Unit Price Calculator
Three sizes, one comparison
Your result will appear here
Fill in the fields on the left and this updates as you type.
Know what this estimate is based on
- Jurisdiction
- General mathematical model
- Scope and limitations
- Planning indicator only. It does not assess every part of a household's finances or replace individualized professional advice.
- Source links checked
- Jul 30, 2026
Built and regression-tested by Smart Tools Lab. It has not been individually reviewed by a licensed financial, tax, or legal professional.
How to use
- 01
Enter Option A: the shelf price, the size printed on the label, and how many items are in the pack. For a single 12 oz can that is $4.29, 12 and 1 — the size box takes ONE item, never the pack total.
- 02
Enter Option B the same way. A three-pack of the same 12 oz cans is 12 in the size box and 3 in the pack box, which the page multiplies to 36 units — this is the step that goes wrong most often.
- 03
Enter the share of the biggest pack you honestly expect to throw out or never use. It is the field that changes answers rather than decorating them, and it charges the waste to the largest pack because that is the one that spoils.
- 04
Open Advanced options for a third option if the shelf offers one, entered as the same price / size / pack trio.
- 05
Also in Advanced options, set the units in one label size: 1 to read ounces straight off the label, 16 to read pounds as ounces, 128 for gallons as fluid ounces, 32 for quarts. It scales all three options together, so convert any oddly-labelled package by hand before you type it.
Formula
Total units in an option = the size on the label × the items in the pack × the units in one label size. That last factor is a scale, not a name: 1 reads ounces straight off the label, 16 reads pounds as ounces, 128 reads gallons as fluid ounces, 32 reads quarts. Price per unit = shelf price ÷ total units. The cheapest option is simply the lowest of those, and the page also reports it per 100 units, because per-ounce prices in tenths of a cent are unreadable side by side. How much cheaper the winner is = (dearest per unit − cheapest per unit) ÷ dearest per unit. The cash difference at the till = the biggest pack's price − the smallest pack's price. A per-unit win and a cash win are different things. Waste is charged to the biggest pack: usable units = its total units × (1 − waste %), and its effective price per unit = its price ÷ those usable units. When that effective figure rises above another option's, the waste has reversed the answer.
Example
Option A is a single can: $4.29 for 12 ounces, so 12 units at $0.358 each. Option B is a three-pack of the same can: $11.49, 12 on the label, 3 in the pack — 36 units at $0.319 each. Option B wins by 10.7%, works out at $31.92 per 100 ounces, and across the 36 ounces in the pack the difference is worth $1.38. It also costs $7.20 more at the till today, which is the part a per-unit figure never shows. Now set the waste field to 25%: 9 of the 36 ounces go in the bin, the usable quantity falls to 27, and Option B's real cost rises to $0.426 a usable ounce — above Option A's $0.358. The bigger pack was 10.7% cheaper and is now 19% more expensive, and $2.87 of it was thrown away. That reversal is the finding the page exists for.
Definitions
- Unit price
- Shelf price divided by the total quantity in the package — the only figure that makes two different sizes comparable. Everything else on this page is built from it.
- Total units
- The size on one item's label multiplied by the number of items in the pack, then scaled into the unit you want the answer in. Forgetting the pack count is the most common way a comparison goes wrong.
- Units per label size
- A scale rather than a unit name: 1 for ounces, 16 to read pounds as ounces, 128 for gallons as fluid ounces, 32 for quarts. It applies to all three options at once and cannot convert between two differently-labelled packages.
- Effective unit price
- Price divided by the quantity you actually USE, after waste. The honest per-unit figure for anything perishable, and frequently a different ranking from the shelf one.
- Shrinkflation
- A package size reduced while the price holds. Invisible to a price comparison and immediately visible to a per-unit one, which is the whole argument for doing the arithmetic.
