How much is Yad Soledes?
Yad Soledes is the amount of heat that would burn the stomach of a baby. Some Poskim rule this is measured by the amount of heat that one’s finger does not feel comfortable in. Others rule it is the amount of heat in a food that one would avoid entering into his mouth.
In degrees: Some Poskim rule Yad Soledes is 40 degrees Celsius [104 Fahrenheit]. Others rule it is 43 degrees Celsius [109 Fahrenheit]. Others rule it is 45 degrees Celsius [113 Fahrenheit]. Practically the custom of many is to measure Yad Soledes by 45 degrees Celsius. However, one is to be stringent regarding reheating foods that have cooled down to consider Yad Soledes 71 degrees Celsius [159 Fahrenheit].
The Halachic ramifications: Knowing whether a food is the heat of Yad Soledes plays a role regarding various cooking related laws on Shabbos, such as a) Heating a food to less than Yad Soledes; b) Reheating a food that is below Yad Soledes.
 See Piskeiy Teshuvos 253/1; Pischeiy Teshuvah 105/7; Minchas Yitzchak 29; Minchas Shlomo 1/91; SSH”K 1 footnote 3; Igros Moshe 4/74; Yoreh Deah 2/52; Midos Vishureiy Torah; Oar Letziyon 2/30; Maor Hashabbos 1/2
 Michaber 318/14
 Vayeishiv Moshe 178; Birur Halacha 318; Piskeiy Teshuvos ibid
 Ben ish Chaiy Bo 5; Kaf Hachaim 253/30; 318/143
 Chazon Ish brought in Piskeiy Teshuvos ibid; Bris Olam Ofeh 92; Chut Shani 29/7; Or Letziyon 2/30-12; Kovetz Mibeis Levi 6/35
 Igros Moshe 4/74
 Minchas Yitzchak Likkutei Teshuvos 29; Rav SZ”A in Minchas Shlomo 1/91; SSH”K 1 footnote 3
 Igros Moshe ibid; Yoreh Deah 2/52; Or Letziyon 2/32 footnote 12; Maor Hashabbos 1/2 in name of Rav SZ”A