_{1}* V

_{1}= G

_{2}* V

_{2}. For example, 6 gallons of wort at 1.042 has 252 total points of gravity. (42 * 6 = 252) Boil that down to 5 gallons and the gravity will be 1.050. (252 / 5 = 50) It works when topping up a concentrated wort as well. Adding 2 gal of water to 3 gal of 1.084 will give me a OG of 1.050 (84 * 3 = G

_{2}* 5).This is extremely useful in hitting your OG because it means you can take a gravity measurement any time during the process and convert it into your final OG. It allows you to measure gravity after the sparge is complete to ensure you have enough sugar in your wort to hit target OG, or occasionally to take excess sugar out of the boil if you get a high efficiency. Using the example above, assume you want that 1.050 OG, but you measure your sparged gravity and only get 1.038 in 6 gallons of sparge water. Total sugars are 38*6 or 228. If we went ahead and boiled that down to 5 gallons, the final gravity would only be 1.046 (228/5). Instead we can add 24 points (252-228) of DME to hit OG exactly. DME generally provides 45 points per pound, so a little more than 1/2 pound would do it. Remember, you can just measure gravity again after the addition.The best thing about this is that you're taking your gravity readings pre-boil when sanitation isn't an issue. You can add the samples back to the boil. It does, however, require that you have accurate volume measurements. A notched brew spoon or dipstick, maybe a marked kettle are essential to making this work. Another thing to remember is that although I've been using 5 gallons for clarity, the real final volume should include what you leave behind in the break and hops. It also contains sugar too that won't make it into the fermenter. The easiest way to account for this is to bump expected final volume to 5.5 or even 6.