Good to know
The unit is the whole job
Every comparison on this page reduces to one division — price divided by quantity — and every error on it happens in the quantity. Three things go into that figure: the size printed on one item's label, how many of those items are in the pack, and the scale that turns the label's unit into the unit you want the answer in. The pack multiplication is where most comparisons die. A twelve-pack of 12 oz cans is 144 ounces, not 12, and entering it as 12 makes a good deal look twelve times worse than it is. A case of six 1-litre bottles is six times the label. A paper-towel pack marked 'six mega rolls equals fifteen regular' is neither six nor fifteen of anything measurable, so use the square footage or the sheet count printed on the side — that is the only number on the package that means the same thing across brands. The scale field handles the rest: 1 reads ounces straight off the label, 16 reads pounds as ounces, 128 reads gallons as fluid ounces, 32 reads quarts. What it cannot do is convert BETWEEN two labels, because it applies to all three options at once. A 2 lb bag typed next to a 24 oz box is out by a factor of 16 and will be ranked confidently wrong. Convert the odd one out by hand — enter the 2 lb bag as 32 — and the arithmetic is safe. Shelves mix units more often than is comfortable, and not always by accident.
The bigger box is often not cheaper
The assumption that buying big is buying cheap is wrong often enough to be worth checking every single time, and there are four reliable reasons why. Promotional pricing lands on small sizes as readily as on large ones, so a single unit on a two-for deal routinely beats the club size that week. Multipacks are frequently priced for convenience rather than value — bottled water, snack packs, single-serve coffee — because the buyer is paying for portioning, not for volume. Package sizes shrink while prices hold, so a per-unit figure that was true last year is not true now, and nothing on the front of the box announces it: a cereal box moving from 18 oz to 16.5 oz at an unchanged $4.99 is 8.3% less cereal and a 9.1% rise in the price of an ounce of it, $0.277 becoming $0.302. And 'family size' or 'value size' are marketing names with no arithmetic behind them; they describe the box, not the price. The page's own default case shows the ordinary version of the trade-off rather than the trap: a $4.29 single 12 oz can at $0.358 an ounce against an $11.49 three-pack at $0.319 — the bigger pack does win, by 10.7%, and it also costs $7.20 more at the till today. Those are two different questions, and a per-unit figure only answers the first.
A per-unit win is not a win if you throw it away
The most common way a correct unit-price calculation still loses money is waste, and it is a bigger factor than any shelf discount. The honest per-unit price is the price divided by what you actually USE, not by what you carried to the car — so a pack with a quarter of it going in the bin costs a third more per usable unit than the tag says, because the price is being spread over 75% of the quantity. Run it on the page's own numbers: the $11.49 three-pack is $0.319 an ounce on the shelf and, at 25% waste, $0.426 a usable ounce. That is worse than the small can's $0.358, so the waste does not merely narrow the margin, it reverses the answer — and $2.87 of that pack is thrown away. USDA and EPA estimates put US household food waste somewhere between a fifth and a third of what is bought, and it is concentrated in exactly the categories bulk pricing pushes hardest: produce, dairy and bread. The practical rule is to enter what you honestly expect to bin rather than what you hope to, and to notice that the answer is asymmetric. Waste can only ever make the biggest pack look worse, never better, which means every bulk decision has a downside the shelf tag structurally cannot show.
What the shelf tag can and cannot tell you
Many stores print a unit price on the shelf tag, and where it exists it is a useful cross-check — but it is not a substitute for the arithmetic, for three reasons. Unit-price labelling is state and local law rather than federal, so some jurisdictions require it and many do not; a chain operating across state lines may show it on one shelf and not on another. Where it is required, two items on the same shelf can legitimately be tagged in DIFFERENT units — one per ounce, one per 100 count, one per pound — which makes them non-comparable at a glance in exactly the way the tag is supposed to prevent. And the tag is often the last thing updated when a package changes size or goes on promotion, so it can be quietly stale on precisely the item you are considering. Beyond the tag, remember the two costs a per-unit figure leaves out entirely. The bigger pack ties up more cash today — $7.20 more in the default comparison — and it needs somewhere to live, which is a real constraint in a small kitchen and the reason bulk buying tends to end at the point where storage runs out. A cheaper unit price bought twice as often is not a saving, and neither is a pantry full of things nobody opened.
