Calculators
Every one names the model it used.
Two brewing calculators giving different answers is usually not a bug in either. It is two different published models, and most sites never say which one they ran. Each of these does.
Molecular SO2 calculator
The only sulphite number that matters. Free SO2 alone tells you almost nothing, because how much of it is actually protecting the wine depends on pH, and the relationship is logarithmic.
Model: Henderson-Hasselbalch on the sulphurous acid pKa of 1.81
Brix, SG, Baume and Oechsle
Convert between every sugar scale in use, and see the potential alcohol as the range it really is.
Model: Standard density relations and the ASBC polynomial
Chaptalisation calculator
Sugar needed to raise a must to a target Brix, accounting for the volume the sugar adds. Regulated or banned in many regions; this is a calculation, not a recommendation.
Model: Mass balance, corrected for the volume the sugar itself adds
Acid adjustment calculator
Tartaric acid to raise titratable acidity, or potassium bicarbonate to lower it. Always bench-trial first: pH movement depends on buffering you cannot predict.
Model: Tartaric at 1 g/L per 1 g/L TA; potassium bicarbonate at 0.9
Fortification calculator
Spirit needed to bring a wine up to a target alcohol, for port-style and vermouth-style wines.
Model: Pearson square
Yield and dilution
Finished volume to expect from a weight of fruit, and water needed to bring a raisined, high-Brix must back into range.
Model: Typical extraction rates and conservation of sugar mass