Refractometer calculater vs ABV calculator

[Mr.Stroganoff]

Hi, hoping I'm not using the wrong forum.

I was wondering why there is such a huge difference in result when I use both the ABV calculator and the refractometer calculator:
http://www.brewersfriend.com/abv-calculator/
and
http://www.brewersfriend.com/refractometer-calculator/

My refractometer displays values in plato, so I had this batch which started with 18 Plato, and ended on 8.4 (for now it's probably growing further on bottle slowly).

In the ABV calculator I get a result of 5.75% of alcohol, with the Refractometer I actually get 8.52%.

Why is this and/or am I missing something? Is it the wort correction parameter I need to change?

 

[Karl]

I think one of them assume that your final Brix reading is the adjusted (for alcohol) Brix value while the other assume that it is the real Brix reading and do the adjustment down to some Brix 3.3. I think the input field should be more specific by defining what Brix value to enter.

 

[Josh (Brewer's Friend)]

Yep, that's correct. The FG input is based on the real reading from the refractometer, which has yet to be adjusted for the alcohol content. The calculator uses this formula:
http://seanterrill.com/2011/04/07/refra ... g-results/
to scale the reading to the adjusted FG and calculate the alcohol content.

Hope that helps. Let me know if you have more questions on this.