View Full Version : The Got Mead calculator

02-01-2013, 11:46 AM
Hey guys, first post here because something is unclear to me about the calculator.

I want to brew a 14%-ish sweat mead. I have 1.8 kg of honey, and the tool advises a final batch of about 5 litres.

Now I have also done my own calculations and come to very different values.

1.8 kg of honey is first multiplied by 0.8 to account for the sugar content (wikipedia). Now I have 1.44 kg of sugar. According to wikipedia this honey sugar is about 55% parts fructose and 45% glucose (and some minor components which don't matter for this calculation). This amounts to a specific gravity of about 1.62 kg/L.

The sugars weigh about 180.16 grams per mole, so I have 8 moles of sugar. One mole of sugar converts to two moles of alcohol, so I have 16 moles of alcohol now.

Alcohol weighs 46.07 grams per mole, so there is 0.74 kg of alcohol. This alcohol weighs 0.79 kg per litre, so there is about 0.93 litres of alcohol.

To get 14% alcohol the total batch now needs to be 6.6 litres. This is 1.6 litres more than the tool advises!

Did I make a mistake somewhere or is the tool wrong? All values are from wikipedia.

02-02-2013, 08:56 AM
Mols! Lol. I doubt many ppl (myself included) will be able to make sense of that. I know WHAT they are, but don't remember how to use them.

Honey usually weighs in at 1.4kg/l on average. So methinks you made a mistake there somewhere with your 1.600 gravity.

02-02-2013, 09:10 AM
There is no definite answer for what gravity you will achieve based on amount of honey, as sugar content varies.
The calculator is, however, an excellent guideline so you have an idea of a ball park figure to aim for.
My advice; buy yourself a decent hydrometer, use the calculator to work out rough values to shoot for and create a must that will achieve 14% (based on the yeast fermenting to bone dry), then stabilise and backsweeten to taste.
Its easy to get caught up in the whole ABV thing, but worry more about making something drinkable and getting your process right.
Best of luck :)

02-02-2013, 12:03 PM
@kudapucat Sorry, that isn't it. I did not use that value in the calculation.

@Cpt. Frederickson That is basically my plan and I already have a nice hydrometer. However, to get the sweetness and choice of yeast right I rely on calculations. Even though it is used as a ballpark figure, the math and values under the hood should be as good as possible.

Is there anyone with acces to the source code or underlying calculation? I would be happy to have a look at it.

Chevette Girl
02-02-2013, 02:54 PM
Hay ghvisser, welcome to the forum!

You do realize that your calculations are only going to be as good as your assumptions, right? All you're ever going to get by running the numbers is "close enough" no matter how carefully you run the numbers unless you find out the EXACT density of your specific honey and the EXACT proportion of glucose to fructose of your specific honey. Nevermind that yeast aren't going to stop EXACTLY at their rated tolerance.

If you want an exact %, go a little low on the initial water addition based on the lower of the two estimations, and add more to bring the SG up to the correct value on your hydrometer (remembering to correct for temperature). I'd be interested in hearing how close your calculation is versus how close the mead calculator is, since I DO remember what a mole is and it wouldn't take me long to remember how to run the calcs myself :p

If you want an exact sweetness at the finish, reserve enough of your honey that you can backsweeten to a certain SG once the yeast have done their thing. Assume it goes to .980 instead of assuming it stops at 1.000 and you should befine.

And remember, your hydrometer reading at the end is going to be affected by the amount of alcohol in solution too so unless you do a spirit indication at the end of it all, your final alcohol content is still going to be an estimation.

Medsen Fey
02-02-2013, 03:41 PM
Welcome to GotMead ghvisser!

Let me start by saying the calculator does work pretty well to provide an accurate estimate that you can plan a recipe around. I ran through your assessment and your thought process is correct, but there are some factors that make the calculator a better tool to use. I don't think I can explain all the difference, but I'll give you a few pieces to include in your assessment.

For one, there are some dynamics with miscible liquids that mean 1+1 doesn't equal 2 when it comes to volume. If you take 1 liter of pure ethanol and 1 liter of pure water and mix them, you won't get 2 liters of liquid. It will come out around 1.92 liters as water and alcohol molecules jam in together closer when mixed. So you won't need 6.6 liters for that amount of alcohol in solution.

