Comparing prices across different units
Bulk comparisons go wrong at the units far more often than at the arithmetic. A 12 fl oz bottle at $4.99 and a 1 quart bottle at $7.49 cannot be compared until both are in the same unit. A quart is 32 fluid ounces, so the small bottle is 499 cents over 12, or 41.6 cents per fluid ounce, and the large one is 749 cents over 32, or 23.4 cents. The large bottle is 44 percent cheaper per ounce.
Set the comparison unit at the top and enter each pack in whatever unit is printed on it. Volume converts within volume and weight within weight; a pack entered in grams cannot be compared against one entered in fluid ounces, and that option is dropped with a note rather than silently converted, because density is not something a calculator should guess.
| Unit | In fluid ounces | Unit | In ounces (weight) |
|---|---|---|---|
| 1 cup | 8 | 1 lb | 16 |
| 1 pint | 16 | 1 kg | 35.27 |
| 1 quart | 32 | 100 g | 3.53 |
| 1 gallon | 128 | 1 oz | 1 |
| 1 litre | 33.81 | 1 g | 0.0353 |
The shelf price and the price you paid
The shelf tag divides price by package size. That number is correct and it answers a question about the package. The question you have is about your household, and it needs one more input: how fast you get through the stuff.
At 24 fluid ounces a month, a quart lasts about six weeks and a gallon lasts five and a third months. Both finish comfortably inside a twelve-month life, nothing is wasted, and the true unit price equals the shelf price. Now drop usage to 4 ounces a month. The gallon needs 32 months to empty, the product is finished at 12, and two-thirds of it goes in the bin — the shelf price is still 18 cents an ounce and you paid 54.
That is the whole mechanism. Waste does not lower what you paid; it lowers what you got. True unit price is price divided by what you actually used, and it is the only figure that compares packs honestly.
Carrying cost is real
Even where nothing expires, holding a year of supply is not free. It occupies space that had another use. It ties up money that could have stayed in your account. It commits you to a brand and a formulation before you know whether you will still want them. And a large opened container is exposed to air, light and temperature for the whole time it takes to empty, so its practical life is often shorter than the printed one.
The storage option puts a crude price on the first of those: name what the space the largest pack occupies is worth per month, and each option is charged in proportion to its size for half the time it sits there, on the reasoning that a container empties gradually. It is a model rather than a measurement, and its job is to stop the comparison pretending the space is free.
Bigger packs get used faster
The second checkbox handles a well-observed effect: give people a larger container and they use more per occasion. Detergent gets poured rather than measured, snacks are eaten from the bag, shampoo comes out in a longer squeeze. The unit price still falls, but the monthly outlay can rise, which means the household spends more while every individual purchase looks like a saving.
Fifteen percent is the default adjustment and it is a guess. The reason to switch it on is not that the number is right but that assuming no effect at all is definitely wrong for anything dispensed by eye. If you measure doses, leave it off.
Reading the sensitivity table
The table re-runs everything at half, three-quarters, one and a half and double your stated usage. It is there because usage estimates are the least reliable input on the page and they are biased in a known direction: people overstate how much of something they get through. If the winner changes between the half-usage row and the current row, the result is resting on that estimate, and the useful next step is to go and look at when you last bought one.
What to check on the label before trusting any of this
Find the once-opened life, not the sealed one. Many products carry both, and only the second clock is running on the container in your cupboard. Cosmetics and household chemicals often print a period-after-opening figure in months; food usually gives a date that assumes the package stays closed. Where a label offers both, the shorter number belongs in the usable-life field, and entering the longer one is precisely how a bulk purchase gets talked into looking sensible.
Questions people ask
How do I compare a bottle in fluid ounces with one in quarts?
Set the comparison unit to fluid ounces and enter each pack in its own unit. A quart converts to 32 fluid ounces, so $7.49 a quart is 23.4 cents an ounce against 41.6 cents for $4.99 per 12 ounces.
Is the bigger pack always cheaper per unit?
Per unit on the shelf, usually. Per unit you actually use, only if you finish it in time. Once a share of the pack expires unused, the price you paid per usable ounce rises in exact proportion to the share wasted, and it can pass the small pack easily.
Why can I not compare grams against fluid ounces?
One measures mass and the other volume, and converting between them needs the density of that specific product. Rather than guess, the mismatched option is dropped from the comparison. Weigh the pack or find its net weight if you want both in the same family.
What usable life should I enter for something that does not spoil?
Enter 999 and the shelf-life limit stops binding. Screws, batteries and paper towels behave this way. Anything with a once-opened figure on the label should use that figure instead, because it is the clock that runs on the container you are using.