Calculating how much extract to use in a partial mash - Does this look right?

I have been having trouble hitting my target gravities in my partial mashes so have decided to use some simple math to help me out (my teachers were right, math is useful).

Does the following seem right? Is there a simpler formula?

My partial mash yields ~3 gallons of wort. So If I were to take a gravity reading and divide it by two, I would have an accurate measurement of the partial mashes contribution to the total gravity, or is it not so simple?

To then calculate how much extract I need to add, I would subtract that number from my target gravity to determine how many gravity points are missing (the points that the extract would need to contribute)

As Liquid Malt Extract will make a wort of ~1.034 specific gravity per pound of extract per gallon of water, I would then divide the missing gravity points by 34, and multiply by 6 (the batch size) to find out how many pounds of extract to use.

Download beersmith. I've found its fantastic for converting AG to PM recipies, or extract to PM.

You put in the recipe that you have to start from, then subtract extract and add base grain (or add extract and subtract base grain) to achieve the desired result.

Plus you get to keep a nifty collection of your recipes.

Your method works.

Rook said:

Download beersmith. I've found its fantastic for converting AG to PM recipies, or extract to PM.

You put in the recipe that you have to start from, then subtract extract and add base grain (or add extract and subtract base grain) to achieve the desired result.

Plus you get to keep a nifty collection of your recipes.

Click to expand...

We are looking to do it mid brew. So we do our PM and as it enters the boil kettle take the Gravity, then figure out how much extract we need to add to hit our desired gravity.

I think it will work, also it is worth mentioning that Dry Extract contributes more gravity points than liquid.

DME will make a wort of ~1.045 specific gravity per pound of extract per gallon of water. Therefore you would need to divide the missing gravity points by 45, and multiply by 6 (the batch size) to find out how many pounds of extract to use.

I will try this technique next brew.