Secondly, your model assumes all the dry weight is fermented. That is never the case. Many times a fermentation will be complete with a fairly large amount of material and residual non-fermentable sugars remaining. As an example, if you take 14 cc of ethanol (with density of 0.79 g/ml)and 86 cc of water (with density of 1.000) and mix them, the resulting solution should have a density of 0.970. You'll never get a mead with 14% ABV with that gravity. The lowest gravity you're likely to see would be 0.990, and most likely, it will end up closer to 1.000. That means there's typically around 30 g/L of "stuff" that isn't fermenting. in a 5 liter batch, that would be around 150 g of "stuff" which would account for almost 1 mole of your sugars, so realistically you aren't going to get more than 14 moles of ethanol. So now you're down to needing only 5.8 liters (not including the volume reduction from the prior paragraph). Your molar calculation requires too many assumptions.

The methodology behind the calculator is based in the principle of reduction in weight (gravity) that is observed. Any fermentation that occurs releases CO2 and the drop in weight from released CO2 correlates directly with the amount of alcohol produced. You can see all the calculation on How Stuff Works (http://recipes.howstuffworks.com/question532.htm). With this method, you are determining alcohol by weight, which can be converted to ABV (though inaccurately) by dividing by 0.79. In practice this method works well, though there are many variables that affect the results including how much glycerin is produced rather than alcohol, and how much ethanol gets dragged out with the CO2 which can be a significant number. There are several different formulas and models that can be used to estimate the ABV based on the gravity drop (see brsquared website (http://www.brsquared.org/wine/CalcInfo/HydSugAl.htm)), and ultimately, your result can easily be +/- 1%.

When planing a recipe, you can know that a starting gravity of 1.105 will produced roughly 14% ABV if fermentation completes. To the extent that you start at a higher gravity, you'll have more residual sugar if the yeast poop out at 14%. You'll have to try the calculator and see for yourself that it works. I wish I could explain it better in terms of stoichiometric equivalents, but you'll need to find someone with a bigger brain than me. ;D

I hope that is of some help.


02-02-2013, 05:32 PM
Thanks guys!
Good explanation (for me anyhow) I was running bets in my head on who of the 4 likely Gotmeaders would respond first.
1&3 came in first and second. So almost a bifecta.

02-02-2013, 05:34 PM
So who were 2 and 4?

02-03-2013, 10:33 AM
Cheers on being so helpful guys! :)

Medsen Fey, if I understand correctly now, the calculator is based on experimental results?

For the points you made; you are correct on the volume reduction when mixing the highly polar water with the alcohol. But this can be fixed by looking at the weights instead of the volumes, which is what I do in the calculation.

The second point is spot on, I forgot to consider that the yeast probably also converts some of the sugars to something else it needs. I suspect this is the mayor factor unaccounted for. Again, I'm curious as to what these compensation factors are.

Tiny edit:
Just read the How Stuff Works thing, it also forgets to compensate for changes in fluid volumes you mentioned (since ethanol is more polar than sugars). It states that the change in specific gravity equals the amount of carbon dioxide bubbled away. A better way would be weighing the whole assembly, instead of just taking the specific gravity. You would need quite an expensive scale for this though.

Medsen Fey
02-03-2013, 10:41 AM
Medsen Fey, if I understand correctly now, the calculator is based on experimental results?

Well, to put into your molar assessment, it is based on the fact that for every 2 moles of CO2 released, there are (approximately) 2 moles of alcohol created. Again, there is some that winds up being converted into other things, and that is why the experimental values tend to vary when they measure the results.

02-03-2013, 11:36 AM
Just did a new calculation with that method and it approximates the calculator better, but it's still of by quite some.

I think I'm just gonna brew this stuff with a <14% yeast then measure it. Will post the results, procedure and recipe when done. I'm going to use anise to spice it. Where I'm from they put that stuff in anything sweet and I think it tastes great. Let's hope for the best!

Thanks for all the explanation.

Medsen Fey
02-03-2013, 03:16 PM
A better way would be weighing the whole assembly, instead of just taking the specific gravity. You would need quite an expensive scale for this though.

Yes, that works. If you search the forums there is a post where Dan McFeeley describes using a postal scale if you want to know more.

02-03-2013, 06:20 PM
A 5 kg kitchen scale is adequate for weighing 1-gallon batches. The total weight matches quite nicely with measured density.

02-05-2013, 08:07 PM
I wanted to let this rest, but I had to math this.

L = desired batch size (L)
M = sugar carrier mass (kg)
S = sugar fraction (-) = ~0.8 for honey
H = yeast yield efficiency (-) = ~0.8 for a homebrewer
F = desired alcohol fraction (-)

The factor 0.647 is the product of all the ideal chemistry equations.

