One ounce (28 g) of Cascade at a mid-range 5.5% alpha acid, boiled the full 60 minutes in a standard 19 L (5 gal) batch at 1.050 boil gravity, comes out to 18.7 IBU by Tinseth or 25.0 IBU by Rager — a gap of about a third, which is normal for these two formulas and covered in full on the Tinseth vs. Rager explainer. Run it yourself with your batch's actual alpha acid and gravity.
Why "5.5% alpha" is a choice, not a fact
Cascade's alpha acid isn't one number — the Cascade hop reference lists it at 4.5-9% alpha acid, one of the widest ranges of any common American variety, because Cascade's alpha content swings more by crop year and growing region than most other varieties. 5.5% is a reasonable mid-range planning figure, but it is not what your actual bag contains. At the low end of the range (4.5%), the same ounce and boil time drops to roughly 15.3 IBU by Tinseth; at the high end (9%), it climbs to roughly 30.6 IBU — very nearly double the low-end figure, from the exact same weight and boil time. This is the single biggest reason a recipe calculated in advance can come out noticeably different in the glass: check the Certificate of Analysis (COA) that came with your actual hops, if you have one, and use that number instead of a published range midpoint.
What actually moves this number, beyond alpha acid
- Batch volume. The same ounce in a 38 L (10 gal) batch contributes roughly half the IBU of the same addition in 19 L — bitterness is a concentration, not a fixed quantity per hop addition, and doubling the volume halves it for an unchanged weight.
- Boil gravity. Both formulas reduce utilization as gravity rises — a denser wort extracts proportionally less bitterness from the same hop charge, which is why the exact same ounce of Cascade contributes noticeably less IBU in a high-gravity boil than in a standard-strength one. See why do my IBUs differ between calculators for how big that gap can get.
- Boil time remaining, not total boil length. A 60-minute addition means 60 minutes of boil time remaining after it goes in, not 60 minutes into a 90-minute boil — a common mix-up that silently changes every IBU figure downstream.
Using this as a planning number
If you're formulating a recipe rather than checking one already brewed, treat 5.5% as a starting assumption you correct once you're holding the actual bag — Cascade's flavour profile (grapefruit, floral, spicy per the hop reference) makes it a common bittering-and-flavour dual-purpose choice, and small differences in alpha acid matter more for a bittering charge early in the boil than for a late aroma addition where the oils, not the bitterness, are the point. If you're chasing a specific target IBU for a style — an American IPA generally wants 40-70, per what IBU should an IPA have — plug your real alpha acid into the IBU calculator rather than scaling this example by eye.