Points Off? Defining gravity and why it matters

Monitoring your wort’s gravity at several check points during brew day can eliminate surprises.

Many homebrewers are number-obsessed types who get panicky when the numbers they predict don’t come to fruition. This nervous numerically driven condition is exacerbated by the comforting presence of equations that promise us certitude where there really isn’t much. The endless swirling demands and interactions of biology, agriculture, and organic chemistry means that short of Croesusian-level expenditures in locking in our starting conditions, we’re rudderless fools lucky that the wind usually blows us home.

Having said that, there are a few things you as a brewer can do to at least keep your gravity numbers somewhat in line. But before we get there — a reminder — if you miss things like your gravity: Congratulations! You’ve made beer! Next time you’ll get closer to your goal, or maybe, just maybe . . . you’ll have a Bob Ross-ian “Happy Accident.”

But why worry about it? Every recipe you see or write is based on the brewer’s efficiency. It’s how you decide how many pounds/kg of grain to use. This is especially important if you use someone else’s recipe. You need to adjust that recipe to reflect your own efficiency. One of the big causes of problems for homebrewers is that they brew someone else’s recipe as is without adjustment. Unless it just happens that your efficiency is the same, your original gravity (OG) will be incorrect.

Set Yourself UP For Success

The First Mantra: Know What the Heck Efficiency Is. The first step in being able to effectively hit your OG is to know your efficiency. Efficiency is a measure of how much of the total extract in the grain you’re able to draw out from the grain. Good news for extract brewers . . . you don’t have to worry about it! Efficiency only comes into play if you’re using grain that has starches that need to be converted into sugar. If you’re brewing an extract batch with a bit of specialty grain, fuggetaboudit! As long as you end up with the volume of wort the recipe was designed for, you WILL hit your OG.
The confusing part about discussing “efficiency” is that it can be measured at three different times. You can check your conversion efficiency as you collect the first runnings of your mash. You can measure mash efficiency once all the wort is in the kettle. Lastly, brewhouse efficiency reflects the wort in your fermenter. Each gives you important clues to how well your process is working.

Conversion efficiency measures how much of the grain starches converted to sugar, and ideally should be near 100%. You can easily check it via the chart at Look up your mash ratio and the gravity of your first runnings. There is also a spreadsheet if you want to dive in a bit deeper. If your conversion efficiency is low, you’ll first want to look at grain crush, then water chemistry, mash temperature, mash duration, mash ratio. . . basically your whole mashing process and any instruments you’re using.

Mash efficiency measures how much sugar, created in the mash, gets into your kettle. Calculate it by multiplying the gallons of wort you’ve collected by the gravity you measure, divided by the potential sugar extracted. Let’s run through a quick theoretical scenario:

Let’s say you have 6 gallons (23 L) of wort with a 1.050 standard gravity (SG) using 10 lbs. (4.5 kg) of grain. That means you have 6 x 50 gravity units (GU) or 300 GU. Rough rule of thumb says perfect extraction is 36 GU per pound (80 GU per kg). (Note: It varies malt-to-malt, lot-to-lot.) For a 10-lb. (4.5-kg) mash, maximal extraction yields 360 GU. In this example, our 300 GU means we pulled 83% of our possible sugar to the kettle (300 kettle/360 max).
If your conversion efficiency is good, but your mash efficiency is low, you’re either leaving wort behind in the mash tun, or your grain absorption calculation is incorrect.

One of the big causes of problems for homebrewers is that they brew someone else’s recipe as is without adjustment.

Finally, there’s brewhouse efficiency — how much beer ends up in the fermenter. This accounts for losses in the kettle in the form of hop absorption, wort left behind due to kettle deadspace, etc.

You calculate it the same way you calculate mash efficiency, by using the ppg (points per gallon) and GU differences between the mash maximums and your fermenter gravities and volumes. With our above example, when we measure the fermenter wort, we find that we have 5 gallons (19 L) at 1.054. We successfully pulled 270 GUs (5 x 54) to the fermenter for an overall efficiency of 75% (270 fermenter/360 max).

If your brewhouse efficiency is low but your conversion and mash efficiencies are good, try to make sure you’re getting all the wort out of your kettle. It may be necessary to start with a larger boil volume, or other adjustments like grist bill or adding a kettle fining for a more compact trub.

The Second Mantra: Know Your Ingredients. Part of the idea of certainty is to mitigate change and drift. Each new lot of malt, and each new harvest year is going to have differences. No matter how great the skills of our maltsters, the numbers will change. Malting technology and skill is pretty incredible — but grain refuses to be cookie cutter.
Drew used to use a particular brand of Maris Otter malt. Swore by the stuff, but then one year it just didn’t work any more. His gravities slumped, and more importantly, the beers developed a consistent haze. For the rest of that year, Drew used other malts, only switching back after his supplier had a new crop year of malt. These days, keeping a bag or two of your favorite base malt can definitely even things out when it comes to the numbers game.

The Third Mantra: Know Your Equipment. If you’ve ever listen to someone talk about their brand new kettle, or system, or other gee-gaw, you hopefully also heard the follow up frustration that occurred when things first went squirrelly. If you didn’t — well, they’re either the luckiest so and so or they’re lying to you.

This is especially true whenever you’re touching your mash apparatus. And we’re not even talking the big changes like switching from a brew-in-a-bag system to a cooler with a manifold. Even small changes, like how you drain the bag or how you add water or run off the wort can change your ultimate efficiency. Are you sure that your sparge water — or mash water for that matter — is getting every bit of malt goodness to your kettle?

