Japanese merchant fleet as of:
01/1940: ~ 1,220 vessels, over 1000 GRT, for 5,256,000 tons of shipping
01/1942: ~ 1,550 vessels, over 1000 GRT, for 5,860,000 tons of shipping
Of the 1550 vessels, they break down as follows:
24% from 1000 to 1999 gross tons
19% from 2000 to 2999 gross tons
16% from 3000 to 3999 gross tons
23% from 4000 to 5999 gross tons, centered at 5000 gross tons
14% from 6000 to 8999 gross tons, centered at 7500 gross tons
04% from 9000 onward gross tons, centered at 10000 gross tons
Centering at 5000, 7500, and 10000 gross tons captures the median about the three main tonnage figures of merit for ocean going, long voyage vessels. Everything below 4000 gross tons, would be intended for near seas routes. Doing the math, you get:
372 ships @ avg. 1500t = 558,000t
295 ships @ avg. 2500t = 737,500t
248 ships @ avg. 3500t = 868,000t
355 ships @ avg. 5000t = 1,775,000t
217 ships @ avg. 7500t = 1,627,500t
062 ships @ >8000t = ~ 620,000t
1550 ships is approximately 6,186,000t
A bit more than 5,860,000, but not much. The difference is about +320,000t (+5.5%) and is well accounted for by slight skews in the percentages and/or the centers of the average values, and/or the reported statistics for Japan for 01/1942.
Please also note that the 01/1942 values, as well as the statistical summary, does not include the second, third or fourth Requisitions. The data is from Western and Japanese shipping company records, which were either not aware of, or did not report, the extent of allocation of vessels to military requirements. Those ships designated for, or actively undergoing conversion to, Naval vessels, as of the beginning of 1941 are accounted for (about 220,000 tons, most from the bottom category), but all ships eventually allocated as fleet auxiliaries, after 1/41, are included in the totals given above (c.f. Kimikawa Maru, and all others). Thus, about 1,200,000 tons (about 150-200 ships) need to be deducted from the above totals to account for all the AVs, AKs, APs, AOs, AEs, ASs, etc, as of 1/42. Notably, these deductions must be made from the bottom 3 categories
This implies the majority of ships > 6000 tons were utilized as auxiliary and/or transport vessels by the IJA/IJN, leaving a scant 600, or so, vessels in the > 3000 ton range, and a very limited few in the > 5000 ton range, as a commercial shipping basis (beginning to see the Japanese shipping crisis??).
It gets worse when modernization is considered. The age of the Japanese merchant fleet, in years, as of 01/1940, is as follows:
45% of the fleet is 15 years old, or less. 41% of the fleet, in 1941 is 4000 gross tons or more; Hmmm. When you take away the allocated vessels, there aren’t many fast, modern, vessels left to ‘acquire’ from commercial routing for duty as transports, etc.. Thus the ‘demand’ for 600,000 tons of shipping by the IGS in Sept-Dec 1942 becomes very important in context. Japan only built 210,000 tons in 1941 (included), and 260,000 tons in 1942 (not included). All figures include tankers!