A method includes operating a downhole acquisition tool in a wellbore in a geological formation. The wellbore or the geological formation, or both, contains first fluid that includes a native reservoir fluid of the geological formation and a contaminant. The method also includes receiving a portion of the first fluid into the downhole acquisition tool and determining a plurality of properties of the portion of the first fluid using the downhole acquisition tool. The plurality of properties includes a mass fraction of a component of the portion of the first fluid and a density of the portion of the first fluid. The method also includes using the processor to estimate a volume fraction of the contaminant in the portion of the first fluid based at least in part on a composition mass fraction function that depends at least on the mass fraction of the component in the portion of the first fluid and the density of the portion of the first fluid.