There is a main body of correspondence, arranged alphabetically by correspondent. This is supplemented by additional letters, which are divided into subject groups:
Tm1 to Tm31. Testimonials written by Kelvin in 1846 regarding the professorship of natural philosophy in Glasgow University.
CS1 to CS708 Compass and sounding machine correspondence, 1877-1880. There are letters from various people and organisations to Kelvin and to his nephew William Bottomley, concerning Kelvin's inventions. Roughly half the letters are to Kelvin, half to Bottomley.
Pr1 to Pr438. Peerage congratulations received by Kelvin and his wife when he was made Baron Kelvin in 1892. Lady Kelvin's mother's death occurred at the same time, and many of the letters include reference to this event.
Jb1 to Jb240. Jubilee congratulations received by Kelvin and his wife for the jubilee of his professorship in 1896.
Rt1 to Rt26. Correspondence relating to Kelvin's retirement in 1899.
OM1 to OM59. Congratulations received by Kelvin and his wife in 1902 when he was awarded the Order of Merit and made Privy Councillor.
DS1 to DS25. Correspondence relating to attempts to have the decimal system of weights and measures accepted in Britain, 1901-1906.
LB1 to LB31. 31 letter books containing drafts (except for LB4 which contains press copies) of letters written to Kelvin: LB1 to LB4, 1878-1884; LB5 to LB31, 1899-1906.
PA1 to PA10. Notes by various people concerning the Kelvin collection.
PA11 to PA17. Kelvin's notes made while reading with William Hopkins at Cambridge.
PA18 to PA41. Cambridge and Glasgow, 1840s and 1850s.
PA42 to PA86. Mainly reprints of Kelvin's published papers.
PA87 to PA179A. Mainly manuscript drafts of Kelvin's papers.
PA180 to PA182. Lists regarding Kelvin's letters and papers.
PA183 to PA188. Material regarding the Lalla Rookh.
PA189 to PA214. Tides.
PA215 to PA231. Telegraphs.
PA232 to PA242. Indian Telegraph Service Examination.
PA243 to PA266. Metric System.
PA267 to PA300. Miscellaneous, c.1850 to c.1900.
PA301 to PA316. Reports from the Physical Laboratory, Glasgow University.
PA317 to PA332. Miscellaneous.
PA333 to PA355. Obituary notes, etc., for Kelvin.
PA356 to PA406. Miscellaneous material.
PA407 to PA446. Material relating to people other than Kelvin.
180 volumes, c.1835-1907, mainly research notebooks, with some additional correspondence.