Solid cylinder of radius $R$ carrying current $I$ uniformly: Outside ($r > R$): $B = \mu_0 I/(2\pi r)$ (same as a wire). Inside ($r < R$): $B = \mu_0 Ir/(2\pi R^2)$ (linearly increasing). At the surface ($r = R$): $B = \mu_0 I/(2\pi R)$ (maximum). For a hollow cylinder (inner $a$, outer $b$): inside the hole ($r < a$): $B = 0$. Between walls ($a < r < b$): $B = \mu_0 I(r^2-a^2)/(2\pi r(b^2-a^2))$. Outside ($r > b$): same as solid wire.