The Fourth Mantra: Know Your Crush. These are probably the two most repeated answers we give to people who are suddenly worried about their gravity. All-grain becomes trickier to diagnose, but the first course is to always check the crush. The malt should be thoroughly cracked with plenty of kosher salt-sized malt chunks with lightly broken, husk material — not confetti! Denny’s crush slogan: “Crush the grain until you’re scared and then crush a little more.” Lastly – check your mill settings before each crush!

Why? A few years back Drew bought a nice three-roller mill to give him better crush control (and replace an old sticking tedious mill). First couple of brews were perfection! Great lauter, great extract, etc., but over time, his mash efficiency plummeted. Drew eventually remembered, “did you check the crush?” Sure enough, one of the set screws on the mill had slipped loose, backed off one side of the rollers during operation, letting a lot of very loosely crushed grain through. About 20 minutes of tinkering with new roller positions, crushing test runs, and he was back in business with a 15% improvement in efficiency.

Many brewers get their grain milled at their homebrew shop so they don’t always have direct control over their crush. One hopes that your shop will give you a good crush, but you can always ask to have the grain double milled. It’s mostly a stop-gap technique that doesn’t work as well as having a proper crush on your first run. If you receive poorly crushed grain, make sure the store knows! A note for the extract brewers — did you stir the wort vigorously to mix the extract all throughout the liquid? 99 times out of 100 that’s your problem. It’s that or your volume readings are inaccurate.

The Fifth Mantra: Know Your Water. Water is a complex topic we’ve covered before. Before taking a bead on water as an efficiency issue, you need to eliminate everything else first, because at the homebrew level, your water is only impactful on efficiency when it’s completely out of whack!
Get your water tested. For the sake of efficiency, make sure to have at least 50 ppm of calcium in your mash and acidify your mash liquor so that it hits a pH between 5.2–5.5. (Don’t assume your calculator is right – measure!) If you’re absolutely convinced that your water is at fault — splurge on distilled water (or reverse osmosis from a trusted source) and build up your calculated mash profile from scratch.

The Sixth Mantra: Know that Bigger Isn’t Better. People are inherently biased that more is more better. We’ve seen people set up to make a BIG beer with assumptions that their efficiencies will remain the same.
Remember that it’s not unusual for your efficiencies to drop 10–20% as you increase your grain to the limits of your mash vessel. Drew had one brew that he did just recently on his system that routinely gets ~78% efficiency. It was a test to see how far the system could go. With a very full vessel, the efficiency dropped from the expected 78% to 65%.

Pre-Boil Check

Just as you’re nearing the boil, take a sample and chill it quickly. Denny puts his sample in an old school metal Cobbler cocktail shaker and dunks the shaker into an ice bath. After a minute of swishing the glass around (or stirring the liquid) the wort is at reading temperature. Drew, on the other hand, uses a refractometer and waits a minute for the tiny sample to cool.
Regardless of method — take the gravity reading and multiply it by the amount of liquid you’ve collected. Take that and divide it by your target volume. Does it line up?

Example: You’ve collected 6.1 gallons (23.1 L) of wort at 1.050 SG. You planned to end up with 5.5 gallons (20.8 L) at 1.055. Congratulations you’re on target (6.1 gallons x 50 ppg = 305 GU; 305 GU/5.5 gallons = 55.5 ppg).
However, if you collected 6.1 gallons (23.1 L) at 1.045, you’re under by 6 points. If instead your 6.1 gallons (23.1 L) are reading 1.055, you’re over by 6 points! Should you decide to correct, what do you need to do?

My OG Is Too High: Aren’t you lucky and blessed? This is the easy one to fix. If your problem is because you have a much lower volume collected, then you can simply top up with the appropriate amount of de-chlorinated water to achieve the appropriate gravity.

If you have the right amount of pre-boil volume and you’re still too high, you’ll want to adjust your efficiency assumptions for the next batch. You can either dilute to achieve the proper gravity (with more volume) or ride with your newer high gravity and adjust your starting hops to provide more balancing punch.

My OG Is Too Low: Okay, this is the one that stresses brewers out. Understandably so — “agggggghhhhh, all my hard work is for naught!” First, chill out, duderino — you’re still on the happy path to beer.

Next, if you’ve already been following along with the wort’s gravity readings for conversion efficiency then you should already have an indication of problems at this point. Over-sparging is the next most common occurrence. Check your liquid levels — are your levels too high? (e.g. I was supposed to collect 6.1 gallons/23.1 L of runnings, but instead I collected 6.5 gallons/24.6 L.) If so, you’ve possibly diluted your wort with too much water. A longer boil can easily correct your issue. To calculate the time, it helps to know your boil-off rate per hour. Drew, for instance, knows on his all-in-one system, he’ll steadily boil a little more than a half-gallon (2 L) per hour. For every half-gallon (2-L) extra, he’d need to boil an extra hour.

Important Note: If you’re boiling longer than expected due to excess volume, make the decision to do so prior to adding your first addition of hops! Wait until you have 60 minutes left before adding your traditional bittering addition!

Adjusting your boil time is always going to be your primary tool, but what if you really messed up and missed your gravity by an unreasonable amount? Don’t hesitate to reach for extract or sugar to boost your gravity – or – and we know this will be “disappointing” – adjust what your end goal is. Instead of an IPA, how about a nice pale ale? Instead of an all-malt strong ale, how about a Belgian-influenced strong ale with sugar to boost the gravity?
There you go — some simple rules to follow and think through when you’re fretting about your gravity! Remember, as with all things homebrewing, you’re making beer, so have some fun with it!

If your conversion efficiency is lower than expected, taking a close look at your grain’s crush may solve for your lower than expected brewhouse efficiency. Photo by Charles A. Parker/Images Plus
Issue: January-February 2021