mg/L to Per Mille Converter

In oceanography and water treatment, distinguishing between volumetric concentration and mass ratio is critical. mg/L measures mass per volume, while ‰ (per mille or parts per thousand) is a dimensionless mass fraction often used for salinity. While they are often treated as roughly interchangeable in casual contexts, strictly professional applications must account for the density of the solution.

This tool performs the conversion with a toggle for Seawater versus Freshwater. Freshwater typically has a density close to 1 kg/L, making mg/L and ppm numerically identical. Seawater, however, is denser (approx 1.025 kg/L), which skews the conversion factor. This utility allows manual density input for high-precision engineering requirements.

The core relationship depends on the density (ρ) of the solution in kg/L.

For standard freshwater, ρ ≈ 1, simplifying the formula to division by 1000. For seawater, ρ is typically 1.025.