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 twenty per cent from 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.
Rock and sand are the big subtraction. Live rock displaces roughly its own volume, and a deep sand bed takes more than people expect, which is why the label figure 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 out by a fifth is out by a fifth 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 100-gallon display with a normal rock pile and a sand bed typically holds 70 to 80 gallons of actual water, and every dose calculated from the label size is 20 to 30 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 rather than to this one, including never dosing calcium and alkalinity together, are on the <a href="/tools">calculators hub</a>.