Kokumori refers to the estimated production output (To-dai; one To was about 18 liters) per tan (an old land area unit; 991.7 square meters) of rice in rice paddies, fields and residential areas in a land survey. Based on this system, a whole Kokudaka (crop yield) was calculated using these measurements. Originally, however, these measurements were used to refer to calculations of To-dai and Kokudaka.
Although standards varied over the ages, it was customary to set basic grades; grades for rice paddies and fields were given according to their merits, such as productivity, on three basic levels: high grade (very fertile rice paddy), medium grade and lower grade (worn-out rice paddy); rice paddies with better production than high grade were "superfine rice paddies" and those with worse production than lower grade were "bottom-ranked rice paddies."
In the Taiko Kenchi (land surveys conducted by Hideyoshi TOYOTOMI), the method for ranking rice paddies was to reduce 2 To (36 liters) when the rank of the rice paddy is downgraded by one (also known as "Nitokudari," literally meaning "reduction of Two To"); rice paddies that produced 1 Koku and 5 To (about 270 liters) was ranked as high grade, those that produced 1 Koku and 3 To (about 235 liters) as medium grade, and those that produced 1 Koku and 1 To (about 198 liters) as lower grade; while the standard for residential areas was to be 1 Koku and 2 To (about 216 liters), other fields known as 'mihakarai,' such as bottom-ranked rice paddies, fields and riverside fields were left to the discretion of the person in charge of the land survey. During the Edo period, a method was adopted using Nitokudari in which rice paddies producing 9 To (162 liters) as bottom-ranked rice paddies and fields producing 1 Koku and 2 To (about 216 liters) as high-grade fields.
Several places in rice paddies and fields would be selected at random and the average of the crop per 3.3 square meters of these locations was calculated after harvesting. When the average crop of unhulled rice was 1 Sho (1.8 liters) per 3.3 square meters, it was equivalent to 3 Koku (30 To = 300 Sho, or about 541 liters) per tan. When rice was threshed by 50%, it was calculated that the amount of brown rice was equal to 1 Koku and 5 To; in other words, it was '1 Koku 5 To-dai' per tan, meaning it corresponded to the 'high grade' rice paddy. A number of daimyo also adopted a method of calculating a harvest yield by reducing it by 20% and enforcing Kangenbiki (subtraction due to losses by drying) (this calculation was done by modifying three koku to 2 Koku 4 To (about 432 liters) by reducing it 20 percent and ending up with '1 Koku 2 To-dai').
Evaluations of land in land surveys depended largely on the judgment of the person in charge, and this sometimes resulted in rice paddies and fields in commercial and industrial business areas being given high estimates and those in important political and military areas as well as remote areas being given low estimates in order to attract support from the public. As this was seen to be unfair imposition on the peasants, there are some cases of these estimates resulting in revolts.
During the middle of the Edo period, Kokumori based on evenness came to be adopted regardless of any conditions; Kangenbiki was abolished and Kokumori taken on residential areas and high-grade fields were altered to be the same amount of lower-grade rice paddies, or 1 Koku and 1 To, while for fields, the method of exercising Nitokudari from this amount was adopted. As productivity advanced in farmlands, actual harvest yields often exceeded the calculated amount of Kokumori, but this calculating method continued until the land-tax reform during the Meiji period.
Furthermore, when Kokumori was to be revised in a land survey, an increase in Kokumori was called "Kokumori-deme" and a decrease was called "Kokumori-chigaibiki"; the term used when upgrading a piece of land (e.g.: from a field to a rice paddy) became called "Kokuma-shukkoku" and downgrading (e.g.: from a rice paddy to a field) became called "Kokuma-biki."