Unit Price Calculator
Shelf labels give a unit price in theory. In practice one is per ounce, the next is per 100 sheets, and the sale sticker covers both. This puts two options on the same footing: enter what each one costs and how much you get, and it tells you which is cheaper and by how much.
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Formula
Bigger is usually cheaper, and sometimes it is not
The assumption that the family size wins is right often enough to be dangerous. Retailers know shoppers believe it, and the large pack is priced against that belief rather than against its cost. Promotions distort it further: a half-price small tin routinely beats the bulk one, and the shelf label showing unit price is calculated from the regular price, not the offer.
The other reliable reversal is the small convenience size, which is never cheaper per unit and is not meant to be. You are paying for portability, and that is a fair trade as long as you know you are making it.
Match the units before you trust the answer
Most mistakes here are unit mistakes, not arithmetic. A label priced per 100 grams sits next to one priced per kilo; toilet paper is sold by rolls that contain different numbers of sheets of different sizes. Convert both sides to the same thing first — the calculator above cannot tell that you typed ounces on one line and pounds on the other, and it will confidently give you a wrong answer if you do.
The saving is only real if you use it
A cheaper unit price on something perishable is a saving on paper and a loss in the bin. Bulk salad, bread and dairy are the classic examples: households routinely throw away a fifth of what they buy, which is far larger than any unit-price advantage. The yearly usage field above exists for this reason. If you get through less than the pack holds before it turns, the small one is cheaper in the only sense that matters.
Storage and cash have a price too
Buying a year of something ties up money now to save a few percent later, and it takes up space you may not have. For a household on a tight month, the small pack can be the correct choice even knowing it costs more per unit. Bulk buying is a strategy for people with slack — cash slack and shelf slack — and it is worth being honest about whether you have either.
Where the gains are actually large
Unit-price differences of two or three percent are noise. Look for the ones in the tens of percent, and they cluster in predictable places: cleaning products, pet food, dry staples, batteries, and anything sold in a single-serve format. Comparing every item is exhausting and pointless; comparing the ten things you buy most often takes twenty minutes once and keeps paying.
Frequently asked questions
How do I work out price per unit?
Divide the price by the quantity. A $4.99 pack of 12 is $0.416 each. Keep three decimal places, because rounding to cents hides real differences on cheap items.
Can I compare different units, like ounces and grams?
Not directly. Convert one side first, or the comparison is meaningless. One ounce is 28.35 grams and one pound is 453.6 grams.
Why does the shelf label disagree with this?
Shelf unit prices are normally calculated from the regular price and are not always updated for a promotion, and some stores round hard. Your own arithmetic on the sticker price is more reliable.
Is the bigger pack always better value?
No. It is more often than not, but promotions, convenience formats and loyalty pricing reverse it regularly enough that checking is worth the ten seconds.
Should I count waste?
Yes, on anything that spoils. Effective unit price is what you paid divided by what you actually used, and for fresh food that can be twenty or thirty percent higher than the label.