Head offset capability is a key parameter for achieving higher track density. We analyze it by using a probabilistic approach. The error rate in head offset positions is calculated using core width of write and read head and mechanical runout. The nonrepeatable runout (NRRO) experiment shows the reasonableness of a Gaussian distribution assumption. The readback signal depends on geometrical overlapped write and read core width. The error rate is calculated by integration of positioning probabilistic density. We also study side erasure phenomena of multiple adjacent track writing by a similar probabilistic approach. The calculation shows the mechanical NRRO affects side erasure in million times adjacent track writing. On these offset-capabilities, the calculated and experimental results are well correlated. These results show this probabilistic modeling is appropriate.