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How To Hit Your Target Original Gravity On Brew Day Every Time (And Why Mash Efficiency Is Irrelevant)

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This article is going to tell you about a simple calculation that will ensure that you hit your target Original Gravity (OG) every single time.

Whether you’re a home brewer or a pro brewer, this trick is for you.

By using this calculation several times during brew day, you will ensure that you have consistent wort and therefore consistent beer.

Consistency is key to product quality and ultimately, brand loyalty.

Your consumers depend on you to brew consistent, quality product.

Jeez Wonka settle down – that’s pretty condescending!

Before You Brew – Set Your Product Specification

If you’re a professional brewer or you run your own brewing business, it’s imperative that you write out a specification of your product before you even think of mashing in.

Maybe you have a brew sheet written out – but that’s not the same as a product specification.

That means writing your beer recipe in a format that’s irrespective of batch size.

So whether you’re pilot brewing a batch or you’ve been brewing the same product for years, you can ensure that your product stays the same without “tweak creep” setting in.

“Tweak Creep” is where a brewer constantly tweaks a recipe from batch to batch – which is perfectly OK – but only if you’re documenting your recipe changes.

If you don’t document your recipe changes, you’ll never be able to recreate that awesome batch you brewed last month.

While there are other numbers you need to specify for your product, for today’s purposes, the wort Original Gravity (aka OG) is the number we are interested in.

Mash Efficiency Is A Number You Cannot Fully Control

I speak to many home brewers and professional brewers and often they get themselves wrapped up in the intricacies of mash efficiency.

They often try and target as higher mash efficiency as they can.

But the thing is, while there are ways that you can skew mash efficiency in one direction or another (e.g. mill gap, liquor to grist ratio etc), it’s really a number you have little control over.

Once you have your equipment dialled in, you’ll find that mash efficiency varies with something as silly as the weather!

So what’s the point of making that number a target?

The same goes for the volume of wort you should collect.

Many brewing recipe apps say something like “collect X litres/gallons of wort.”

But what’s the point if those X litres/gallons of wort aren’t at the gravity you targeted?

The only number you have full control over on brew day is your OG.

Maybe a YouTube video of Metallica’s “Nothing Else Matters” should go here.

You’re going to get whatever mash efficiency and volume you’re going to get on brew day.

So it’s your job as a brewer to hit your product specification’s OG.

Period.

Enter The Dilution Calculation

I’ll let you in on a secret – wort gravity is merely a ratio of water to sugar in your wort.

If you think about wort in this way, then you can extrapolate a calculation to determine what volume of wort at your target gravity if you know the volume and gravity of a sample of wort in your kettle.

The beauty of a ratio calculation is that units of measure become irrelevant.

So for gravity measurements, you can use SG, Plato, Brix, Baume or kg/m3 – it really doesn’t matter.

Same goes for volumetric measurements.  Litres, BBLs, hL, Firkins, Hogsheads – same goes here.

As long as you are consistent with your units of measure, this calculation will work for you.

To make this calculation, you will need the following data and you’ll need to be able to accurately measure where required:

  • Current Gravity (CG) – Use a refractometer or density meter to measure wort gravity quickly hot side.  A hydrometer probably won’t cut it here because your wort may be too hot for an accurate hydrometer reading.
  • Current Volume (CV) – Maybe your kettle has a measuring stick, sight glass or flow meter.  Make sure this is calibrated and accurate.
  • Target Gravity (TG) – This comes from your product specification.

The formula is as follows:

Nail That OG Every Time

So let’s run through an example of this calculation on a hypothetical brew day.

Our data is as follows (note, with SG we’re only going to use the gravity “points” i.e. SG – 1):

  • CV = 1500L
  • CG = 1.060 (that’s 60 points)
  • TG = 1.056 (that’s 56 points)

Target Volume @ TG = 1500 x 60 / 56 = 1607L

Now, if we’re at the end of the boil, we need to adjust our wort gravity to hit our target spec of 1.056.

By subtracting the current kettle volume, we can determine the amount of liquor required to hit our target OG.

1607L – 1500 = 107L of liquor should be added to the kettle to make the wort 1.056.

Let’s try it again with US Units of Measure such as Plato and Barrels (bbl):

  • CV = 12.82 bbl
  • CG = 14.7P
  • TG = 13.7P

Target Volume @ TG = 12.82 * 14.7 / 13.7 = 13.75bbl

And therefore our dilution volume would be 13.75 bbl – 12.82 bbl = 0.93bbl liquor dilution required to hit target gravity.

As you can see, the numbers for both calculations are identical.

Could be the Illuminati controlling our units of measure.

Simple!

Conclusion

There’s no reason that you shouldn’t be hitting your target OG for your product each and every time.

Personally, during a brew day, I use a refractometer to make this calculation during lautering so I can determine how much run off there is to go.

