Ab initio calculations have been made on [n]helicenes and their planar, zigzag isomers, the [n]phenacenes, at the HF/6-31G* and B3LYP/6-31G* levels for n = 6-10 and 16 and at B3LYP/6-311G** for n = 6-10. The energies and magnetic susceptibilities of the helicenes and phenacenes are found to vary linearly with n in both series; comparison of them indicates only slight loss of aromatic character in the helicenes despite their large departures from planarity. Proton chemical shifts of [7]helicene and [7]phenacene are in good agreement with experiment. For [16]phenacene, the eight inner rings have nearly identical bond lengths and chemical shifts, showing their convergence tp the zigzag infinite-polymer limit. While geometrical convergence is also evident in [16]helicene, the magnetic shielding, subject to effects of nonadjacent rings, requires a larger value of n for convergence.