The measurement must be done with a density meter like an alcoholmeter or a pycnometer. The calculator will not work with a refractometer measurement.
Temperatures from 0 - 40°C (32 - 104°F) can be entered. The measurement is then temperature corrected by the calculator. Thus, what is displayed on the alcoholmeter and the measurement temperature must be entered. When using a pycnometer, one of our two pycnometer calculators, either the one for alcohol solutions or the one for sugar solutions, must be used first, and then the output value "kg/lt density at measuring temperature" and also the measuring temperature must be entered here.
If the calculation result is "Calculation of sugar content not possible", this indicates a sugar content that is actually far from soluble at the indicated alcohol strength. So it is most likely a measurement or input error.
If spirits are sweetened, other substances such as glycerol are usually added in addition to sugar. This also affects the density and thus falsifies the calculation of the sugar content. However, only slightly, because these other substances usually make up only 1/100 compared to the amount of sugar added, in extreme cases a maximum of 1/10. It is often illegal to add these substances. Then one takes either at most only so much that it cannot be proved so easily, or one describes the product in such a way that these substances are permitted, thus for example "with Rum" instead of "Rum".
Wood aging, even with a sherry-soaked barrel of usual size, can only add a maximum of 5g/lt of sugar to the liquor. So one should subtract up to 5g/lt from the calculated value for barrel-aged spirits, depending on their coloring.
Since the calculation result is suitable to question the quality of the spirit, one should use good measuring instruments and know them well.
For some types of spirits it is regulated that no or almost no sugar may be added, for others not. And if it's allowed, it doesn't necessarily have to state how much sugar was added.
Especially with Rum, a lot of sugar is used and also other substances that flatter the palate, but as a contrast to this, the product is often sold as particularly authentic. For this reason, counter-movements were formed some time ago, both producers who offer Rum without additives, as well as websites that deal with this issue. There was even once an online petition with the goal of having to label additives in Rum. Someone then came up with the idea of determining the sugar content based on the difference in the printed alcohol strength and a measurement with an alcohol spindle, and then creating lists of which rums contain how much sugar. For example, here:
https://thefatrumpirate.com/hydrometer-tests-2
http://www.drecon.dk/
The second link also contains the formula. It is a simple rule of thumb, and without temperature correction: Each g/lt of sugar increases the density by so and so many g/lt. And this constant was determined by the value, by how much 10gew% sugar increases the density of pure water. But it is not that simple. Even the two-part mixtures sugar-water and ethanol-water do not have a linear density curve. And in the case of the three-part mixture ethanol-sugar-water, there is an additional factor: Ethanol and sugar repel each other. As a result, the volume increases when you mix them, and so the density does not increase as much as you might suspect. And this effect is stronger the greater the concentrations of ethanol and sugar. We made our own measurements for this, because unfortunately you can't simply look up the density of ethanol-sugar-water mixtures in any books.
Our calculator thus calculates about 5% more sugar for low sugar amounts and about 10% more sugar for high sugar amounts than with this rule of thumb. Whether one evaluates a schnapps differently, if it contains 33 instead of 30g/lt sugar, that must of course know everyone himself.
By the way, since 2021 there has been a spirits regulation that stipulates a maximum sugar content of 20g/lt for Rum throughout the EU. Anyone who does not comply with this may no longer call their product Rum. One of the best-known sweet Rums, the Don Papa Rum, has really had its sugar content reduced from just under 30g/lt to this 20g/lt. With other rums it is simply no longer written that it is Rum, but as an example with the A.H.Riise Non Plus Ultra very rare it says "Superior spirit drink made from premium matured Rum". And so that you can see even without reading that it should be a Rum and not a Whiskey or Cognac, palm trees are shown.
Of course, it is not necessarily a matter of identifying sweetened rums. Fruit schnapps and spirits are also often sweetened. And it's not fundamentally wrong if you like the taste. But as a distiller, you should try to know why you like something. Especially if you think you can't get the same smoothness as your favorite spirits, you could check them for sugar.