Quite frankly, Johnny Drejer deserves the credit here. Many have expanded on his works, but if they could see further, it is only by standing on the shoulders of a giant. The underlying methodology is sound. Hydrometers measure density, and if we can know with a high degree of accuracy what the ABV truly is, then if there is a difference between the observed and stated density, something inside the rum must be causing it. Aside from the simplification that sugar density is treated as a convenient average, the method is remarkably useful. Though he makes it abundantly clear that density is not the same as sucrose, in practice, sucrose is often the dominant factor aside from ethanol content, and other contributors, such as glycerin, are usually small enough to ignore for calculations. DuRhum's lab tests, for example, showed that Ron Zacapa contains around 0.3g/L of glycerol(https://durhum.com/here-we-rum/), which would increase its density by less than 0.1%. My hydrometer is only accurate to about 0.5 ABV, which is roughly a 1.25% change in density. But let's do a deep dive into Drejer's methodology. We know how sugar affects the density of water: Curve of density of water based on Brix, from Drejer. For his calculation, Drejer used a rate of 4.00331 ΔBrix⁄Δg/cm³ for his calculation, which is the average slope from 0 to 100 g/L of sugar added.^1 That is a broad range, since most rums seem to stay at or below the EU legal maximum of 20 g/L, but the line is fairly flat, so the approximation works well enough. Sugar adds a pretty consistent change in density to water. The same approach is then applied to the effect of alcohol on water: Curve of density of water based on ABV, from Drejer. From those two curves, Drejer derives the final formulation: (observed density - stated density⁄4.00331) × 10 The factor of 10 converts Brix to g/L. That gives us this reference table: Table used to convert observed and stated ABV into added sugar, from Drejer. Why it is imperfect 1. As stated earlier, sugar is not the only thing that affects density. Drejer makes that clear from the start. Glycerin is usually minor, but it does have an effect. Other sweeteners or additives may also change density in ways that do not behave exactly like sucrose. Sugar is still probably the biggest factor, but it is not the only one. 2. The method depends on the label ABV being accurate. By law, ABV has to be very close to what is stated on the bottle, and both the EU and the American TTB allow only a maximum deviation of 0.3%. That makes the stated ABV a solid anchor, but not a perfect one. 3. The method combines two separate curves: sugar in water and alcohol in water. It assumes that the same relationship holds in a mixed ethanol-water solution. That is probably close enough for a practical estimate, but alcohol and water behave in non-intuitive ways. For example, mixing 250mL of water, with a density of 1g/mL, and 250mL of ethanol, with a density of 0.79g/mL, results in a solution with a volume of 480mL and a density of 0.93g/mL (not 500mL of 0.895g/mL as we might expect).^2 Once sugar is added too, the system becomes even more complicated, because sugar dissolves differently depending on how much water and alcohol are present. 4. Measurement error can easily dominate the result. If you are using a cheap 0-100 hydrometer with tight lines, you can be off by a few ABV just from reading the instrument by eye or from small calibration differences. A 1-2 ABV error may not sound like much, but at 40% ABV, a reading of 38% instead of 40% would suggest roughly 8 g/L of added sugar. You ideally want the ABV reading to be accurate to within about 1%, which corresponds to roughly 0-5 g/L of added sugar. That is one reason many people treat 5 g/L or less as effectively no added sugar. Why it still works But those issues might not even matter. Despite its errors, it is a solid estimate and has proven itself useful. All things considered, 20g/L is really not a lot of sugar in terms of total volume. As such, it may be possible that the effects of saturating the water with sugar such that it affects the bonds with the ethanol content would be pretty minimal at these ratios. Drejer also tested the method by dissolving 40g/L of sugar in a 37.5% ABV spirit and measured it at 25% ABV, which matched the chart closely. I would still be cautious about extreme cases outside the chart, such as a 15% liqueur or a 151 proof rum. Can it be improved? A