L = 0.647*M*S*H/F

This is the formula I'm using now. The yeast yield efficiency (experimental data) is the correcting factor. Now it matches up quite closely to the calculator. However, there is still a small discrepancy, even with the factor tuned at some values. It seems that the formula used by the calculator is not a simple first degree polynomial. Anyone have a hunch?

02-06-2013, 12:56 PM
At one point there was a thermodynamics thread about the change in temperature of the must due to sugar metabolism. I think there was a 50% efficiency factor in there for the yeast sugar-to-alcohol conversion, with the other 50% going to waste (heat). Not sure if that helps, or just makes things more complicated.

02-06-2013, 08:21 PM
Something to consider about the yeast efficiency:
Honey, on average, has 38.19% fructose, 31.28% glucose, and 1.31% sucrose. These are the readily fermentable forms of sugar with the Disacharides going through glycolysis before the sucrose (sucrose requires the extra step of inverstase, which is present in honey and therefore just as easily fermented as glucose and fructose). There is maltose present in honey, and many wine yeasts cannot metabolize maltose, but some can. 70.78% (glucose fructose and sucrose), with half the maltose yielding ~75% should, in theory, equall the fermentables in honey: honey should be 75% fermentable by weight.
Why is this not the case?
This (http://www.meadmadecomplicated.org/science/calc_alcohol.pdf) paper seems to agree with me. ???

02-07-2013, 09:04 PM
Sorry for weighing in (pun intended?!) on this a bit late.

I think (correct me if wrong, ghvisser), but your calculations for 14% alc didn't include that you want a sweetish mead, so there will ALSO be residual honey
(sweet means say 1.015 to 1.025 SG at the end, depending on your tastes. 1.015 is approx 54 g/L, 1.025 is approx 86 g/L).

I would go for 1.015 SG since 14% will have a sweeter mouthfeel after a year of aging (YMMV).

An extra point to note (part of Dan's weighing-the-carboy method) - CO2 stays in solution for a while, so at the end of fermentation, you need to allow for the (little bit extra) weight of CO2 - or else vacuum remove the CO2 before weighing you carboy :cool: since you don't want to aerate it by stirring. Who says you can't be obsessive/compulsive about handling your mead :o

RE formulas:
It is based on experimental, on BRSquared's web site info, reverse formulating an hydrometer scale, etc.

I am glad Medsen explained it - did a better job than I could, and I was the one who cobbled the calculator together out of wax and sealing paper and ....

Have a look at the javascript behind the calc page for some assumptions.
(SG to BRIX is 3rd degree equation. Temp is 3rd degree also, but the compensation which doesn't change things too much)


BRIX/Balling/Plato are the same scale ( just different accuracy and reference temp )
Baume is PA (approx)
BRIX = 135.997(SG^3) - 630.272(SG^2) + 111.14SG - 616.868
SG = 1.00001+(BRIX/(258.6-0.89*BRIX))
Baume (Bates et. al. 1942 @ 20 degC) = 145*(1-(1/SG)) => SG=145/(145-Baume) ( also Brix = 0.033431522+0.5532*Plato to match 145-145/SG formula )
%ABW (Alc by weight) = 0.8 * % ABV (Alc by Volume)
%ABV = Baume
g/L sugars from SG = 10*SG*Plato where Plato is the Brix 3rd order quadratic eqn defined above
sugars from g/L conversion is derived to 3 significant figures using brute-force solving of the g/L to sugars formula (guessing sg values)
Temp correction for hydrometer (SG correction) = 1.313454 - 0.132674*T + 2.057793e-3*T**2 - 2.627634e-6*T**3 for 59F (15C)
C = 5*(F-32)/9
F = C*9/5 + 32
1 US Gal = 1.2 Imp Gal = 0.263963098 Litres
for honey: 1 Pound = 12 US Gal = 14.4 Imp Gal = 1.65345 Litres = 2.2046 Kg = 0.062501 Oz = 0.39186765 Cups = 0.04889252 US Fl Oz = 0.04695798 Imp Fl Oz = 0.7771215 Us Pints = 0.7771215 Imp Pints = 1.5641637 US Quarts = 1.8783192 Imp Quarts

SO, in summary, the calculator does g/L honey to SG and vice versa using a couple of formulas.

It doesn't do it from chemical molecular equations (although you could argue that the SG/BRIX/... formulas are derived from that).

It also tries to be nice to you and allow you to use volume rather than weight of fruit (puree your fruit to get the volume as dense as possible will help).

02-23-2013, 04:56 PM
Thanks JamesP, that was exactly what I was looking for.

In the mean time, the mead is fermenting quite nicely, today I transferred it to another carboy. The taste is already very fine, after the clearing I'm adding the spices and expecting a nice drink :-)