This article is from the WineMaking FAQ, by malak@CAM.ORG (Don Buchan) with numerous contributions by others.
Hydrometers are calibrated to read pure water at 1.000 at typically 60F
or 67F. Typically you can take any tap water and expect to reasonably
find 1.000; if your tap water reads significantly from this -- but the
distilled water from your supplier still reads 1.000 -- it probably
wouldn't be a good idea to use your tap water (but that's another
discussion.)
This is taken from the instructions sold with a hydrometer:
"This hydrometer gives an accurate reading when the temperature of the
liquid is 60 deg. F. The following tables show how to correct for
temperature difference.
F SG Correction 50.....Subtract 1/2 60.....Subtract 0 70.....Add 1 77.....Add 2 84.....Add 3 95.....Add 5 105....Add 7
 
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