Methodology

The IBO standard rates a bow at 70 lb draw weight, 30″ draw length and a 350-grain arrow with a bare string. The estimator adjusts that rating for the ways a real setup differs, using the three shop rules of thumb that have priced this gap for decades:

• Draw length: ±10 fps per inch away from the reference draw.
• Draw weight (compound): ±2 fps per pound away from 70 lb.
• Arrow weight: ∓1 fps per 5 grains away from the reference arrow weight.
• String accessories: −3 fps for a peep, −4 for peep and D-loop, −6 with silencers fitted.

Recurve and longbow ratings follow a different convention — quoted at a 28″ draw with a reference arrow of 9 grains per pound of draw weight (gpp) — so the same rules are applied from that baseline instead:

Both constants are the standard archery forms of the physics definitions — kinetic energy as ½mv², momentum as mv — with the grains conversions folded in. Unit conversions use the exact definitions: 1 fps = 0.3048 m/s, from which the mph (×0.681818) and km/h (×1.09728) factors follow.

Drag spends an arrow's speed at a roughly constant percentage rate. This model spends 1.2% of remaining speed per 10 yards — the middle of the published range for typical hunting arrows — and recomputes kinetic energy at each distance from the reduced speed. It is an approximation: large fletching and light shafts decay faster; heavy, low-drag arrows slower.

FOC follows the standard Easton definition, balance measured from the nock throat on the finished arrow. The weight build-up is a plain sum of published component figures; grains per pound is the total divided by draw weight. The component ranges on the weight page are the makers' published spans for current shafts and parts.

Static spine is the shaft's measured deflection under the standard test (28″ span, 1.94 lb centre load), expressed in thousandths of an inch. The selection chart groups draw weight and finished arrow length into the common 600 / 500 / 400 / 340 / 300 groups, following the structure of Easton's published shaft-selection charts. Point-weight and cam-aggression adjustments are stated on the spine page itself, and the page says plainly to confirm against your own shaft maker's chart before buying — makers' labels differ at the margins.

Crossbow ratings carry no common standard, so the model starts from the maker's own rating and its stated test-bolt weight, then applies the industry rule of one fps per three grains of bolt-weight difference. Energy and momentum are then computed from the adjusted speed with the same formulas as everywhere else on the site.

It is an estimate. For a well-tuned compound with honest inputs, expect roughly ±5 fps against a chronograph. Cam design, string condition, tuning state and release consistency all move the real number in ways no adjustment table captures.

Inputs dominate. An IBO rating measured generously, or an arrow weight guessed instead of summed, moves the answer more than any refinement of the model would. The tools are built so the honest input is the easy input.

Traditional bows vary most. Maker ratings for recurves and longbows are not standardised; treat those estimates as ±10 fps territory.

Guidance bands are cited, not invented. The KE, momentum and FOC bands shown across the site reproduce ranges published in manufacturer guidance and the Ashby Bowhunting Foundation's research summaries. They are rules of thumb for well-placed shots with suitable broadheads — never a substitute for local hunting regulations, which always win.