We have explained previously the amount of Matzah and Maror every individual is obligated to eat on the Seder night. Some say this amount must be determined by volume of the Matzah, not its weight. (For instance, by inserting Matzah into a baby bottle until it reaches the twenty-seven-milliliter mark, which would indicate twenty-seven grams of volume.) Indeed, this is the correct way to measure the amounts of Matzah, Maror, and any other food for that matter based on volume and not weight, for this is the method of measuring handed down to us by our Sages throughout the entire Talmud.
Nevertheless, the great Rishon Le’Zion Hagaon Rabbeinu Yitzchak Yosef Shlit”a writes (in his Yalkut Yosef- Pesach, Chapter 486) that already from the times of the Geonim (over one-thousand years ago), people were instructed to measure by weight since there is not usually that great of a distinction between a given food’s volume and weight, among other reasons. Similarly, Maran Rabbeinu Ovadia Yosef zt”l replied to us ten years ago when we asked him this question that although in theory one should measure by volume, nevertheless, since this is difficult to do, it is already the prevalent custom to measure by weight. Thus, whether with regards to hard Matzah, soft Matzah, or Maror (romaine lettuce) eaten on the Seder night, one should measure these amounts by weight. (A weight of twenty-seven grams of Matzah or lettuce certainly contains a twenty-seven-gram volume within it since both hard and soft Matzah as well as lettuce are lighter than water. One who wishes to act stringently should take approximately thirty grams of soft Matzah.)
The question therefore becomes: Is it permissible to keep a mechanical (non-digital) food scale next to the table on the Seder night to measure the appropriate amounts of Matzah and Maror for all those present?
Indeed, our Sages forbade measuring or weighing anything on Shabbat or Yom Tov, for these acts resemble acts resemble weekday actions of merchants conducting business. It is therefore forbidden for one to weigh a piece of cake on Shabbat in order to ascertain how much it weighs. It would seem then that it should be forbidden to measure or weigh amounts of Matzah or Maror for the Seder.
Nevertheless, our Sages (Shabbat 126b) taught that it is permissible to measure things in the context of a Mitzvah on Shabbat. For instance, one may measure a Mikveh on Shabbat in order to ascertain whether or not it is lacking water. The Poskim therefore write that it is permissible to weigh Matzah and Maror on Yom Tov or Shabbat (when the Seder night coincides with Shabbat). Maran zt”l rules likewise in his Chazon Ovadia- Shabbat, Volume 6, page 26. Needless to say, this applies only to a mechanical scale, not a digital one.
Summary: The Kezayit of Matzah and Maror for the Seder night should be measured by the weight of these foods. There is no need to measure based on volume. It is permissible to weigh the Matzah and Maror with a mechanical scale during the Seder.