Sight Tape Calculator (Pin Gap and Point-On)

Pin gap is what your sight picture actually looks like: how far apart your 30 and 40 yard pins sit, whether a fifth pin will fit in the housing, and how far a single pin slider can reach before it runs out of travel. All of it comes from two numbers, arrow speed and sight radius, and a bit of trajectory geometry.

Where the formula comes from

An arrow leaves the bow and gravity immediately starts pulling it down at a constant rate. How far it falls depends only on how long it is in the air, so drop grows with the square of distance. Written out for archery units, with distance in yards and speed in feet per second:

• Drop in inches = 1,738.8 times distance squared, divided by speed squared. At 280 fps and 40 yards that is 35.5 inches of drop.
• Sight angle, as a tangent = 48.3 times distance, divided by speed squared. Dividing the drop by the distance collapses the square, which is why the sight angle grows in a straight line with distance even though drop does not.
• Pin gap in inches = sight radius times 48.3 times the yardage difference, divided by speed squared.

That last line is the whole tool. At 280 fps with a 32 inch sight radius, ten yards of distance is 32 times 48.3 times 10, divided by 78,400, which is 0.197 inches. About three sixteenths of an inch between pins, which will be familiar to anyone who has set up a five pin sight.

The useful and slightly counterintuitive consequence is that pin gaps are evenly spaced in this model even though drop is not. The distance from 20 to 30 yards on the sight bar is the same as from 60 to 70. Real tapes widen slightly at the far end because of drag, which the section below covers.

Pin gap per 10 yards, by speed and sight radius

Read the table as a design tool rather than a scoreboard. A hunter running 290 fps on a 32 inch radius gets 0.184 inches between ten yard pins, so a 1.5 inch housing has room for roughly nine pins geometrically and about five that are genuinely distinguishable once fibre optic diameter and low light are accounted for. A target archer at 250 fps with a 40 inch radius gets 0.309 inches per ten yards, which is why long bars and slower target arrows go together.

Speed versus sight radius

The two inputs pull in opposite directions and both are legitimate strategies:

• More speed compresses the pins. Gaps shrink with the square of velocity, so going from 260 to 300 fps cuts pin gap by about a quarter. Pins stack closer, more of them fit in the housing, and a range estimation error costs less. It also gets harder to tell the 40 from the 50 in poor light.
• More sight radius spreads them. Gaps grow in direct proportion, so moving from a 30 to a 40 inch radius spreads them by a third. Small aiming errors become visible and correctable, which is the whole reason target sights use long bars.

Hunters generally want the compressed picture, because a single sight picture covering 20 to 40 yards is worth more in the field than resolution. Target archers want the spread. Neither is more accurate in the abstract, and it is worth knowing which one you have chosen rather than inheriting it from whatever sight came on the bow. If you have not measured your arrow speed yet, start with the arrow speed calculator, because every number on this page scales with the square of it.

Pin gaps of two tenths of an inch are precise, and that precision is entirely wasted if the animal is at 37 yards and you thought it was 30. This is the recurring theme across every calculator here: the arithmetic is easy, and the input everybody actually gets wrong is distance. A rangefinder with angle compensation removes both errors at once, which is why it consistently outperforms any equipment upgrade of similar cost.

Single pin sliders and how far they reach

A movable pin sight trades the multi-pin picture for a single crisp aiming point, and the limit becomes how much vertical travel the housing has. Rearranged, the same formula gives the maximum reach:

Maximum yardage = near yardage plus travel times speed squared, divided by 48.3 times sight radius. At 290 fps with a 32 inch radius and 1.5 inches of usable travel below the 20 yard mark, that comes out at roughly 101 yards. This is why single pin sights reliably print tapes out to 100 yards and why a slower traditional or crossbow setup runs out of tape far sooner.

A practical caution: the reach figure assumes every inch of travel is usable. Housings hit the riser, cable guards get in the way, and some sights lose the last quarter inch to the mount. Measure the real usable travel with the sight on the bow rather than reading a catalogue number.

Point-on distance, and why barebow archers care

Point-on is the barebow equivalent of a sight mark: the distance at which putting the tip of the arrow on the target puts the arrow in the middle. It falls out of exactly the same geometry, with the eye taking the place of the peep and the arrow point taking the place of the pin:

Point-on distance in yards = eye height above the arrow, times speed squared, divided by 48.3 times the eye to point distance. A recurve archer shooting 200 fps, with an anchor that puts the eye 1.5 inches above the shaft and an arrow point 26 inches from the eye, points on at about 48 yards.

That single number organises the whole gap shooting system. Below point-on you hold under the target by a decreasing amount as you approach it, and above point-on you hold over. Archers who face-walk, moving the anchor down the face to change the eye offset, are directly manipulating the first term in that equation, which is why a lower anchor shortens point-on. Increasing arrow speed pushes point-on further out, one of the few genuine arguments for a lighter traditional arrow.

What this model does not include

The calculation treats the arrow as a projectile with no air resistance, which is a good approximation and a well understood one. Two things it leaves out:

• Drag. Arrows lose speed in flight, more so with large vanes and fixed broadheads. Inside 50 yards the effect is small. Past 70 it grows, so real sight tapes have marks spaced slightly wider at the far end than the model predicts. Never extrapolate a tape you calibrated at 20 and 30 yards out to 90.
• Shot angle. Every mark assumes a level shot. Angled shots play shorter, and no amount of tape precision fixes that. Run those through the angle compensation calculator first, then use the corrected distance on the tape.

The right way to build a real tape is to shoot two widely separated known distances, typically 20 and 60 yards, and calibrate between them. Every major sight manufacturer's app works this way for exactly the reasons above. Treat this calculator as the tool that tells you what to expect and whether your sight has the travel for what you want, then let real arrows settle the marks.

Related tools

• Arrow speed calculator: pin gap scales with the square of this number
• Angle compensation calculator: correct the distance before you use the mark
• Total arrow weight calculator: arrow mass sets speed, and speed sets the tape
• Best archery rangefinders: the input that matters more than the tape