Making this calculation at the start of the boil is a great way to dial in more accurate hop charges in the kettle to hit target IBU.

Making the calculation at the end of boil helps you to work out how much liquor to dilute with so you hit your target OG.

And remember, it’s more difficult to take water out of wort than it is to put it in.

So it’s a good idea to boil your wort at about 10-20% over target gravity and dilute on every brew so you know you will hit your specified OG every time.

Happy brewing!

If you’d like your products specified in a way that ensures product quality and consistency each and every time, contact me to discuss your requirements.

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16 thoughts on “How To Hit Your Target Original Gravity On Brew Day Every Time (And Why Mash Efficiency Is Irrelevant)”

  1. What if you found out yesterday that your hydrometer is two points out… Kept missing my OG by a couple of points. Stupid hydrometer! ????

  2. Manuel Cortez Salazar

    Hi Hendo. Question. How the dilution of beer affects the final IBUS. If I add water I will get less IBUS???

    1. Yes. But it’s so small that it won’t make much of a difference to your final beer.

      If you’re really concerned about IBU drop then simply hop the beer as if the volume was going to be slightly higher. I call it the “virtual kettle”.

      Thanks for your comment 🙂

  3. G’day Hendo, hope you are well. I have been using this calculation the last half dozen brews with some interesting results. Brewed a batch yesterday and had to add 25% to the final volume to meet the gravity. I adjusted the brewhouse efficiency to suit and got much closer on the second brew.

    Also, you mention in your article at measuring near the end of the boil, so I assume that it is okay to get the volume at a high temperature. Sorry, the reason I ask is that I was reading an article that mentions that all measurements should be taken at room temperature.

    Thanks again, the calculations have definitely helped!
    Cheers Jase

    1. Hi Jase,

      Glad to hear that the calculation is helping your brewing accuracy 🙂

      With the near-end-of-boil sample, the volume measurement will be largely correct even though there will be some shrinkage of the wort when it’s cooled. It’s not 100% accurate but it’s good enough to hit a standard gravity tolerance of +-0.2 Plato/0.002 SG points from your target OG.

      That said, measuring gravity on the other hand, will require a cooled sample. That’s why I use a refractometer for all hot side gravity measurements.

      I hope that helps!

  4. G’day Hendo,
    Have only been brewing AG for 1.5 years and the learning curve has been slow. I am always looking for ways to improve my technique to try and make beer of a quality that I am happy with. Am just accepting that I need to pay more attention to my OG and FG and this article is absolutely brilliant thank you. Whilst I understand your intention is to plan your brew so that CG is higher than your TG so that you can lose those points with dilution (which is a great idea!), it would be good to also explain what to do if your CG is lower than your TG just to complete the picture.
    Cheers!

      1. Hey Hendo – picking up on this question, I just completed my first AG brew and ended up with an OG reading of 1.039 and target OG was supposed to be at 1.049. Now I understand the theory behind your approach if this happens again should I continue to boil until I get the correct reading or is there another way to approach this like using dextrose? cheers

        1. Hey David,

          If you got to the end of the boil and found out that your OG was 1.039 instead of your target of 1.049 then you might want to regularly check your kettle gravity during the lauter and sparge rather than waiting all the way until the end of the boil to check gravity.

          It looks like you over sparged on this batch. Checking the kettle gravity regularly means that you won’t over-dilute your wort and you’ll hit your target OG.

          I check gravity more than a dozen times on brew day so I don’t encounter this problem. Try it and see how it works for you.

          I hope that helps! 🙂

          Hendo

          1. Hendo – in response, just confirming that the 1.049 should be before boil. In Brewfather it shows a pre boil gravity of 1.039 (which I achieved) and then final OG of 1.049. Does the boil increase the OG result? A real newbie question I’m sure !! cheers David

          2. Gravity increases during the boil because you’re evaporating water out of your wort. Less water & decreased kettle volume but the same amount of sugar = Higher gravity!

            I would ignore Brewfather for your start of boil gravity and just aim for the correct end of boil gravity.

            Don’t forget to monitor your kettle gravity during lauter and sparge 🙂

  5. This makes so much sense and can’t wait to try it. Quick question on this process though, are you starting at a lower pre-boil volume to be able to adjust OG down to hit target or just boiling longer as mentioned to drop the volume but increase sugar?If the former, is there a calculation for that? Or is it just knowing grain absorption and boil off volumes to offset. Thank you for this!

    1. A good and quick way to deal with this is to start your boil at your OG knowing that you’ll boil off whatever (it no longer matters) then top up water to your target OG. The relation to water and sugar in your wort is always constant no matter how long you boil!

  6. Hi Hendo!

    Quick question. What effect could this have on pH and SO4/CL ratio? I would assume that it would be minimal if you haven’t missed gravity by a landslide, but I was curious upon reading. I know this is an old post, but I’m interested to see your thoughts. Thanks!

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