Why unit price beats package price
The unit price is simply the package price divided by how much is in the package. It is the only number that lets you compare a 18-ounce box against a 32-ounce bag, or a four-pack against a twelve-pack, without guessing. Shelf tags in most states already carry a unit price, but the units are inconsistent (per ounce here, per pound there, per 100 sheets somewhere else), and they are not updated for coupons or promotions, which is where a quick calculation pays off.
Worked example with the defaults: item A costs $4.99 for 18 ounces, which is 4.99 / 18 = $0.2772 per ounce. Item B costs $7.49 for 32 ounces, which is 7.49 / 32 = $0.2341 per ounce. Item B is the better value: it costs 15.6 percent less per ounce, and buying 32 ounces at A's rate would have cost $8.87 instead of $7.49. Converted to metric, that is $0.978 per 100 grams for A against $0.826 for B.
Weight, volume and count are different things
A fluid ounce is a measure of volume, about 29.57 milliliters, while an ounce is a measure of weight, 28.35 grams. They are not interchangeable, and a package sold by weight cannot be compared with one sold by volume unless you know the density. Ice cream, chips and cereal are the usual traps: US ice cream is sold by volume, so a half gallon and a 48-ounce carton are different products from a pricing perspective. This calculator groups items into weight, volume and count families and compares only inside a family.
For counted goods (paper towels, capsules, razor blades, diapers) the right unit is often not the package count but the usable amount. Paper towel prices are best compared per 100 square feet, not per roll, because roll sizes differ. Supplements are best compared per serving or per milligram of active ingredient, since capsule strengths vary. Convert to the unit you actually consume before comparing.
When bigger is not cheaper
The assumption that the larger size always costs less per unit is wrong often enough to be worth checking. Retailers price club packs on the expectation that shoppers do not verify, and promotions on small sizes routinely beat everyday pricing on large ones. Surveys of supermarket shelves regularly find that roughly one item in four has a cheaper unit price in a smaller package. Sale prices, digital coupons and store brands all change the ranking week to week.
Bigger is also worse when you cannot use it in time. A 64-ounce bottle at 10 percent less per ounce is a false economy if you throw a quarter of it away, so compute the effective cost per ounce actually consumed. Storage space, freshness after opening and expiration dates belong in the decision alongside the arithmetic. Two more practical habits: enter the after-coupon price rather than the shelf price, and watch for shrinkflation, where the package looks the same but the net weight has dropped and the unit price has quietly risen.