Estimate vacuum bags needed to portion your food.
Bag count follows straight from total food divided by portion size, but roll consumption has a detail that catches people out: each bag needs roughly 3 cm more film than the food occupies, to give clean material for the bottom and top seals. On short bags that overhead is over 10 percent of the film, which is why a roll yields noticeably fewer bags than dividing its length by the bag length suggests. Sizing portions to how you will actually cook also matters — vacuum bags cannot be resealed once opened, so a portion that is too large forces you to thaw more than you need.
Bags needed
Bags = ceiling(total food g / portion g)
Roll yield
Bags per roll = floor(roll length cm / (bag length + 3 cm seal allowance))
A vacuum sealer needs clean flat film at both ends: one seal at the bottom and one across the top after the air is drawn out. About 3 cm total covers both. Omitting it means the last few bags on a roll come out short and the seal runs too close to the food to hold.
By how you cook, since a vacuum bag cannot be resealed. Portion to a single meal for your household so nothing is thawed unnecessarily. Around 400 grams suits two to three people for a main protein; 150 to 200 grams suits single servings.