Sight Tape / Pin Gap Calculator
Archery pin spacing from your arrow speed and sight geometry, with a to-scale pin-stack diagram so you can see the gaps before setting them.
Gap math from projectile-drop geometry scaled to your sight radius (verified in tests: a 280 fps bow runs ~0.2″ between the 20 and 30 pins at 33″). The model ignores air drag, so real gaps run a few percent wider at 50+ yards — set your first two pins by shooting, and these numbers predict the rest closely. The universal truths hold: gaps grow with distance, and every 10 fps of speed tightens the whole stack.
The drop explodes, and the pin gaps do not move at all
At 280 fps an arrow falls 8.86 inches by 20 yards and 141.83 by 80 — sixteen times the drop for four times the distance, exactly the square. Which sets up the expectation that the long pins must bunch up at the bottom of the housing.
| Distance | Arrow falls | Hold angle | Pin sits | Gap from the last pin |
|---|---|---|---|---|
| 20 yd | 8.86″ | 0.012311 | 0.0000″ | — |
| 30 yd | 19.94″ | 0.018467 | 0.2031″ | 0.2031″ |
| 40 yd | 35.46″ | 0.024623 | 0.4063″ | 0.2031″ |
| 50 yd | 55.40″ | 0.030779 | 0.6094″ | 0.2031″ |
| 60 yd | 79.78″ | 0.036934 | 0.8126″ | 0.2031″ |
| 70 yd | 108.59″ | 0.043090 | 1.0157″ | 0.2031″ |
| 80 yd | 141.83″ | 0.049246 | 1.2188″ | 0.2031″ |
They do not. A pin is not set by the drop but by the angle you hold, which is the drop divided by the distance — and dividing a square by its own base leaves something linear. So every ten-yard gap is 0.2031 inches, and sweeping 20 to 200 yards the largest and smallest gaps differ by 2.7e-15 inches, which is floating-point noise rather than a trend.
Which is why a sight tape can be a printed strip
Because the offsets are linear in distance, 25 yards sits exactly halfway between the 20 and 30 marks — not approximately, but to an error of 0e+0 inches. Interpolating between two known pins is exactly right in this model, for any pair and not just neighbours, which is what lets a tape be a ruler instead of a lookup table.
The two hardware knobs are exact proportions too
| Change | Ten-yard gap | Ratio | Predicted |
|---|---|---|---|
| 28″ bar | 0.1724″ | 0.8485 | 28/33 = 0.8485 |
| 33″ bar | 0.2031″ | 1.0000 | 33/33 = 1.0000 |
| 40″ bar | 0.2462″ | 1.2121 | 40/33 = 1.2121 |
| 240 fps | 0.2765″ | 1.3611 | (280/240)² = 1.3611 |
| 280 fps | 0.2031″ | 1.0000 | (280/280)² = 1.0000 |
| 320 fps | 0.1555″ | 0.7656 | (280/320)² = 0.7656 |
| 400 fps | 0.0995″ | 0.4900 | (280/400)² = 0.4900 |
Sight radius enters once and speed enters squared, both to the last digit the model carries. Which explains the real trade: a longer bar spreads the pins for finer aiming, and a faster bow packs them together. From 260 to 340 fps the whole 20-to-60 yard span shrinks from 0.94 to 0.55 inches of housing — a fast bow is more forgiving of a ranging mistake and harder to aim precisely between pins, at the same time.
What the model leaves out
All of the above is exactly true of a vacuum trajectory, which is what this page computes and says it computes. Real arrows shed speed to drag, so real gaps open slightly at the long end rather than staying perfectly even. The uniformity is a property of the model rather than a measurement — the honest version is that gaps are even enough that interpolating is a good habit, and the error grows with distance in the one direction the model cannot see. Shoot the long pins to set them.
How to use
- Enter your arrow speed and sight radius.
- Set your pin distances.
- Read the spacing and check it against your sight housing.
- Verify by shooting at each distance.
Frequently asked questions
What determines pin gap?
Arrow speed, the distance between pins, and the geometry of your setup — sight radius, peep-to-sight distance and anchor. Faster arrows shoot flatter, so the pins sit closer together, which is the main reason speed is pursued for hunting setups.
Why do my pins run out of room in the housing?
Because the gaps at longer distances grow quickly with a slower setup. A slow arrow with pins out to sixty yards can need more travel than the housing allows, at which point the options are fewer pins, a slider sight, or a faster arrow.
Should I use a fixed pin sight or a slider?
Fixed pins are faster to use and need no adjustment under pressure, which suits hunting inside moderate ranges. Sliders give one clean sight picture at any distance and require setting the range first, which suits target archery and known-distance shooting.
How accurate is a calculated gap?
Close enough to set up from and not to trust blindly. Real arrow flight depends on the specific arrow, fletching, broadhead, form and conditions, so the calculation is a starting point. Shooting at each distance and adjusting is what actually sets the pins.
Does gap shooting mean I do not need pins?
It is a different method rather than a shortcut — gap shooting uses the arrow point against the target at known distances, and it demands consistent form and a great deal of practice. Plenty of traditional archers use it well; it is not easier than pins.
Why do my broadheads hit differently from field points?
Because fixed-blade broadheads have surface area that steers the arrow, amplifying any tuning error. An arrow that flies well with field points and poorly with broadheads is telling you the setup is not tuned, and the fix is tuning rather than a separate sight mark.
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