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Sea water does not freeze until about −2 °C (28 °F) if you add more salt you can lower the freeze point to -21°C.

We know from How much sea water can I safely drink? that drinking salt water is not good. But adding any substance to water that will form a solute will lower the freezing temp. You can drink as much sugar water as you want, the only risk is added calories which you likely need anyhow.

The higher the concentration of salt, the lower the freezing point drops. Any foreign substance, such as sugar, alcohol, or any chemical salt, added to the water, forms a solute, which will lower the freezing point and melt ice. Source

So if I want to add sugar to my water to keep it from freezing how do I calculate how much sugar per unit of water I need to add, to keep it liquid at a given temperature?

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This can be calculated using a property called cryoscopic constant Kf which links the concentration of a solved substance to the freezing point depression Td:

Td = m * Kf

where m is the molality which is the amount of mols of solved substance per kg of solvent (here water). For water Kf is 1.86K*kg/mol and the molar mass of sugar (sucrose) is 342g/mol. So to calculate the freezing point depression do:

Td = 1.86 / 342 * mass of sugar in g / mass of water in kg

Some examples for grams of sugar added to 1 liter water:

g/l     Td 
10      0.054  
50      0.27  
500     2.7

I found a science-fair-project that measured this.

So you need a huge amount of sugar for a small effect, not very practical.

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    Yep. Sugar is far heavier per mole than salt, and also only produces one mole of particles when one mole is dissolved in water (salt dissociates into ions, producing two moles of particles). These two factors together make sugar incredibly impractical.
    – etherealflux
    Jan 18, 2016 at 21:47
  • @etherealflux would a monosaccharide be any better (e.g. glucose)? Presumably a factor of ~2, so not really.
    – Chris H
    Jan 20, 2016 at 9:29
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    @ChrisH Yes it would be about twice as effective, as the molar mass of e.g. glucose ("grape sugar") is 180.2g/mol (sucrose is a disaccharide) and the freezing point depression is inversly linear in the molar mass. Still loads of it required for a significant effect.
    – imsodin
    Jan 20, 2016 at 9:34
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    @imsodin, I wonder if you could make something akin to oral rehydration solution: NaCl+ KCl (i.e. lo-salt), glucose, maybe a few other compounds (food additives) in small quantities (malic acid, even CaCl₂) to get the total molar concentration up.
    – Chris H
    Jan 20, 2016 at 9:39
  • @ChrisH Sure you could optimize, in the end it is all up to the amount of individual molecules/ions solved in the water. So you are looking for lightweight substances that ideally split into several parts when solved (thats why salt is quite good at it). I do not know, but I guess with standard "household" measures you won't get very far. Oral rehydration solution has some grams of salt, potassium and about 10 grams sugar, the net effect is 0.58degC for standard ORS (from wikipedia osmolarity value).
    – imsodin
    Jan 20, 2016 at 9:48

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