Unit Price Calculator
Shelf tags make the package price loud and the per-unit price small, which is exactly backwards for deciding. Enter one package's price and quantity for its unit price — or fill in both options and the calculator names the cheaper one and says by how much per unit.
Compare two package sizes
Example: 3.89 for 24 oz vs 5.49 for 36 oz — the larger box is 5.91% cheaper per ounce.
For the percentage arithmetic behind the coupons and price changes that quietly reshape a unit-price comparison, see the companion guide, Commission and discount arithmetic; the full set of method guides sits on the guides hub.
One division that stores hope you skip
Unit price = package price ÷ quantity. A 4.5 six-pack is 0.75 per can; 3.89 for 24 oz is 0.1621 per ounce while 5.49 for 36 oz is 0.1525 — making the larger box 5.91% cheaper per ounce. Those figures are computed at build time by the same engine as the form, so the worked example is guaranteed to match what the calculator would tell you.
That is the whole of the single-option mode: one price divided by one quantity, with the price allowed to be zero or more — a free sample is a valid entry — and the quantity required to be greater than zero, since dividing by no quantity has no defined meaning. The engine carries out the division at full precision and hands the raw figure to the display layer; the four decimal places you read back are a presentation choice, not a rounding the arithmetic itself performed.
What the comparison actually reports
Fill in both options and the calculator does more than divide twice. It computes each unit price, decides which is smaller, and expresses the gap as a percentage of the dearer option — so "5.91% cheaper per unit" means every ounce bought at the better price costs 5.91% less than the same ounce would at the other. Measuring against the more expensive option is a deliberate convention: it answers the shopper's real question — how much do I avoid paying by switching — rather than the inflated figure you would get by dividing the gap the other way.
The engine also refuses to invent a winner where there is none. When the two unit prices land exactly equal — 2 for 8 against 3 for 12, both 0.25 apiece — it reports a tie and a saving of 0%, instead of splitting a dead heat on floating-point dust. A genuinely equal pair stays equal.
Because the saving is always taken against the dearer of the two, which option you type into the first set of fields and which into the second makes no difference to the answer: the same 3.89-for-24 and 5.49-for-36 pair returns the larger box as cheaper by 5.91% whichever order they are entered. That symmetry is what lets you work down a long shelf two packages at a time — keep the current cheapest, bring in the next contender, and the pair you compare never has to be in any particular order for the surviving winner to be the true one.
The same-units rule
The comparison is only as good as its denominators. Quantities must be in the same unit before dividing: a pound jar compared against an ounce jar needs the pound converted to 16 oz first, and "per 100g" shelf labels need both packages on the 100g basis. When labels disagree, convert the quantity — never the computed unit price — and re-run the division.
The failure is worth seeing in numbers. Take a 5.6 jar holding one pound and a 3.6 jar holding 12 oz. Entered honestly, the pound becomes 16 oz, so the first is 0.35 per ounce and the second 0.3 per ounce — the 12 oz jar is the cheaper of the two by 14.29%. Leave the first jar labelled "1" for one pound and the division instead returns 5.6 "per unit," a figure sixteen times larger that makes the smaller jar look like a landslide bargain it is not. The engine has no way to know a "1" meant a pound; it divides exactly what you type, which is why aligning the units is your job and not the calculator's.
When the bigger package costs more per unit
The reflex that bulk is always cheaper is the assumption the calculator exists to test. A store can price the larger size at a higher unit cost — a quantity surcharge — trusting that the bigger number on the shelf reads as the better deal. Compare a 4 box of 16 oz against a 9.5 box of 32 oz: the small box works out to 0.25 per ounce and the large one to 0.2969, so here the smaller package wins by 15.79% per ounce. Nothing about doubling the contents guaranteed a lower rate, and the only way to know is to run the division.
The same test applies to countable items, not just weights and volumes. A 18 bottle of 200 tablets is 0.09 per tablet; a 39 bottle of 500 is 0.078 per tablet — so the larger bottle is 13.33% cheaper per tablet. This time the bulk is rewarded, but only because the arithmetic said so, not because it was bigger.