little, yes. Problems 1 and 2 cannot really be solved with a hydrometer. A hydrometer measures density, and that is all it measures. If the liquid contains other density-altering substances, or if the bottle is mislabelled, that is simply outside of the method's control. Problem 3 is the one area where a better model is possible. To improve it properly, I would need highly accurate measurements across many data points, with carefully controlled sugar amounts and ABV levels, so that I could determine whether the change in density for sugar in an ethanol-water mixture really matches the change shown in the pure-water charts. That would give a more defensible model, but it is more work than I want to take on right now. What I can do instead is use a more specific formula. Drejer's formula is as follows: sugar (g/L) = (ρobserved ​− ρstated​⁄4.00331)×10^1 This uses one fixed constant, 4.00331, regardless of the ABV. That is a linear approximation, combining the whole typical ABV range into an average. It's a simplification, which makes it easier to use, at the cost of assuming that every unit of density change corresponds to the same amount of added sugar regardless of ABV. We know that isn't true. Adding 40 g/L of sugar to a 37.5% ABV spirit drops the apparent reading by about 12.5 percentage points, but adding the same 40 g/L to a 60% ABV spirit only drops it by about 8 points. A more granular formula would be: sugar (g/L) = 1000 × (ρobserved ​− ρstated​⁄1 − ρstated × ​v) I added 4g of sugar to 37.5% ABV rum, to a final volume of 100mL. This equals 40g/L, and my hydrometer reads 25% ABV. Here, ​v is the apparent specific volume of sucrose in the spirit blend, which is approximately 0.625cm³/g. The amount of volume occupied by a gram of sucrose actually changes based on how much sucrose is already present in the solution, but scientific measurements show that the noise is greater than the measured difference. So, in practice, treating ​v as a constant is a defensible simplification given the data.^3 Also, just to double check Drejer, I took some 37.5% ABV rum (and made sure the initial hydrometer reading is 37.5% ABV), and then carefully dissolved 4g of sugar into it, then topped it up to exactly 100mL and stirred well (equal to 40g/L of sugar). My hydrometer floated at precisely 25% ABV. So, this is a nice agreement, because that means my hydrometer agrees with Drejer's, and my methodology gives an answer very close to Drejer's as well. I added sugar in increments of 20g/L and got the following readings: 55%, 50%, 45%, 40.5%, 36% But what if we explore another end of the scale? One where my formula and Drejer's pretty substantially disagree? I chose 60% ABV and high sugar content because that's the most extreme scenario, and actually, both formulas held up reasonably well. Science is supposed to be reproducible, and that was really the point of this exercise. For the most part, they do. At ABV and sugar content so high that you won't even plausibly encounter it, it appears that my formula is a bit more accurate, but only within a couple percentage points compared to Drejer's. At 37.5% ABV with 40 g/L of added sugar, my formula, Drejer's formula, and the measured result all landed in the same place to within a fraction of a percentage point. If you've been relying on Drejer's hydrometer method for a typical bottle of rum, this is good news! While my formula does not completely align with my data, it is around twice as close as Drejer's formula. But your formula is scary! Here's a calculator: The calculator does not produce radically different values from the Drejer table. What it does do is save you from having to read the chart row by row, and it uses the exact formula so you do not need to guess between the lines when the ABV is not an integer. Hydrometer tests > Note: values below 5 g/L can effectively be treated as no added sugar, because measurement error and natural barrel-derived compounds can easily account for that difference. For a complete table of hydrometer test results, see the . --- ^1: Drecon, Drejer's website(http://www.drecon.dk/) ^2: The Lost Volume Demonstration - Carolina Knowledge Center(https://knowledge.carolina.com/professional-growth/activities/the-lost-volume-demonstration/) ^3: Apparent Specific Volumes of Sucrose in Different Aqueous Cosolvent Mixtures at 298.2 K - Pharmaceutical Sciences, p. 325(https://pdfs.semanticscholar.org/20e7/6d53984a0a9bc45abbc257d97a0912b1393f.pdf)