The expression of grain sizes on a logarithmic scale. The phi value (ϕ‎) is related to grain diameter (d) by the expression ϕ‎ = −log₂d. Increasing positive phi values are for grain diameters progressively finer than 1000 μ‎m, and increasing negative values for grain diameters progressively coarser than 1000 μ‎m (e.g. 2ϕ‎ = 250 μ‎m; 1ϕ‎ = 500 μ‎m; 0ϕ‎ = 1000 μ‎m; −1ϕ‎ = 2 mm; −2ϕ‎ = 4 mm).