? By making a really heartfelt case for using a kitchen scale, because I absolutely am 100% convinced that this one simple change to a coffee-brewing routine can make a world of difference.
Let me show you!
Weight Matters (Or, "Matter Weighs")
There are several reasons to consider weight instead of volume—not just with coffee beans, but with just about any amount of dry ingredients you might use in the kitchen.
With coffee beans specifically, there are the important questions of size and density. Take a handful of beans out of a coffee bag and look at them: They're not all perfectly uniform, right? Not only that, but they're round instead of flat: They'll never—no, never—lay perfectly evenly or compacted in a coffee scoop, a tablespoon, or a measuring cup, which means that those measures aren't terribly accurate.
(Furthermore, my definitions of "heaping" or "rounded" or whatever kind of tablespoon I scoop might be different from yours, and most such instructions are vague at best: There's really no telling.)
For example: That (heaping) tablespoon of beans up there? Conventional wisdom tells us one of those will weigh 7 grams, but that one, as you can see, weighed 8.7 grams. Other tablespoon scoops I weighed came out at 7.5 grams, 9 grams, 9.4 grams (!), and 7.3 grams.
Sure, each discrepancy taken on its own seems like a small thing, but your coffee recipe only uses about 30 grams of beans per 12 to 16 ounces of water: Nearly 2 grams difference per tablespoon will throw your whole recipe out of whack—or at the very least, make each batch taste completely different.
I did a little test, aiming to measure out enough coffee to brew about 18 ounces—some for me, some for a friend. By weight, that would be about 50 grams; by volume, conventional wisdom says it should be about 7 tablespoons.
But look what happened when I scooped out 7 tablespoons:
Measuring ground coffee by volume was no better: In the following picture I scooped out 4 tablespoons each of ground coffee and whole beans, and look at what I got: Nearly 5 grams difference—and neither the 30 grams I was shooting for on that particular batch.
To make matters even worse, all ground coffee is not equal: A coarser grind will yield more volumetrically than a fine grind will, but the latter will typically be denser, so you'll wind up with more coffee. (See below: The fine grind on the right was scooped out using the "same" 4 tablespoons as the coarser stuff on the left.)
Why on earth would you measure the weight of your water? Again: Consistency and accuracy. If you measure your water's volume in a measuring cup before you boil it, you're not accounting for for what might convert to steam and evaporate out of the batch. Furthermore, if you boil your water and then measure it out volumetrically into a measuring cup before pouring it into your coffee, you're wasting heat as well as your own precious time.
Placing your readied coffee-brewing aparatus—be it a French press with the grounds in the bottom or a cone dripper resting on a mug—directly onto a scale and hitting "tare" before adding your water will let you know exactly how much hot water you're pouring in, which will give you much more control over the strength and extraction rate of the brew.
Consistency Is Key
With an inexpensive kitchen scale, all inconsistency is put right to bed: Every time I go to make a batch of coffee, I know exactly how much of my dry ingredients I've got, and I can apply that knowledge to the amount of wet ingredients (e.g. water) I will need to finish the brew.
That is to say, if I wanted a 10-ounce mug of coffee, I'd use 280 grams of water to do the brewing. Using my (cheap! durable! small!) scale, I can measure out 20 grams of coffee for a stronger cup, or 16 grams for a weaker one, and know exactly how much coffee I'm adding, because there won't be any tablespoons to make my measuring cattywompus. Using a scale allows me to keep my cup tasting just as good as the last time—or better, since I'll know just what I started with and what I'm aiming for.