Archery Calculators

FOC calculator (with reverse: find the shaft GPI you need for a target FOC) and stabilizer MOI planner. Arrow spine now has its own full calculator. All run in the browser; nothing is sent anywhere.

Arrow Spine → Advanced Spine Selection → Arrow Node & Oscillation → FOC Stabilizer MOI Group Size Projection → Sight Tape Generator → Score Tracker →

Arrow Spine Calculator

The spine calculator has moved to its own page — compound and recurve, with cam profile, rest type, GPI, insert weight, FOC estimate, and Axial's light-arrow recommendation. Includes a full methodology section on what spine charts are actually based on.

Open the spine calculator →

FOC Calculator

Front of center percentage — and its reverse: given a target FOC, find out what shaft GPI you need.

Arrow cut length (inches)

Shaft GPI (grains per inch) Check manufacturer specs for your shaft

Point weight (grains)

Insert weight (grains)

Nock weight (grains)

Total fletching weight (grains) All vanes or feathers combined

Wrap weight (grains, 0 if none)

Target FOC (%) What FOC do you want to hit?

Arrow cut length (inches)

Point weight (grains)

Insert weight (grains)

Nock weight (grains)

Total fletching weight (grains)

Wrap weight (grains, 0 if none)

Total arrow weight —

Balance point from nock —

FOC —

Zone —

0% 7% 11% 15% 20% 33%

Stabilizer MOI Planner

Moment of inertia for each rod and weight combination — shows how much each component contributes to rotational resistance. MOI = m × r²; longer and heavier beats short and heavy. Units: oz·in²

Total MOI —

Component	MOI (oz·in²)	Share

MOI is calculated about the grip pivot. Rod contribution: ⅓ × rod_mass × length². Tip weight: mass × length². Higher total MOI = more resistance to rotation during the shot and aiming — but also slower recovery after the shot. There is no universal target; compare configurations against each other.