No such empirical formula exists to obtain Clay % from Fine content (silt+clay). The A-line in the plasticity chart enables you to see if the majority of (silt+clay) fraction is clay if it plots above the A-line and silt if it plots below the A-line. However, it could be organic soils as well!

I feel correct estimate clay percentage is only by conducting both combined dry and wet sieve analysis. you want estimate clay percentage so wet sieve analysis is to be performed. Clay % also to be obtained from activity number ..activity = plasticity index/clay percentage..i recommend you to conduct sieve analysis for soil samples and obtain the correct value of clay %.

I guess there is no such empirical formula to calculate clay from silt+ clay. Even at all it exists it may not give a correct value of clay%. Although it is a tedious method, particle size determination following the International Pipette method is the best method and that too after the removal of all cementing agents like organic carbon, free Fe2O3 and CaCO3. You may refer to Bhattacharyya, T., Sarkar, Dipak & Pal, D.K. (Eds.) 2009. Soil Survey Manual. NBSS&LUP Publication No. 146, India, 400pp.

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Agree with Karthigeyan S. Precise estimation of clay content can be obtained by adopting appropriate laboratory analysis method. The relationship between clay content and Atterberg’s limits had been reported. Similar relationship with dispersion ratio had also been reported. The feasibility of using such relationships may be considered in view of operational range of soil texture and precision. Nevertheless, it may provide qualitative idea about clay content.

https://www.jstage.jst.go.jp/article/sandf/47/5/47_5_887/_pdf

https://icce-ojs-tamu.tdl.org/icce/index.php/icce/article/viewFile/2059/1731

https://file.scirp.org/pdf/OJSS_2016012013263226.pdf

http://horizon.documentation.ird.fr/exl-doc/pleins_textes/pleins_textes_7/sous_copyright/010033767.pdf

No such empirical formula exists to obtain Clay % from Fine content (silt+clay). The A-line in the plasticity chart enables you to see if the majority of (silt+clay) fraction is clay if it plots above the A-line and silt if it plots below the A-line. However, it could be organic soils as well!

Karthigeyan . S : Thanks for your suggestion Sir. I have the sieve analysis data from 10 to 0.075 mm but don't have hydrometer data (

There is no empirical formula to calculate clay from silt & clay.

Yes, I agreed with Boriana Tchakalova. Further I can suggest hydrometer analysis is the accurate test to distinguish the percentage of silt and clay.

Pipette method is also suitable. It is generally assumed that pipette methods are more accurate than the hydrometer methods although the latter are more rapid .