What is one to do if he does not remember if he recited Birchas Hamazon?
[A man] who is unsure if he already recited Birchas Hamazon or not, then if he ate to the point of satiation [i.e. he is full from the meal, and drank liquid during it-See Q&A], then he must repeat the entire Birchas Hamazon due to the doubt. If, however, he did not eat to the point of satiation [i.e. is still hungry for more food, or is unsure], then he does not repeat Birchas Hamazon. [The above case refers to one who does not have any more bread to eat, or is to full to eat anymore. If, however, one has more bread to eat, and is able to eat more, then he should escape the doubt and rewash and eat a Kezayis of bread within Achilas Peras, and thus recite Birchas Hamazon without question.]
Summary [includes Q&A]:
A man or woman who does not remember if he/she recited Birchas Hamazon, is to try and eat another Kezayis of bread and recite it. If one is unable to do so, then by a man, if he was satiated from the meal and drank liquid during it, then he must repeat the entire Birchas Hamazon [if he is Ashkenazi, and if Sephardi he repeats only the first three blessings].
Does the above law also apply to women? If a woman is unsure if she recited Birchas Hamazon, must she repeat it if she is satiated from the meal?
Although women are obligated in reciting Birchas Hamazon, it is questionable as to whether they are Biblically obligated similar to men, or are merely Rabbinical obligated. Accordingly, a woman who is unsure if she already recited Birchas Hamazon is not to repeat it even if she ate to the point of satiation. Nevertheless, if there is more bread available, and she is able to eat more, then she should rewash and eat a Kezayis of bread within Achilas Peras, and recite Birchas Hamazon according to all.]
Must one be full only from the bread, or does all the food in the meal join for satiation?
All the foods eaten during a meal which consists of bread is included in the measure of satiation, even if one did not eat enough bread to be considered satiated from it alone. [Thus, if one washed on bread, and ate at least a Kezayis within Achilas Peras, then if he is satiated from the meal, he is Biblically obligated in Birchas Hamazon, and must repeat it in a case of doubt. If, however, he did not even eat a Kezayis of bread within Achilas Peras, then he is not obligated to recite Birchas Hamazon at all.]
If one is unsure whether he is satiated from the meal, is he to repeat Birchas Hamazon?
If he is unsure whether he is satiated, then he is not to recite Birchas Hamazon.
If one did not drink during a satiating meal and is in doubt if he recited Birchas Hamazon, is he required to repeat it?
Some Poskim rule that in such a case he is to have a drink and then repeat Birchas Hamazon. Other Poskim, however, rule that if he was thirsty during the meal and did not drink, then he is not to repeat Birchas Hamazon, although if he is able to, he is to eat another Kezayis of bread and recite Birchas Hamazon.
 Admur 184:2; Michaber 184:4; Beis Yosef 184 in name of Rosh Brachos 3:15; Sheilasos of Rav Achaiy 53; M”A 184:8; Ketzos Hashulchan 44:2; See Kaf Hachaim 184:15-27; Piskeiy Teshuvos 184:4-6
 See Q&A!
 Admur ibid; M”A ibid
 Including the Rabbinical blessing of Hatov Vihameitiv. [Admur ibid; Maharash Chiyon; Mahariy Halevi 44; Kneses Hagedola, brought in Beir Heiytiv 184:7; Aruch Hashulchan 184:5; M”B 184; See Chochmas Shlomo 184:7; See Kaf Hachaim 184:15]
Other opinions-Sephardi custom: Some Poskim rule he is not to recite the forth blessing of Hatov Vihameitiv. [See Beir Heiytiv 184:7 that from the Michaber ibid it is implied that one does not repeat Hatov Vihameitiv; Chesed Lealafim 184:2; Ben Ish Chaiy Chukas 1:9; Torah Lesham 65; Kaf Hachaim ibid; Or Letziyon 2:13]
 The reason: If he was satiated from the meal then the obligation to recite Birchas Hamazon is Biblical [Admur ibid; Seder 2:1; M”A 184:8; 11; Brachos 20b; Tosafus Brachos 49b; Rosh Brachos 7:24], and hence in a case of doubt we apply the rule of Deoraisa Lechumra. [Admur ibid]
 See Q&A!
 Admur ibid; M”A ibid; Beir Heiytiv 184:7; Omitted from Michaber and Poskim ibid; See M”B 184:15; Piskeiy Teshuvos 184:5
The reason: If he was not full from the meal then the obligation to recite Birchas Hamazon is merely Rabbinical, and hence in a case of doubt we apply the rule of Safek Brachos Lihakel.
