Calculating OG from FG and Est. ABV

Discussion in 'Beginners Brewing Forum' started by The Grim Beaker, Jun 3, 2020.

  1. The Grim Beaker

    The Grim Beaker New Member

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    Hi
    So I've just got a new malt extract kit for a golden ale which will brew 23 litres (6 gal) from 1.7kg (3.7lb) of liquid extract, however looking at the info on the package there is no information on the OG. The package does state the approx final gravity (1.006) and the approx ABV will be (4.4%). I intend to tinker with the brew and not just brew as is, so knowing the OG would be very helpful. Is there a way to calculate an approximate OG from the FG & ABV?
     
  2. Nosybear

    Nosybear Well-Known Member

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    Yes, there's a way: ABV=(OG-FG)*131.25. Solving for OG: OG=FG + ABV/131.25.

    There's a more complicated, more accurate formula but this one will get you a good enough answer at low (less than 10%) ABVs.
     
    Steve SPF and The Grim Beaker like this.
  3. The Grim Beaker

    The Grim Beaker New Member

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    Nice one.
    I'm currently calculating my ABV like this:

    (76.08*(OG-FG)/(1.775-FG))*(OG/0.794)/100

    And when i tried juggling it around to work backwards so to speak it just didn't work. I assume I had something in the wrong order.
     
  4. Nosybear

    Nosybear Well-Known Member

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    That formula is more accurate for higher ABV solutions (beer, wine, you know). The simpler formula will get you close enough for our purposes.
     
  5. The Grim Beaker

    The Grim Beaker New Member

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    Yeah, definitely works a treat. It's given me an estimate OG so now I've got a base for my tinkering. Cheers
     
  6. Megary

    Megary Well-Known Member

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    Are there more fermentables in the kit besides the 3.7# of LME? Seems light to me, but I'm not super familiar with extract.
     
  7. The Grim Beaker

    The Grim Beaker New Member

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    That is all that is in the kit, but it does need an additional 1kg of brewing sugar / 1.2 LME, so I've had to factor the gravity of that as well.
     

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