Convert every package in the category to one unit — fluid ounces for liquids, ounces or pounds for weight, plain count for counted goods — then divide the shelf price by that number. Compare those two figures and nothing else. "Loads," "uses," "sheets" and "rolls" are selling quantities, not measurement units. NIST's Unit Pricing Guide is blunt about it: unit prices "must be based on legal measurement units such as those for declaring a packaged quantity or net content as found in the Fair Packaging and Labeling Act," and "the terms 'loads,' 'uses,' and 'servings' are not acceptable units of measure."

The unit a fair comparison is allowed to use

NIST Special Publication 1181, the federal unit pricing guide, sets out the permissible units in its Table 1. Liquid volume gets gallon, quart, pint or fluid ounce. Weight gets ounce or pound. Count gets piece, count or each. Area gets square foot or 100 square feet.

The guide also handles the terms printed in giant type on household packaging: "Other terms such as sheets, rolls, bags, napkins, and tablets should be used only to supplement unit pricing information and not in lieu of it, as they are not units of measure."

One more rule saves you from the commonest shelf-tag trap. Every size in a category is supposed to carry the same unit: "The declaration of the unit price of a particular category of product in all package sizes offered for sale in a retail or online establishment should be uniformly and consistently expressed in the same unit of measure." When one tag reads price per quart and the tag beside it reads price per fluid ounce, the store has broken that rule. The two numbers are not comparable until you redo them yourself.

A worked example: three sizes of one detergent

Seventh Generation's Free & Clear liquid detergent page lists three sizes: 45 fl oz at 33 loads, 90 fl oz at 66 loads, and 135 fl oz at 99 loads. Divide each one. 45 ÷ 33, 90 ÷ 66 and 135 ÷ 99 all come to 1.36 fl oz per stated load. The load count is the volume restated, so it adds nothing, and price per fluid ounce is the whole comparison.

Say your store shows $13.49 for the 90 fl oz bottle and $18.99 for the 135 fl oz bottle. That is 15.0 cents per fluid ounce against 14.1 cents. The big bottle wins by about nine-tenths of a cent per ounce, which is $1.24 across the whole bottle — real, and far smaller than the jump in package size suggests.

Now check the dose, because the dose is what turns ounces into laundry. The same page puts line 1 of the cap at 1.5 fl oz for a medium load and line 2 at 2.2 fl oz for heavily soiled or larger loads. Fill to line 1 every time and 135 fl oz yields 90 doses, nine short of the 99 loads on the front label. Fill to line 2 and it yields 61. At 14.1 cents per ounce, a line-2 load costs 31 cents and a line-1 load costs 21 cents. That spread beats any bottle-size decision on the shelf.

Counted goods need an equivalence, not a roll count

For paper products, the package count is the number designed to mislead. Charmin's Ultra Soft page publishes the equivalence chart that makes a comparison possible: one Mega Roll equals four Charmin regular rolls, one Mega XL equals six, one Mega XXL equals eight, and one Forever Roll equals 32.

So a 12-pack of Mega Rolls is 48 regular-roll equivalents, and a 6-pack of Mega XXL is 48 as well. Those are the same purchase. Convert to regular-roll equivalents or, better, to total square feet or total sheets, then divide the price. Do it once for the brand you buy and write the multiplier on your replenishment list so you never redo it in the aisle.

Trash bags need the same discipline. NIST notes that products "sold in terms of count, dimensions, thickness, and capacity" force you to "choose the unit that would be the most helpful and meaningful to the consumer." For bags that is usually price per bag at a fixed gallon capacity and mil thickness. A 13-gallon 0.9 mil bag and a 13-gallon 1.1 mil bag are different products, not different prices.

Where the bigger package stops being the better buy

Three conditions reverse the default, and all three are checkable before you reach the register.

  1. The unit price is flat or higher. This happens often on a promoted mid-size. Once you have cents per fluid ounce for both, the answer is arithmetic.
  2. It will not fit or will not keep. A 135 fl oz detergent bottle is tall on a shelf and heavy to tip when full, so measure the space first. Check the space before the price, the same way you would compare a container's dimensions against its stated capacity.
  3. It is a variety or combination pack. NIST exempts these from unit pricing entirely — a variety package holds "two or more individual packages or units of similar but not identical commodities." Stores may skip the unit price on those tags, so you build it yourself from the net contents.

Before the next trip, write down the unit you are using for each category you buy in bulk: fluid ounces for detergent, regular-roll equivalents for toilet paper, sheets for paper towels. Add the best unit price you have seen, so you have a benchmark instead of a feeling. Add the dose or size you actually use, because that is what turns a unit price into a cost per wash or per wipe. Three numbers, one line each. They turn a shelf full of competing package sizes into a two-second comparison. Then check them against the stock you already have, the way you would take a pantry inventory before writing a shopping list.

Sources and references