So I've just completed my fifth BIAB brew, and this time I decided to try and calculate the mash efficiency of my setup since I'm fairly sure it's not as high as it should be. I also think at the moment its the most useful efficiency measure, but I'm not sure!

I tested the wort after the mash and after putting it into the fermenter to get gravity readings.

The readings were 1.020 after the mash (67C), and 1.030 into the fermenter (~25C).

The recipe for this brew is here: http://brewerschoice.com.au/little-cheapers-pale-ale-2/

To determine the post-mash gravity, I plugged the values into this calculator and got this:

enter image description here

I then plugged the recipe and this gravity reading into this calculator (approximating the recipe as close as possible) to get this result:

enter image description here

34 litres was my strike volume - I'd estimate the volume post mash was probably more like 32 litres, and I think my keggle has about a 3L trub, which when accounted for brings the calculated efficiency down to 67%.

Key questions:

  • Am I doing this right?
  • Is mash efficiency the usual measure people quote?
  • What can I do if the gravity is too high or too low pre-boil?

Any assistance is greatly appreciated!

3 Answers 3


Looks like you are doing it right to me, using those calculators.

The only thing that might change your actual # is the calibration temp of your hydrometer. Be sure that it is 20C. Some hydrometers are closer to 15 or 18C, depends on the manufacturer. Although that won't change your OG much.

67% mash efficiency does seem like a normal for BIAB.

If the gravity is to high preboil, you can always do a calculation of how much sugar you have and dilute the boil with water. In the end, you'll just be leaving more wort behind to still hit the same batch size. If the gravity is to low preboil, you can try and squeeze your grain bag some more, sparge with a little water or do a calculation and add a little dry extract to compensate.

Your OG seems low, but based on the recipe it makes sense. As long as you were trying to make a low gravity beer it seems like you are on track.


I can see that your post mash and post boil gravities are really off.

For example you state an estimated post mash gravity 1.037 after correction, but then have 1.030 post boil. This would only be possible if you diluted with water. Post boil will always be higher from the boil off evaporation.

In any case, I think the hydrometer calculator is setting you in a wrong direction by it being too far of a temp spread, and you really didn't have 1.037. All hydrometers are different, and have their own temperature correction but only are accurate if you're within their tolerance usually +- 10°F or so.

It's best to cool a gravity sample to within tolerance then take a reading and adjust based on your hydrometers specific instructions. Refractometers are useful as an alternative.

Only a couple efficiencies matter. Mash and brewhouse. Mash is the big one, and where you get your fermentables so dialing this in helps a lot. Brewhouse is basically everything combining mash efficiency with end volume of beer. With the ultimate goal of buying exactly the amount of grain needed for X gallons and product and get the desired ABV.

Post mash/Pre boil and post boil reading are useful to hit numbers, Gravity correction calculators help with adjucts like DME to raise SG or water to lower SG. An alternative to adding adjuncts to raise SG is to boil off more at the sacrifice of volume, but take care hop times need adjusted too.

Also you have about 4 liters of over sparge there which will reduce your mash efficiency. Recipe calls for 20 strike, 10 sparge. For 26 pre boil.

1.030 post boil is really bad for that recipe. Check and make sure all your grain was milled and the wheat was flaked.

  • I had the grain double milled to be sure. I suspect my problem is that the bag isn't reaching very far into my kettle, and so most of the water is just sitting at the bottom not doing anything. Unfortunately I put my thermometer half way up, so it gets caught on the bag! I suspect I might need to rejig it. Thoughts?
    – George
    Feb 3, 2016 at 9:47
  • @George I think correcting the mash water volume will help a lot. Getting a bag that fits the pot evenly will help too. It's possible you didn't hit the best pH or temp range with the diluted mash, reducing the sugars extracted. Different enzymes are active at different pH and temp ranges. Feb 3, 2016 at 12:41
  • @George a short probe thermometer will help to keep it out of your grain bag. Feb 3, 2016 at 13:28

First, I can't see your images from my work computer so I'll explain it from scratch.

Each type of grain (base malt and adjuncts) have a potential yield of sugar. For simplicity's sake, we'll use the "point system" (that is Specific Gravity minus 1 times 1000), so for a Specific Gravity of, say, 1.040, we would say that you have 40 point (1.040-1 = .040, .040 * 1000 = 40).

Brewing software, such as BeerSmith or Brew Target have the potential points built in (it's also available from the manufacturer and often on the bag). US 2 Row has a potential of 36 points (if you had 100% efficiently, and put 5 pounds in a 5 gallon batch, you'd have a measured SP of 1.036).

You're going to measure your gravity at the end of the boil (hops don't add a lot to this). So, Efficiency = Measured Gravity Points / Potential Gravity Points. Let's take this a step further though because sometimes we don't always boil off what we want.

Efficiency = (Measured Gravity Points * Actual volume)/(Potential Gravity Points * Target Volume).

FWIW, Brad Smith does an excellent job of explaining this here.

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