Alpha acid is the single biggest driver of a hop's bittering potential, and the published range for a given variety can be wide enough that "just use the average" produces a real, noticeable error in a bittering charge — see how many IBU is 1 oz of Cascade at 60 minutes for exactly how much that range moves a real IBU calculation.
A sample across common varieties
| Variety | Alpha acid range | Typical use |
|---|---|---|
| Cascade | 4.5-9% | dual-purpose |
| Amarillo | 7-11% | aroma |
| Centennial | 8-11.5% | dual-purpose |
| Citra | 10-15% | dual-purpose |
| Mosaic | 10.5-16% | dual-purpose |
| Simcoe | 11-15% | dual-purpose |
| Chinook | 11-14% | dual-purpose |
| Nelson Sauvin | 12-13% | aroma |
| Columbus (CTZ) | 13-17% | bittering |
| El Dorado | 14-16.5% | dual-purpose |
Full figures for every variety this site covers are on the hop reference, including flavour profile and common substitutes — this table is a quick lookup for the alpha-acid number alone.
Why some ranges are wide and others are narrow
Cascade's 4.5-9% span is nearly double its own low end — one of the widest ranges of any common American variety — while Columbus's 13-17% range is proportionally much tighter. This isn't random: some varieties are simply more sensitive to growing conditions, crop year and region than others, and a variety bred more recently for bittering consistency (Columbus, developed with high, stable alpha acid as an explicit breeding goal) tends to hold a tighter range than an older variety like Cascade, developed in the 1970s before alpha-acid consistency was as much of a breeding priority.
Why the published range is a planning figure, not a lab result
None of these ranges represents your actual bag of hops — alpha acid varies by crop year and growing region even within the same named variety, and the only way to know your specific hops' real figure is the Certificate of Analysis (COA) that should come with a fresh, reputable purchase. Treat this table (and the fuller hop reference) as a starting point for recipe planning before you're holding the actual hops, and switch to the COA's real figure once you are — especially for a bittering charge early in the boil, where alpha acid has the most direct effect on the final result.
Using this with the IBU calculator
Whatever figure you use — a table midpoint while planning, or a COA figure once you're brewing — the IBU calculator takes alpha acid directly as an input alongside weight and boil time, computing both Tinseth and Rager IBU from whatever number you enter, so updating your recipe's alpha acid assumption from a planning estimate to a real COA figure is a one-field change, not a full recalculation.