This paper uses a complete washing and sieving method and accurately determines the clay mass contents in the loess deposits at the eight regions of Xianyang, Ili, Xi'an, Yan'an, Lvliang, Linfen, Xining, and Lanzhou in China. The method uses nylon cloth sieves with apertures from 0.0008 mm to 0.048 mm and standard steel sieves with apertures from 0.063 mm to 28 mm. It uses a rotary vibration machine to wash and sieve the loess into many clay, silt, sand, and gravel subgroups. The masses of the separated materials construct the complete mass-based particle size distribution (PSD) curves for the tested loesses. The results show that the Xianyang, Ili, Xi'an, Yan'an, Lvliang, Linfen, Xining, and Lanzhou loesses contain 65.28%, 56.73%, 56.76%, 38.7%, 31.78%, 30.55%, 30.1%, and 26.29% clay in mass. These clay contents are 1.3-14.9 times higher than the clay contents from the past publications for the eight types of loess. On the other hand, the plasticity data in past publications show that loess belongs to the clay type of soil. The clay contents in the publications are underestimated, which is caused by the inseparability of fine soils (or clay and silt mixture) from the existing PSD test methods. Macro-photographs, micro-photographs and SEM photographs present the separated materials of individual clay, silt, sand, and gravel particles. Particle sizes measured from the SEM photographs confirm their particle sizes within their size limits. The clay particles exhibit strong internal cohesion, while the silt, sand, and gravel particles are individual and non-cohesive. Atterberg limits test results further demonstrate the clay particles' high plasticity features and the silt particles' non-plasticity features. (c) 2026 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/ 4.0/).