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.
ABV calculator
Alcohol by volume from original and final gravity, with both the simple and the high-gravity formula so you can see where they diverge.
Model: Standard (OG-FG)x131.25 and the Cutaiar alternate for strong beer
IBU calculator
Bitterness from hop weight, alpha acid, boil time, volume and boil gravity, using the Tinseth utilisation model.
Model: Tinseth
Beer colour calculator
Colour in SRM and EBC from a grain bill, using MCU and the Morey correction, with a swatch of the approximate result.
Model: Morey, from Malt Colour Units
Strike water calculator
The temperature to heat your mash water to so the mash lands on target, given the grain temperature and your water-to-grain ratio.
Model: Standard mash thermodynamics, malt specific heat 0.41
Priming sugar calculator
Grams of sugar to bottle-condition to a target carbonation, accounting for the CO2 already dissolved in the beer.
Model: Residual CO2 by temperature, dextrose at 3.9 g/L per volume
Hydrometer temperature correction
Corrects a gravity reading taken at the wrong temperature back to the hydrometer's calibration point.
Model: Standard 4th-order calibration polynomial
Dilution and blending
How much water to add to bring an over-strength wort down to the gravity you actually wanted.
Model: Conservation of gravity points