Knowing Your Original Gravity

This is a tough one. I had to put my thinking cap on before sitting down to write this response out. I need to get one thing clarified before I jump into my suggestions. It’s tempting to jump down the rabbit hole of brewing efficiency when discussing nailing target gravity, but being efficient, or not, has nothing to do with hitting the OG target. There is a foolproof method to achieve your goal that only requires a good hydrometer or refractometer, a calibrated wort kettle, a uniform sample, and some simple math. Let’s tackle this task one layer at a time.

But before I jump into my method to hitting target original gravity readings I need to take a brief moment to discuss grain crush and mash uniformity. If you’re an all-grain brewer buying grains already crushed or crushing your own grains, just note that how fine your malts are crushed as well as grain size will have an effect on your mash efficiency. Consistency is key here so you have a general sense where you should land. Also, make sure your mash is well stirred when mixing brewing water into the grains. Dough balls will have a negative impact on your brewing efficiency. 

Step 1

Do whatever you do to prepare your wort before boiling. Whether that’s a partial mash with extract added, a triple decoction, or an infusion mash with a few pounds of sugar added to bump your OG, do it. After you have all extract-contributing components added to your wort and all water in the kettle, including any last runnings dripping in from the mash tun, thoroughly, and I do mean thoroughly, mix your wort with a spoon or paddle. The goal is to make sure there are no dense pockets or weak pockets of wort. No need to splash about or make a big to-do while you stir, just be thorough.

Step 2

Take a sample of wort and measure your pre-boil volume. I am old-school and have three hydrometers, each with an 8 °Plato range (0–8, 8–16, and 16–24/gravity point equivalence: 0-32, 32–66, 66–102) and each with a built-in thermometer. Did I mention I like hydrometers? Once you take a sample, cool it down to room temperature or whatever temperature your device requires for proper use. While your sample is cooling, go ahead and begin heating your wort to boiling. Ideally, you want to cool your sample before you start boiling so you can adjust your hop weights if required or desired, but if you don’t plan on tweaking your hop calculations the wort sample and pre-boil volume is all you need.

Step 3

Calculate how much extract is contained in your kettle. This value is a constant and does not change during boiling unless you add malt, sugar, or extract, or spill part of your kettle. If you followed the directions in Step 1, the only reason your extract will change is through a spill. Let’s just agree that it’s a safe assumption that extract is a constant. An easy way to determine extract is by referring to the handy data shown in Table 1 and multiplying kg extract per liter by liters of wort (1 gallon = 3.785 liters; if your kettle is calibrated in gallons just divide your volume by 3.785 to convert to liters or calibrate it in liters).

If you want to do this long hand or write a spreadsheet, kg extract = (wort density) x (wort °Plato) x (liters of wort). Note that mathematical products of wort volume and wort strength (Plato), and products of wort volume and wort density don’t work as shortcuts for calculating extract; density and strength are both required.

Here is an example. Before the boil, you have 22 liters of wort, and your hydrometer reading is 10.5 °Plato/1.042; how much extract is in the kettle? We can see from the data table that the wort sample contains 0.109 kg extract per liter. And multiplying this by 22 gives us 2.4 kg of extract. We’ll come back to this value in a bit.

Step 4

Determine your final wort volume before boiling. To nail the target gravity, the only variable we can change is wort volume because extract is constant at 2.4 kg in our example and we want to hit our target, which, by the way, is 12 °Plato/1.048. Start by using the data table to determine that there is 0.126 kg extract contained in a liter of 12 °Plato/1.048 wort. We know we have 2.4 kg extract in the kettle and can determine the wort volume at 12 °Plato/1.048 wort by dividing 2.4 kg extract by 0.126 kg/liter. We want 19 liters after the boil. There are a few tips to make this all work out.

Tip 1 – Wort contracts when cooled but it’s easier to measure volume when it’s hot when we are still brewing. Nineteen liters of 68 °F (20 °C) wort equates to about 19.8 liters of 208 °F (98 °C) wort (19 ÷ 0.96).

Tip 2 – It’s a lot easier to add water at the end of the boil to bump up wort volume than it is to boil excess water away because too much was added in Step 1. This is where experience comes into play and helps determine how much wort to collect before boiling. And when cool water is added to hot wort, the wort contracts some and makes it hard to estimate cool wort volume. Using hot water to top up your kettle helps hit your wort volume and gravity targets when things cool down.

Conclusion

That’s it to this nailing your gravity discussion. If you have a well-mixed, pre-boil wort sample and know your pre-boil wort volume, you can determine your extract. Armed with this information and your target gravity you can determine how much wort you need to produce. Consistent brewing practices help keep adjustments minor and you can decide whether hop additions should be changed to track

Response by Ashton Lewis.