Comparing after a coupon
A discount changes the effective price of one option and can flip the ranking outright, so the comparison belongs after the coupon, never before. Return to the 3.89 box of 24 oz and the 5.49 box of 36 oz, where the larger box started ahead at 0.1525 against 0.1621 per ounce. Put a 20% coupon on the smaller box and its price falls to 3.11 — a saving of 0.78 — which is 0.1297 per ounce once divided. The 24 oz box is now the cheaper of the two by 14.97% per ounce: the winner has changed hands on the strength of a single coupon. Apply any reduction to the affected package's price first, then let the calculator divide.
Why a stacked "20% then 15%" coupon is not 35% off, how a cut and a later rise fail to cancel, and the rest of the percentage arithmetic that governs these reductions are worked through in the companion guide, Commission and discount arithmetic. The discount calculator will compute the reduced price for you before you bring it back here to compare.
When cheap per unit isn't cheap
Unit price measures cost, not value: the giant tub only wins if you finish it before it expires, and a promotion can flip the ranking overnight — apply any discount to the affected package first, then compare. If a package shrank while its price held (the quiet trick sometimes called shrinkflation), the price change calculator can quantify the effective increase per unit you're now paying.
Close contests and the digits on screen
When two options nearly match, the decimals decide the winner, and too few of them hide it. A 3.29 pack of 20 is 0.1645 per unit and a 4.89 pack of 30 is 0.163 — rounded to the cents on a shelf tag both look identical, yet the second is 0.91% cheaper per unit. Unit prices here display with up to four decimal places precisely so a contest this tight stays visible instead of collapsing into a tie that is not real.
That displayed figure is a clean presentation of a more precise value the engine holds internally: it rounds nothing between steps and normalises away binary floating-point artefacts before showing a result. If you copy a per-unit figure into further calculations by hand, remember you are copying the rounded display, not the full-precision number — the methodology page sets out exactly how that rounding works.
Where the unit price stops
A unit price is a general mathematical definition — price divided by quantity — and it knows nothing beyond the two numbers you give it. It cannot judge quality or freshness, how much of a perishable you will use before it spoils, the storage the larger size demands, or a membership price that applies to only one option. It does not read the units for you, convert between them, or know that a "1" meant a pound. Inputs are held to a documented range so the arithmetic stays dependable for everyday figures, and a quantity of zero is refused rather than answered with an infinity. The calculator tells you which package is cheaper per unit; whether cheaper per unit is the right basis for this particular purchase is a judgment it leaves to you.
Frequently asked questions
What is a unit price?
The price divided by the quantity: a 4.5 six-pack costs 0.75 per can. Reducing every package to the same denominator is the only fair way to compare a 24 oz jar against a 36 oz one.
What if the two packages use different units?
Convert one of them first — comparing a per-ounce price against a per-pound price makes the smaller unit look sixteen times cheaper than it is. Put both quantities in the same unit (all ounces, all grams, all count) and the comparison becomes honest.
Is the bigger package always the better deal?
Usually, but not reliably — stores sometimes price the large size at a higher unit cost (a "quantity surcharge"), betting shoppers assume bulk is cheaper. In the example above, the larger box does win: 0.1525 against 0.1621 per ounce. The calculator checks instead of assuming.
Should I compare before or after a coupon?
After — a discount changes the effective price of one option and can flip the winner. Apply the coupon or promotion to that package's price first, then compare the resulting unit prices.
What does the savings percentage refer to?
The gap between the two unit prices, measured against the more expensive one. "7% cheaper per unit" means every ounce or item bought at the better price costs 7% less than it would have at the other.
What happens when two packages have the same unit price?
The engine reports a tie rather than guessing. Two options at 2 for 8 and 3 for 12 are both 0.25 per unit, so it returns a saving of 0% and names no winner — a genuinely equal pair stays equal instead of being split by a rounding artefact.
Why is the saving measured against the more expensive option?
Because that answers the question a shopper is actually asking — how much do I avoid paying by choosing the cheaper one. In the 3.89-versus-5.49 example the gap is 5.91% of the dearer unit price. Dividing the same gap by the cheaper price instead would produce a larger, flattering number that describes the wrong direction of the trade.
Can I compare three or more packages at once?
The engine compares two options at a time. For a longer shelf, find the unit price of each, then pit the current cheapest against the next contender — the winner of each pair carries forward. Because the comparison only ever needs the two unit prices, the order you test them in never changes which package finishes cheapest.
Comparisons happen entirely on your device; nothing is uploaded. Unit prices display with up to four decimal places so close contests stay visible — rounding rules are on the methodology page.