Assuming the seven rods are arranged such that the 7th rod is surrounded by the other 6, then the strap must have a length equalling 2.5 cm + 6 * 1.8 cm + 2 * pi * (1.8/2)^2 cm which is
13.3 + pi * 1.62 cm or approximately 18.4 cm to 1 decimal place.
