Crossbow Arrow Speed Calculator
The speed on the box was measured with the lightest bolt your maker allows. Your hunting bolt is heavier — this is what it actually shoots, and what that costs (and doesn't cost) in energy.
How Crossbow Ratings Work
There is no single crossbow equivalent of the IBO test, so each maker rates speed with the lightest bolt it considers safe — and advertises that number. Your bolt, with a hunting broadhead fitted, is almost always heavier. The adjustment the industry uses is simple:
One fps per three grains. The honest surprise is what happens to energy: it barely moves, because the heavier bolt keeps the energy the speed figure gives up — and momentum climbs outright. Vertical-bow shooters run the same maths through the arrow speed calculator; the principle is identical, only the baseline differs.
Bolt Weight vs Speed, KE and Momentum
| Bolt weight | Real speed | Kinetic energy | Momentum |
|---|---|---|---|
| 400 gr | 400 fps | 142.1 ft-lb | 0.71 |
| 425 gr | 392 fps | 144.8 ft-lb | 0.74 |
| 450 gr | 383 fps | 146.9 ft-lb | 0.77 |
| 475 gr | 375 fps | 148.4 ft-lb | 0.79 |
| 500 gr | 367 fps | 149.3 ft-lb | 0.81 |
Read the KE column carefully: the 500-grain bolt is the slowest in the table and carries the most energy. Speed is the only thing a heavier bolt actually costs you.
Crossbow Worked Examples
Rated 400 fps at 400 grains, hunting bolt 450 grains: 400 − (450 − 400) ÷ 3 = 383.3 fps, and the energy is 146.9 ft-lb — effectively unchanged from the rated bolt.
Rated 370 fps at a 380-grain bolt, shot with a 425-grain hunting bolt: 370 − 45 ÷ 3 = 355 fps. Fifteen fps is the entire gap between the box and the field.
Limits of the 3-Grain Rule
The 1-per-3-grains rule is an industry rule of thumb, not a law of physics: individual bows vary by a few fps either way, and the rule drifts at the extremes of bolt weight. Never shoot a bolt under your maker's stated minimum weight to chase the rated number — the minimum exists to protect the limbs, the string and you. A chronograph remains the only exact answer; this estimate exists so the box number stops misleading you.
Crossbow FAQs
Why is my crossbow slower than its rated speed?
Because the rating was measured with the lightest bolt the maker allows — often 350–400 grains — and your hunting bolt with a broadhead weighs more. The standard rule is about 1 fps lost per 3 grains above the rating weight, so a bolt 60 grains heavier costs roughly 20 fps. Nothing is wrong with the bow; the rating was never measured with your bolt.
How much kinetic energy do I lose with a heavier bolt?
Surprisingly little, and sometimes none. A 400 fps crossbow with a 400-grain bolt makes about 142 ft-lb; with a 500-grain bolt at roughly 367 fps it makes about 149 ft-lb — the heavier bolt is slower but carries similar or better energy and clearly more momentum, and it is kinder to the limbs and string.
Can I shoot a lighter bolt to get the rated speed back?
Only down to the maker’s stated minimum bolt weight — never below it. An underweight bolt absorbs too little of the shot’s energy, which is hard on limbs and string and can void the warranty; at the extreme it approaches a dry fire. The rating is a ceiling measured at the minimum, not a target to chase.
Does bolt length change the speed?
Only through weight. A longer bolt of the same construction weighs more, and the weight rule prices it in. Length itself matters for fit and safety — the broadhead must clear the rail and foot stirrup — not for the speed maths.
Is 350 fps fast enough for deer?
Easily. Crossbow energy figures dwarf vertical-bow numbers: 350 fps with a 425-grain bolt is about 116 ft-lb, several times the commonly cited deer requirement. At crossbow speeds, accuracy, broadhead flight and a quiet bow decide far more than another 15 fps.
- Archery Trade Association — industry standards body for bow and arrow measurement (ATA draw length and weight)
- Easton Archery — shaft selection charts — the reference spine-selection and arrow-building charts this site’s groupings follow
- Gold Tip — arrow university and spine charts — manufacturer spine charts and arrow-weight data used to cross-check the tables