Other opinions: Some Poskim rule that if one ate a mere Kezayis of bread and drank after being thirsty then he is Biblically obligated to recite Birchas Hamazon even if he is not satiated from the meal. [Opinion in Admur 197:7; opinion in Rama 197:4; Opinion in Darkei Moshe 197:6; Opinion brought in Beis Yosef 197; Bach 197 [rules like Yireim]; Peri Chadash [rules like Yireim]; Yireim 253; Kol Bo 25; Mordechai Brachos 177; Shivlei Haleket 154; This opinion understands the opinion of Rebbe Meir in Brachos 49b to be Biblical; Rebbe Meir in Brachos ibid, as also explained in Tosafos Brachos ibid; Evident from Admur 197:7 towards end and M”A 197:12 and M”B 197:28 that according to this opinion one who drank but did not eat to satisfaction should do the Zimun over one who is satisfied but did not drink; See Machatzis Hashekel 184:11] Other Poskim rule that one who ate a Kebitza of bread is Biblically obligated to recite Birchas Hamazon even if he is not satiated. [Levush 184; Sefer Hachinuch Parshas Eikeiv, and Rabbeinu Yerucham and Rabbeinu Yonah, brought in Elya Raba 184:9; See M”A 184:11 in name of Olas Tamid and Levush; See Kaf Hachaim 184:22 in length] According to this approach, one would be obligated to repeat Birchas Hamazon if they ate a Kezayis/Kebeitza of bread and are unsure if they already recited Birchas Hamazon. [See Kaf Hachaim ibid]
 Kaf Hachaim 184:15; See M”B 184:15; Piskeiy Teshuvos 184:5
 Admur 186:1; Michaber 186:1; Rambam Brachos 5:1; Rosh Brachos 3:13; Safek in Brachos 20b
The reason: On the one hand, it is a non-time dependent positive command, and hence women should be Biblically obligated. On the other hand, the verse states, “On the good land that you were given” and since the land was not given to females, as an inheritance, but rather to the males, therefore perhaps they are not Biblically obligated. [Admur ibid; Rashi Brachos ibid]
 Birkeiy Yosef 186:4; Machazik Bracha 184:1; Gloss of Rav Akiva Eiger 184 and Teshuvah 25; Shaarey Teshuvah 184:7; P”M 186 A”A 1; Kol Eliyahu 6; Binyan Shlomo 13; Kitzur SHU”A 44:1; Ketzos Hashulchan 44 footnote 3; Kaf Hachaim 184:25
The reason: This is due to a Sfek Sfeika, as perhaps she already Benched, and even if she didn’t perhaps she is only Rabbinically obligated. [Poskim ibid]
Other opinions: Some Poskim rule that women are obligated to repeat Birchas Hamazon if they ate to the point of satiation and are unsure if they already recited Birchas Hamazon. [Shaareiy Efraim 11:12, brought in Beir Heiytiv 184:7 and Shaariey Teshuvah ibid; See Sdei Chemed Brachos 3]
 Kaf Hachaim 184:25 in name of Chesed Lealafim and Ben Ish Chaiy Chukas 1:10 [regarding being Yotzei with another]
 Admur 177:2 “Everything eaten during the meal is included in the satiation, as the verse states Veachalta Vesavata.”; See also Seder 2:3 regarding Mezonos bread that the satiation includes also foods eaten with the bread; This is unlike the possible implication from Admur in Seder 2:1 who writes “When he eats bread to the point of satiation” that one must be satiated from the bread alone.
Other opinions: Some Poskim rule that one must be satiated from the bread alone, excluding the other foods, in order to be Biblically obligated to recite Birchas Hamazon, and hence if he is not satiated from the bread alone he does not repeat Birchas Hamazon in a case of doubt. [P”M 184 A”A 8; Possible implication of Admur in Seder 2:1, as explained above]
 Admur 184:2 “Less than a Kezayis is not considered eating at all.”; See P”M ibid who uses this to prove that one must be satiated from bread alone in order to Biblically need to recite Birchas Hamazon, otherwise one who ate a crumb of bread and became satiated from other foods would be Biblcially obligated to recite Birchas Hamazon. However, the wording of Admur ibid answers his question, as less than a Kezayis is not considered eating at all, and is not within the fulfillment of Viachlta!
 Kesav Sofer 28; Binyan Shlomo 13; Sdei Chemed Brachos 9; Kaf Hachaim 184:20
 Birchas Habayis 17:9
 The reason: As according to some opinions he is not obligated to recite Birchas Hamazon if he did not drink, and hence even if he is satiated it is not a Safek Deoraisa in their opinion. [Birchas Habayis ibid footnote 13]
 Ben Ish Chaiy Chukas 1:9; Kaf Hachaim 184:26