A tank sold as a hundred gallons does not hold a hundred gallons of water. Take off the rock, the sand, the space above the waterline and add back the sump, and the number that actually matters can be about 15 per cent short of the one on the box. Every dosing calculation on this site is only as right as this figure.
Worked from your tank's real water volume rather than the number on the box. The calculator itself needs JavaScript; everything it assumes is written out below.
The space above the waterline and the sand bed are the big subtractions. Rock takes out less than it looks, because only its solid skeleton displaces water: 120 lb of it is about 5 gallons. Either way the label runs high rather than low.
Do not forget the sump, the return plumbing and whatever is sitting in a frag tank. That is water your dose is diluted into whether or not you counted it.
If you only ever measure this once, measure it properly once. A dosing figure that is 18 per cent out is that far out every single time you use it, and it will read as the tank behaving unpredictably.
Because the label is the volume of the empty glass box. Rock, sand, the space above the waterline and any equipment inside all displace water, and a sump adds some back. A 120-gallon box (48 by 24 by 24 inches) filled to 22.5 inches, with 2 inches of sand and 120 lb of rock, holds about 101 gallons of water by this calculator, and a dose worked from the label size is about 18 percent too high.
Aragonite sand is about 60 percent solid grain by volume and 40 percent water between the grains, so a sand bed displaces roughly 60 percent of its own volume rather than all of it. The calculator applies that fraction to the bed's dimensions, which is why entering the sand depth matters and why guessing it as solid overcorrects.
3.785 litres. The calculator works in litres internally and shows whichever unit you pick, and every dose on the other pages is computed from the litres this page produces. An imperial gallon is 4.546 litres, which is a 20 percent error if a British tank's label is read as US gallons.
The rules that apply to every dose, not just this one, are on the calculators hub: never adding calcium and alkalinity at the same spot or the same minute, what to do when your readings fall by the same amount every week, and what these numbers assume about your additives.