Radius of turn rule of thumb

[Le Flaneur]

From the since rescinded US Air Force AFMAN 11-217, Volume 3, page 65:

Turn Radius Calculation. The following two relationships provide the distance required to turn an aircraft 90° using 30° of bank. This distance is the aircraft’s approximate turn radius. These formulas are particularly useful when determining lead turn points when planning to perform a radial-to-arc or arc-to-radial portion of an instrument procedure

Formula 1: Turn radius [in nm] = (True Airspeed [in knots] / 60) - 2 or (Mach × 10) – 2
Formula 2: Turn radius [in nm] R = (True airspeed [in knots] ÷ 60)^2 / 10 or Mach^2 × 10

If you are flying in against a ground-fixed reference (e.g. DME arc, VOR radial, FMS Track-to-fix or course-to-fix leg), use ground speed in lieu of true airspeed