Frequently asked questions
How do I compare two packages of different sizes?
Normalise both onto one per-unit figure: shelf price divided by total units, where total units is the label size multiplied by the number of items in the pack. A $4.29 single 12 oz can is $0.358 an ounce; an $11.49 three-pack of the same can is 36 ounces and $0.319 an ounce, so the three-pack is 10.7% cheaper per ounce. Per-unit prices in cents are hard to read side by side, so the page also shows the winner per 100 units — $31.92 per 100 ounces here — which is the same number in a form you can compare at a glance.
Is the bigger box always cheaper per unit?
No, and it is wrong often enough to be worth checking every single time. Promotional pricing on a small size, a multipack priced for convenience rather than for value, and package sizes that shrink while the price holds all break the assumption, and the shelf tag is exactly where that hides. When the bigger pack does win, it wins on a per-unit basis and loses on cash today: the three-pack above is cheaper per ounce and $7.20 more at the till.
How do I handle a multipack?
Put the size of ONE item in the size box and the count in the pack box, and let the page multiply. A twelve-pack of 12 oz cans is 144 ounces, not 12. A case of six 1-litre bottles is six times the label. A paper-towel pack marked 'six mega rolls equals fifteen regular' is neither six nor fifteen of anything measurable — use the square footage or the sheet count printed on the side, because that is the only figure on the package that means the same thing across brands.
One label is in pounds and the other in ounces. What do I do?
Convert one by hand before you type it. The units field scales all three options together, so it cannot resolve a comparison between two DIFFERENT labels: a 2 lb bag entered next to a 24 oz box is out by a factor of 16 and the page will confidently rank them wrong. Enter the 2 lb bag as 32 and leave the units field at 1. The conversions worth memorising are the four the field is built around — a pound is 16 ounces, a gallon is 128 fluid ounces, a quart is 32.
Why does the page ask what I will throw away?
Because a per-unit win is not a win if you bin a quarter of it. Waste is charged against the biggest pack, and the effect is larger than people expect: at 25% thrown out, the $11.49 three-pack costs $0.426 a usable ounce rather than $0.319 — which is worse than the small can's $0.358, so the waste reverses the answer entirely and $2.87 of that pack goes in the bin. USDA and EPA estimates put US household food waste somewhere between a fifth and a third of what is bought, concentrated in exactly what bulk pricing pushes hardest: produce, dairy and bread.
Can I just read the unit price on the shelf tag?
Where it exists, it is a useful cross-check — but do not rely on it. Unit-price labelling is state and local law rather than federal, so some jurisdictions require it and many do not, and where it is required two items on the same shelf can legitimately be tagged in different units, one per ounce and one per 100 count. It is also the last thing updated when a package shrinks. Treat it as a second opinion on the arithmetic, not a substitute for it.
What is shrinkflation and how does it show up here?
A package that gets smaller while the price stays the same. It is invisible to anybody comparing prices and obvious to anybody comparing per-unit figures, which is precisely why this arithmetic is worth doing at all. A cereal box going from 18 oz to 16.5 oz at an unchanged $4.99 is 8.3% less cereal and a 9.1% rise in what an ounce of it costs — $0.277 an ounce becoming $0.302 — and nothing on the front of the box says so. The size on the label is the number that moves; the price is the number designed not to.
Which unit should I compare in?
Whichever one both packages already use, and if they use different ones, the smaller. Weight for solids, fluid ounces for liquids, sheets or square feet for paper, count for anything sold by the item. The absolute unit does not matter to the ranking — dividing every option by the same constant cannot reorder them — so the only rule that matters is that all three boxes are read in the SAME unit.