# Why is it necessary to approximate starting and finishing gravity?

In the book "Home Beermaking", by William Moore, it talks about calculating starting and finishing gravity in chapter 6. I believe that he's talking about when I get good enough at homebrewing to make my own recipes, as the current recipes I'm using all have an approximate starting and finishing gravity already listed.

My question is: "why is this important?" Is the only purpose of calculating starting and finishing gravity to relate how much alcohol content the beer has to another individual looking to make beer from your recipe? If so, why can't one simply measure the starting and finishing gravity when first making the recipe (since undoubtedly the only way to determine if a recipe is good is to make it at least once), and then relate this to others?

Is there something about the approximate starting and finishing gravity that will tell a brewer whether or not the beer will balance before making it?

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The starting gravity and ending gravity serve many purposes, but ultimately will only tell you one thing, the percentage of alcohol. Some of the purposes it may serve are:

• Beer style guidelines
• Mouthfeel, flavor, bitterness, even aroma (FG)
• Yeast tolerance (SG)
• Efficiency of sugar extraction in all-grain brewing (pre-boil gravity)
• Hop efficiency
• How much grain/extract to purchase.

It really is one of the most important measurements of the beer (others include IBU's, and SRM).

If you formulate a recipe, quite often you may wish to create a style. For example, you may wish to brew a farmhouse ale, specifically a saison, you can see that it requires an original gravity between the ranges of 1.048 - 1.065, and a FG between 1.002 - 1.012. In order for your beer to technically be considered a saison by the BJCP's standards, you would need to land your measurements between those ranges. If you want to pull a Dogfish Head and brew whatever you want, toss out any guidelines, and brew what you want.

If you do go with the "screw it, I brew what I want" approach, it still serves other purposes. Your yeast will only be able to tolerate so much alcohol before it becomes toxic for the yeast, and it cannot tolerate it. For instance, you can't brew a Russian Imperial Stout with a low tolerance yeast, because of the tremendous starting gravity of the style. The OG will help you determine what yeast you should choose. If you go too low, the yeast will try to do its magic, but may die out too early and final gravity will be much higher than you want, making it unbearably sweet to drink.

When you all-grain brew, you "mash" the grains, which is soaking them in warm water to extract sugars from the husks. Each grain has a maximum efficiency, and while you'll never be able to reach 100% max, it's nice to not have to compensate with more grain because you had low efficiency. The way you measure the efficiency of your mashing process, is by taking a gravity reading (which will ultimately become your original gravity upon completion).

So as you can see, the starting gravity, and ending gravity is probably the most important measurement in brewing great beer.

As an aside, you shouldn't interpret William Moore's use of approximation as "winging it". While I don't own a copy of the book, I assume he's using approximate readings in one of two ways (or maybe both).

EDIT: Now that I understand what he is talking about, what he's explaining is essentially going to help you determine how much fermentables you need to use to essentially achieve a certain ABV (long-term goal). Essentially you'd be stone-age brewing if you didn't bother with gravity readings. What you'd get is what you get. What he's describing is one of the most basic steps in formulating a recipe.

With that said, while having knowledge of how to calculate your original gravity is very valuable and a great way to impress and confuse your assistant brewers and friends, there are plenty of tools that will do the hard work for you, freeing you up to get back to doing the fun part of homebrewing.

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Thanks for the thoughtful and clear answer! – jwir3 Jul 23 '13 at 21:57
I'm curious in what William Moore was using approximate SG and FG readings for? – Scott Jul 23 '13 at 22:00
Although this answer is fairly comprehensive, I don't agree with "mouthfeel" as the sole indicator of the impact of overall attenuation on the sensory response of a beer. A beer with high attenuation (low FG) will be less sweet than the same beer with less attenuation (higher FG). This can impact the flavor, bitterness, and potentially aroma of the beer. – Galapagos Jim Jul 24 '13 at 0:26
I may be misunderstanding your disagreement, but I didn't intend to state that mouthfeel is the only thing that would result from a low final gravity, much the same as yeast tolerance isn't the only related element in a high starting gravity. They're merely some of the indicators. With that said, excellent additions, I've added them to the list. – Scott Jul 24 '13 at 13:57
Ah, now that I have more context, I've edited my question. Nothing really changes. Although it should be said that by using the term "approximate gravity", as long as your numbers and quantities add up properly and you know your brewhouse efficiency, evaporation rates, etc, you should be able to get near exact gravity readings. There's a lot of factors that play into it, but as long as you know all the variables, you should be able to say with certainty that you will get 1.### reading from this recipe. – Scott Jul 25 '13 at 17:09