The three Fu periods of summer
Sanfu (三伏) means the three Fu periods: Chufu (初伏), the first Fu; Zhongfu (中伏), the middle Fu; and Mofu (末伏), the final Fu. Together they identify a traditional hot-season interval. Their dates combine two solar terms with the repeating names of days in the Ganzhi, or stem-and-branch, cycle. They are not fixed lunar month-and-day observances.
The counting rule
What is a Geng day?
A Ganzhi day name pairs one of ten heavenly stems with one of twelve earthly branches. Geng (庚) is the seventh stem. It returns every ten days, although the complete stem-and-branch name returns only every sixty. The Geng days within that cycle are Gengwu, Gengchen, Gengyin, Gengzi, Gengxu and Gengshen. Counting Fu periods requires the Geng stem, not a particular one of those six complete names.
The two solar terms used in the count are Xiazhi, Summer Solstice, and Liqiu, Beginning of Autumn.
| Period | Pinyin | Start | Last included day |
|---|---|---|---|
| First Fu | Chufu | Third Geng day counted on or after the Xiazhi calendar date | Day before the fourth Geng day |
| Middle Fu | Zhongfu | Fourth Geng day counted on or after the Xiazhi calendar date | Day before the first Geng day on or after the Liqiu date |
| Final Fu | Mofu | First Geng day on or after the Liqiu calendar date | Ninth day after its starting date |
These are calendar-day rules. When Xiazhi itself falls on a Geng day, that day counts as the first Geng day. When Liqiu falls on a Geng day, that day begins the final Fu. Each period starts at the beginning of its calendar date, regardless of the hour at which the solar term occurs.
The name fu carries the sense of hiding or concealment. In a traditional five-phase explanation, the metal associated with Geng withdraws before the power of summer fire. This symbolism connects the choice of stem with the hot season.
Period lengths
Because Geng recurs every ten days, the first Fu always lasts ten days. The final Fu also lasts ten. The middle Fu continues until the qualifying Geng day at Liqiu, so it lasts either ten or twenty days. The complete sequence therefore lasts thirty or forty days.
A twenty-day middle Fu is one extended middle period, not a fourth Fu period. Its length depends on where Liqiu falls in the ten-day stem cycle. Leap months do not enter the calculation.
Sanfu in 2026
For the UTC+8 calendar dates used in this example, Xiazhi falls on June 21, 2026, and Liqiu on August 7, 2026. The following Geng days determine the three periods.
Counting the relevant days
| Gregorian date in 2026 | Ganzhi day | Role in the calculation |
|---|---|---|
| June 25 | Gengwu | First Geng day after Xiazhi |
| July 5 | Gengchen | Second Geng day |
| July 15 | Gengyin | Third Geng day: first Fu begins |
| July 25 | Gengzi | Fourth Geng day: middle Fu begins |
| August 4 | Gengxu | Before Liqiu, so it does not begin the final Fu |
| August 14 | Gengshen | First Geng day after Liqiu: final Fu begins |
| August 24 | Gengwu | First day outside the completed Fu sequence |
The August 4 Geng day comes too early to begin the final Fu. The final Fu must wait for August 14, making the middle Fu twenty days long.
The complete 2026 periods
| Period | Starts | Ends, inclusive | Length |
|---|---|---|---|
| First Fu | July 15, 2026 | July 24, 2026 | 10 days |
| Middle Fu | July 25, 2026 | August 13, 2026 | 20 days |
| Final Fu | August 14, 2026 | August 23, 2026 | 10 days |
| Whole sequence | July 15, 2026 | August 23, 2026 | 40 days |
August 23 is the last included Fu day; August 24 is the first day after the sequence. The distinction matters when a calendar uses a short label such as “end of Fu.”
Summer weather
Sanfu is traditionally associated with the heat of midsummer and late summer. It falls after the Northern Hemisphere's longest day, when accumulated heat can remain high even as daylight shortens.
“Forty-day Fu” describes the longer version of the counting pattern. It does not mean forty consecutive days above a temperature threshold. Local weather determines the intensity of heat and the timing of a place's hottest days.