Original gravity (OG) is a measurement of how much dissolved sugar is in your wort, the unfermented liquid that will eventually become beer. It’s taken before fermentation begins, and it tells you how much potential fuel the yeast has to work with. A typical OG might range from 1.028 for a light lager to 1.080 or higher for a big imperial stout, with pure water sitting at a baseline of 1.000.
How Original Gravity Works
When you mash grain or dissolve malt extract in water, sugars and other soluble compounds enter the liquid. These dissolved solids make the wort denser than plain water. Original gravity is simply a measure of that density, expressed as a number relative to water’s density of 1.000 at standard temperature and pressure. A wort with an OG of 1.050 is 5% denser than water, meaning it contains a substantial amount of fermentable material.
The “gravity points” system is a shorthand brewers use to make the math easier. An OG of 1.050 equals 50 gravity points. This makes it simple to scale recipes up or down and compare different batches at a glance.
The Two Main Scales: Specific Gravity and Plato
You’ll see original gravity expressed in two ways depending on where you’re reading. The specific gravity scale (written as 1.040, 1.060, etc.) is the most common in homebrewing. The Plato scale, favored by many professional breweries, measures sugar content as a percentage of the solution by weight. One degree Plato means the liquid is 1% sugar by weight.
The two scales aren’t perfectly linear with each other, but here are some common conversions:
- 1.040 SG = 10.0°P
- 1.048 SG = 12.0°P
- 1.056 SG = 13.8°P
- 1.064 SG = 15.7°P
- 1.072 SG = 17.5°P
- 1.080 SG = 19.3°P
You may also see degrees Brix, which is functionally equivalent to Plato for brewing purposes.
How to Measure Original Gravity
The most common tool is a hydrometer, a weighted glass tube that floats in your wort sample. Denser liquid pushes the hydrometer higher; you read the number at the liquid’s surface. Hydrometers are inexpensive and reliable, but they’re fragile, and they need a fairly large sample (usually a few ounces). They also require a temperature correction if your wort isn’t at the hydrometer’s calibration temperature, which is typically 60°F for older models or up to 70°F for newer ones. Warmer liquid is less dense, so a hot sample will read lower than its true gravity.
A refractometer is the other popular option. It works by measuring how light bends as it passes through a drop of liquid. Denser, sugarier liquid bends light more. The advantage is speed and sample size: you only need a drop or two, and you get a reading in seconds. This makes refractometers ideal for checking your wort on brew day before fermentation starts. The drawback is that once fermentation begins, the alcohol and dissolved carbon dioxide in the liquid distort the reading, making refractometers unreliable for tracking fermentation progress unless you use a correction calculator.
Digital densitometers exist too, offering lab-grade precision, but at a cost of a couple thousand dollars they’re really only practical for commercial operations running hundreds of samples.
Why Original Gravity Matters
OG is the starting point for calculating alcohol content. Once fermentation finishes, you take a second reading called the final gravity (FG). The difference between the two tells you how much sugar the yeast consumed and converted to alcohol. The standard formula is straightforward:
ABV = (OG − FG) × 131.25
So if your OG is 1.050 and your FG is 1.010, you get (0.050 − 0.010) × 131.25 = 5.25% ABV. That’s a typical strength for a pale ale.
Beyond alcohol, OG gives you insight into body and sweetness. A beer that starts at a high OG but finishes with a relatively high FG will taste fuller and sweeter, because residual sugar remains. A beer that starts at the same OG but ferments down to a very low FG will be drier and thinner on the palate, even though it’s higher in alcohol.
Typical OG Ranges by Beer Style
Different beer styles call for very different starting gravities. The Beer Judge Certification Program (BJCP), which sets the standard guidelines for competitions, specifies OG ranges for every recognized style. A few examples give you a sense of the spectrum:
American light lagers sit at the low end, with an OG of 1.028 to 1.040. These are designed to be crisp and easy-drinking, with minimal residual sweetness. Standard American lagers and wheat beers typically land in the 1.040 to 1.055 range. IPAs and amber ales push into the 1.056 to 1.075 territory, where you start getting noticeable malt backbone alongside hop bitterness. Imperial stouts, barleywines, and Belgian strong ales can climb above 1.080 or even 1.100, producing beers with 8 to 12% alcohol or more.
Hitting Your Target OG
If you’re brewing and your OG comes in lower than expected, it usually means you extracted less sugar from your grain than planned. Common causes include a mash temperature that was too low, not enough grain, or sparging too quickly. You can correct on the fly by boiling longer to concentrate the wort or adding a small amount of dry malt extract.
If your OG is too high, the fix is simpler: add water to dilute the wort down to your target. This is one reason experienced brewers check gravity before pitching yeast. It’s much easier to adjust at this stage than to fix a finished beer that’s too strong or too weak.
Consistency in your OG readings across batches is one of the clearest signs that your brewing process is dialed in. Small variations of a few points are normal, but if you’re regularly missing your target by 10 or more points, something in your process needs attention, whether that’s your grain crush, your water volume, or your mash technique.