What the estimate is built from
The calculator treats every set as the standard seven polyhedrals (d4, d6, d8, d10, percentile d10, d12 and d20) at ordinary 16 mm size, and gives each set about 18 cm³ of solid volume. Weight comes from the material you choose: about 25 g a set for acrylic, 35 g for heavier resin and 95 g for metal. Both figures are simple multiples of the set count, so 15 acrylic sets is 270 cm³ and 375 g.
Those per-set figures are averages. Oversized, sharp-edged or inclusion-filled resin dice can be well above them, and metal sets vary with the alloy and whether the dice are hollowed, so if you know the weight of one of your own sets, scaling from that is more accurate than the material preset.
Solid volume is not bag size
The volume shown is the dice themselves, not the space they take up loose. Polyhedral dice leave gaps when poured together, so a bag or box needs noticeably more room than the solid figure, which is why the page suggests about 30% spare. A bag sold as holding "about 10 sets" is also usually measured that way, so compare its stated set count rather than its nominal capacity.
Weight matters more than volume once metal sets are involved: a few metal sets weigh as much as a large acrylic collection, and a thin drawstring bag or a magnetic tray can sag or tear under them. For shipping, weigh the bag and packaging too; the calculator counts only the dice.