Q: I’m having difficulty in hitting my OGs. Can you help?

A: Finding your pre-boil gravity is an easy way to detect whether you are going to hit your original gravity, and gives you time to course correct if things are looking a bit off. A pre-boil reading gives you a chance to extend your mash times to extract more sugars from your grain bill, if your reading is lower than expected. If your readings are higher, increase your sparge or pre-boil volumes to hit your targets. 

To calculate the pre-boil gravity from the target original gravity (OG) and your target final gravity (FG), you can use the following steps:

Determine the total gravity points: Subtract the FG from the OG. Gravity points are usually expressed in thousandths (e.g. OG of 1.050 is 50 points).

Gravity Points = (OG − FG) × 1000

Calculate the volume change: If you know the volume of wort before and after the boil, you can use this to adjust the gravity points.

Adjust for volume:

Pre-boil Gravity = (Post-boil Volume /Pre-boil Volume)​ x Gravity Points

Here’s an example:

OG: 1.050 | FG: 1.010

Post-boil Volume: 20 litres | Pre-boil Volume: 25 litres

Calculate the gravity points:

Gravity Points = (1.050−1.010) × 1000 = 40

Adjust for volume: 

Pre-boil Gravity = 20/25 × 40​ = 32

So, the pre-boil gravity would be 1.032. If you haven’t hit this, then adjust accordingly as per above. ■