Your sight radius is the distance from the centre of your peep sight to the centre of the pin housing on your bow sight, measured at full draw. It's the single most critical input for generating an accurate sight tape — get it wrong and every yardage mark will be off by a consistent, compounding amount.
A flexible tape measure or a length of string and a ruler. You do not need to remove any equipment from your bow. A partner makes it easier, but it's doable alone with a draw board or wall-mounted bow holder.
Draw your bow to full draw. While at full draw, have your partner place the end of the tape measure at the centre of your peep sight hole — the small hole you look through. Run the tape straight along the string and riser to the centre of the pin housing (the circular housing that holds your sight pin). Read the measurement.
If you're measuring alone, anchor your bow in a draw board or vice, draw it with a rope release to full draw, then measure with a rigid ruler or tape. Never measure while relaxed — the bow geometry changes significantly between at-rest and full draw, and your measurement will be wrong.
Most compound setups measure between 5 and 9 inches. Write the number down to the nearest quarter inch.
When you draw a compound bow, the cam system rotates and the string and cables shift. The physical angle and distance between your peep and pin housing changes. A sight radius measured while the bow is relaxed can be off by 0.5 to 1 inch from the true at-draw measurement. On a 70-yard tape, a half-inch error in your radius moves every mark by several millimetres — enough to miss a vital zone at long range.
Enter your sight radius into the sight tape generator along with your arrow speed and distances. The generator uses your radius to convert arrow drop (in inches) into the physical amount your sight needs to move — which is what gets printed on the tape.
Re-measure any time you adjust your sight. Moving a sight forward or back even a quarter inch changes the radius and shifts every mark on your tape. A new sight position always means a new tape.