Sefer Ha’mitzvos Ha’Katzar (the concise book of mitzvos)
By the Chofetz Chaim
This book lists the Torah mitzvos that can be observed today
First Section – the Positive Commandments
13. It is a positive commandment to say Grace after Meals after eating bread.
As it says in Devarim (Deuteronomy) 8:10, “And you shall eat and you shall be satisfied, and you shall bless Hashem, your G-d.” According to the Torah Law, you are only required to say Grace if you have eaten until you are full. According to Rabbinic law, even if you only ate a kazi’as (olive amount) of bread, you are obligated to say Grace after Meals. Before eating you shall bless, as we learn in a Kal V’chomer (from less to more). If one makes a blessing when he is full, of course he should also make a blessing when he is hungry.
This commandment is obligated at all times and in all places, for men. For women, it is in doubt if this is a Torah obligation or not.