A scale for the thickness of layers in regular ''diffusive''-type thermohaline staircases, derived from dimensional analysis, is found to collapse oceanic data. Combining this scale with laboratory-derived double-diffusive flux laws, we formulate effective diffusivities for salt, heat, and density. The diffusivities depend on the Turner number R&rgr;, but are independent of the brunt-Vaisala frequency. For R&rgr; near 1 the diffusivities for salt and heat are approximately equal (≂10-4 m2 s-1). They decrease roughly as R&rgr;-4 and R&rgr;-2, respectively, over the oceanic range 1≤R&rgr;10.