Rum wash is usually made from molasses and water.
Molasses is juice pressed from sugar cane, from which a large part of the sugar and water has been removed by various techniques. In the process, the other substances are concentrated and new substances are created by heat processes. This syrup thus acquires a characteristic, very intense flavor. Depending on how sour the molasses is, it tastes more like licorice or also has something fruity.
Rum is usually made from "blackstrap" molasses. In this case, as much sugar as possible has been crystallized and removed. It is therefore less sweet, has more flavor per sweetness, is somewhat bitter and contains many unfermentable substances. This type of molasses is available mainly as animal feed and soil fertilizer. In "fancy" molasses, less and gentler sugar has been removed. It is therefore sweeter, less aromatic, less bitter and can be used used for baking, for example. When you buy sugarcane molasses for human consumption at an organic market, it's usually fancy molasses.
Molasses usually contains around 20% water. However, it is sometimes offered somewhat diluted, especially blackstrap molasses, because it is otherwise very thick and therefore difficult to pump due to the many unfermentable substances. Blackstrap molasses, starting from 20% water content, has about 50% sugar and 30% unfermentable substances and fancy molasses about 70% sugar and 10% unfermentable substances.
Many products have both the sugar content and the water content listed. You can then enter that into the calculator. Sometimes, however, a sugar content and density is also given. This can also be entered into the calculator. Molasses has a density between 1.35 and 1.45kg/lt. Between 1.2 and 1.59 can be entered.
Legally permitted for rum production, however, are all other sugar cane products:

• Cane sugar:
  However, this is more expensive for rum producers than molasses and has less flavor per sugar. Therefore, it is used only by hobby distillers.
  There are very different cane sugars. White cane sugar has no appreciable amount of water or unfermentable matter and is indistinguishable in taste from white beet sugar. Brown cane sugar may be white cane sugar with some molasses added. In this case, it has perhaps 0.5% non-fermentable substances. Real raw cane sugar has between 2 and 7% non-fermentable matter. Depending on how dark it is, it also tastes very different. These sugars are called "Muscovado", "Turbinado" or "Demerara", among others. One often notices a certain moisture. In granulated brown sugar this is 0.5 - 5% water. Often, however, raw sugar is offered as a block, depending on the country of production called, for example, "panela", "jaggery", "gurr", "rapadura" or "piloncillo". These blocks contain up to 20% water.
• Sugar cane juice:
  The "rum agricole" or "cachaça" distilled from it has a very different taste from usual molasses rum. Since probably no hobby distiller in our latitudes has sugar cane juice available, we did not include it in the calculator.
• Dunder:
  This is the name given to the stillage in rum distillation, that is, the contents of the still after distillation. If the wash was made from blackstrap molasses, its dunder has a high density. 1.1 kg/lt is an estimate. If you want to wash with a lot of dunder, you should measure its density beforehand. You can then enter the SG, Oechsle, kg/lt density or Brix reading into the calculator.
• Skimmings:
  This is foam that is produced during sugar production. Because it is expensive to transport due to its large volume per kg and also because it cannot be stored for long, skimmings is only used by rum producers who also produce sugar or who have a neighboring sugar refinery. Since no hobby distiller has skimmings available, we did not include it in the calculator.
• Sugar cane vinegar:
  Sugar cane vinegar: Used in rare cases to increase the ester content of rum. However, because this is rather uncommon and vinegar is then only used in small quantities, we did not include it in the calculator.

In addition to cane sugar products, water and yeast, yeast nutrients are sometimes added to the wash.
The unfermentable substances of sugar cane products have a high density. The calculator assumes 1.7kg/lt. However, this value can be changed here:
Density of unfermentable substances:  kg/lt
If a lot of molasses or a lot of dunder is used, rum mashes will have a high density challenging the yeast, even if not too high an alcohol strength is targeted. High density means high osmotic pressure for the yeast. That is the main purpose of this calculator: To calculate the density of the mash, on the one hand to prevent too high amounts of molasses or dunder from being used, and on the other hand to prevent confusion caused by the typical high density readings and thus wrong decisions. For normal yeasts, problems can start at a density of about 1.08 kg/lt. Turbo yeast can tolerate about 1.13 kg/lt. By "problems" is meant a slow, delayed and incomplete fermentation with increased amounts of fermentation by-products. Within limits, however, this can be quite intentional. Such high levels are quickly achieved with the use of molasses and dunder. For example, 10 liters rum wash consisting of 2 liters blackstrap molasses, 2 liters dunder and water has a density of about 1.1 kg/lt. Although a maximum of only about 8.5%abv alcohol can be achieved with this wash, which is actually a well manageable alcohol strength for most yeasts, the yeast will likely be affected by the high density.
The calculation of the maximum achievable alcohol strength does not take into account whether the yeast can reach this alcohol strength at all and under the given conditions.
It is possible to switch between SG, Oechsle, kg/lt density, Brix and Plato. The calculated reading after fermentation refers exclusively to a density meter. A refractometer shows a much higher value after fermentation.