Gravity readings are taken to gauge fermentation. You need to buy a hydrometer. Take a reading of your beer before you send it into the fermenter. To do so, fill the tube up with wort (the beer) until it's about 2-1/2" from the top of the tube. Insert hydrometer. Spin to release the bubbles and place on flat surface. Read the number that corresponds to the wort line. That is your OG, or original gravity. Think of it as a starting point. Your recipe will have an OG, that is your goal, to reach that number. If your number is lower than the OG on the recipe, your beer has less sugars than the recipe. Your efficiency is low. If your OG is higher than the recipe, you have more sugars that the recipe called for, your efficiency is high. The higher the OG, the more alcohol will be in your final product, for the most part.
So, you've got the OG, wort goes into the fermenter, in goes the yeast and after 2 weeks, you take a gravity reading with your hydrometer to see if the yeast is all done working. The number should be nearing your FG, or final gravity. The difference in numbers OG-FG, is the amount of sugars your yeast ate and turned into alcohol. If your beer remains at the same FG for 2 days, about 3 gravity readings, it's done, and ready to be bottled. You can use the OG-FG to measure ABV.
Your gravity readings tell you many things. Efficiency, abv, attenuation, whether your beers fermented out, and many other things. Get a test tube or vial of some sort(sanitized) and fill it with wort, then put the hydrometer in it and see where it levels out at.... Ex. 1.060 or 1.042. Check what temperature the sample your taking and use hydrometer temperature adjustment chart, most hydrometers come with a sheet that has it. Many websites can do it for you just google it. Punch all your readings and their temperatures into the chart and you have your correct reading. Also make sure your sample is chilled to 100 F or less. Hope this helps.
Hey, I'm fairly new myself, but as far as i am aware, most home brewers like to do gravity readings, firstly for calculating the abv (which is nice to know), and also for telling when the brew is ready to bottle/finished fermentation.
So you take your first reading (OG - original gravity) you take this reading with your hydrometer before you add the yeast, and write down the number. (example 1.055).
then (after a week or two of fermentation) check the gravity again, and if the gravity stays the same over 3 or so days it's ready to bottle. =D
I have probably left something out, so if I have let the other people fix it up, and most of all, GOOD LUCK =D
Can you brew beer without taking a gravity? Absolutely.
Now that that's out of the way.
Why take a gravity reading?
It really depends what your goals are. If you are a beginner, it's less important (IMHO) as you're concentrating on sanitation and pitching temperature and not leaning over your kettle and dropping a pen in your boil.
Gravity readings become important when you're designing recipes, going for specific styles, measuring brewhouse efficiency, determining alcohol %, and more. It becomes especially important when you brew all-grain to ensure you're hitting your preboil gravity (good conversion, efficiency, sparge gravity, etc).
On the other end, you want to check your gravity to ensure your beer has fermented out all the way. If you start at 1.060 and you get a stuck fermentation at 1.050, you're probably not going to want to drink the result unless you pitch some more yeast. So measuring gravity on the back end is probably more important for producing a drinkable result (although you're only going to have a ballpark of how well your fermentation went if you don't have a starting gravity).
To measure your gravity, you want your wort close to room temperature. Fill a graduated cylinder about 7/8 or slightly less. Take a temperature reading of the wort (you'll need this later to add to a program like Beersmith to adjust the gravity for temperature). Gently lower your hydrometer into the wort and gently release and spin it. The goal is not to smack it off the bottom of the cylinder and break it. It's pretty fragile. Wait about 30 - 45 seconds and then see where the wort level is on the hydrometer. Plug this into Beersmith or other hydrometer adjust tool, then plug in the temperature and it'll give you your gravity reading.
Awesome info. This really helps! I'm just going to do kits for a while then kick it up a notch. I have a red ale LME for batch. Number one. Wife let me buy it today, then told me I can't make it Gil we get back from a trip we r going on. We don't leave Gil next week
If you figure out what recipe you want to make you can order the ingredients from a Homebrew supply website for cheaper than kit of the same beer costs. Also think about bulk ordering your LME or DME since you will be using it in all your beers. Doing this will also save you money and it's good to always have some on hand.