Some brewing software applies a utilization bump for pellet hops, commonly cited around 10%, on the theory that the pelletizing process — grinding hop cones and compressing them under pressure — breaks down cell walls and increases the hop material's surface area, releasing alpha acid into the wort more completely than a loosely-packed whole cone does over the same boil time.
What this calculator actually does
This site's IBU calculator applies no separate pellet-vs-whole-cone adjustment — 28 g of a 10% alpha hop at 60 minutes in a 19 L batch at 1.050 boil gravity computes to 34.0 IBU by Tinseth and 45.4 IBU by Rager regardless of which physical form you're actually using. Check this baseline. Some other brewing software applies roughly a 10% pellet bump on top of a comparable baseline — for the same addition, that would put pellet-form utilization closer to 37.4 IBU. That specific gap is a large part of the answer to why do my IBUs differ between calculators if you've noticed a consistent 5-10% offset between this site and a different tool on identical hop schedules.
Why this site doesn't apply the adjustment
The Tinseth and Rager formulas as originally published were not derived with a built-in pellet/cone distinction — the utilization curves both formulas use come from general studies of boil-time and gravity effects on isomerization, not a controlled comparison between hop forms. The commonly-cited pellet bump is a widely-adopted convention layered on top of the base formulas by various software authors over the years, not part of either formula's original derivation, and the size of the real effect is genuinely debated among brewers — some report a noticeable difference switching between forms with an otherwise identical recipe, others report none they can reliably attribute to form alone rather than to other batch-to-batch variation.
What this means practically
If you brew consistently with one hop form (pellets, which the overwhelming majority of homebrewers use, or whole cones, more common in commercial and some traditional homebrewing) the absolute IBU number matters less than consistency — your own tasting calibration against your own calculated numbers, brewed with your own consistent hop form, is the most reliable guide regardless of which convention a calculator applies. If you're switching from whole cones to pellets (or the reverse) for the first time, expect the actual bitterness in the glass to shift slightly even at an identical calculated IBU, and treat your first batch in a new form as a chance to recalibrate your own expectations rather than a sign the calculator got something wrong.
Related reading
See how many IBU is 1 oz of Cascade at 60 minutes for a worked single-hop example, and the Tinseth vs. Rager explainer for the other major source of cross-calculator disagreement, unrelated to hop form.