Tank volume

Every dose and every water change is a fraction of your water, so the number has to be the water, not the box. A "20 gallon" tank holds noticeably less once you take off the glass, the gap under the rim, the substrate and the rocks. Enter what you can measure; this gives you the water column you should dose against, and passes it to the other calculators on this site.

Li = L − 2t,  Wi = W − 2t,  Hi = H − t     (outside dims, t = glass;
                                            bottom pane sits under the sides)
hw = Hi − gap                               water height
d  = (d_front + d_back) / 2                 mean substrate depth (linear slope)

nominal     = L × W × H                     what the box says
glass box   = Li × Wi × hw                  filled to the waterline
substrate   = Li × Wi × d                   bulk (grains + pores)
pore water  = substrate × porosity
WATER COLUMN = glass box − substrate − hardscape      ← dose to this
all water    = water column + pore water

cm³ ÷ 1000 = L;  1 US gal = 3.785411784 L;  1 UK gal = 4.54609 L

Notes

1. Which number to dose against. Water inside the substrate trades places with the water column slowly, so it doesn't dilute a dose the way open water does. We dose against the water column and show pore water separately; if you'd rather count it, use the "all water" line.
2. Porosity is typical, not measured. Packed sand and gravel run about 0.35–0.40. Aquasoil granules are porous themselves, so the figure is higher and varies by brand; 0.55 is a midpoint. It only affects the pore-water line, not the water column.
3. Outside measurements assume a rimless build (bottom pane under the side panes). Framed tanks and tanks with a raised bottom differ. If you can measure inside the glass, do that and the glass thickness is ignored.