Compare Products, Verify Unit Prices, and Detect Shrinkflation
Compare two or more product offers on a common per-unit basis, verify a retailer's shelf-tag unit price against the arithmetic, or measure shrinkflation between an old and new pack size.
All calculations run in your browser — no offer names or prices are sent back to CalcDomain, and the share link uses a URL fragment so nothing is stored server-side.
Enter the pack details and the retailer's displayed unit price to check whether the arithmetic matches within rounding.
Enter the previous and current pack details to measure the change in unit price.
Nominal vs usable unit price
The nominal unit price divides the effective price by the full package quantity — the figure a retailer typically prints on the shelf tag. The usable unit price divides by the quantity you actually expect to consume before waste, spoilage or expiry. For shelf-stable products the two are equal, so the distinction is invisible. For perishables it is decisive: a bulk pack with a low nominal unit price can carry a much higher usable unit price once realistic waste is subtracted.
The calculator asks for a 'usable percentage' so you can encode this judgement explicitly. Leave it at 100% when comparing tinned goods, cleaning supplies, batteries or anything else that keeps indefinitely. Lower it for fresh produce, dairy, bakery items and any large pack that outstrips your usual consumption. The winner in comparison mode is chosen by the usable unit price first and the nominal unit price only as a tiebreaker, so the answer follows the money you actually spend per unit consumed.
Arithmetic price verification
The verify mode does one thing: it recomputes the expected per-basis unit price from the entered pack details using the same effective-price formula the compare mode uses, then reports whether the retailer's displayed number matches within a small tolerance. A match tolerance of max(0.005, 0.5% of the recalculated price) absorbs rounding at the fourth decimal without hiding meaningful mistakes.
This is an arithmetic check, not a legal audit. It does not certify compliance with any state or federal unit-pricing regulation, does not verify that the displayed shelf price is accurate at checkout, and does not distinguish deliberate mislabelling from rounding. It answers a narrower question: given the price and pack details in front of me, does the printed unit price add up? A 'displayed higher' or 'displayed lower' verdict is a prompt to look more closely or ask the store, not an accusation.
Shrinkflation measurement
Shrinkflation is a package downsize at a constant (or less-than-proportionally-raised) shelf price. Because the anchor shoppers usually notice is the sticker price, the change is easy to miss and the unit price is where it becomes visible. The calculator quantifies it by converting both the old and new packs to a shared base unit (gram for mass, millilitre for volume, matching subtype for counted items) and reporting the percentage change in unit price, quantity and shelf price, plus the extra amount you would now pay to buy the previous quantity.
Two familiar cases: a 500g box that drops to 430g at the same $4.29 shows a roughly 16.3% rise in unit price hidden behind an unchanged sticker; a 12-roll paper towel pack that becomes 10 rolls at the same $9.99 shows a 20% rise in unit price for the same reason. The tool does not judge — a smaller pack at the same price is only shrinkflation if the retailer has replaced the old one, not if it is simply a new SKU sold alongside the original.
Frequently Asked Questions
What is the usable unit price?
The usable unit price is the effective price divided by the usable quantity — the amount of the package you actually expect to consume before it expires or is thrown away. It differs from the nominal unit price, which uses the full package quantity. For non-perishables the two are equal; for perishables, waste raises the true cost per unit.
How do I represent a '2 for $X' promotion?
Set 'Number of packages' to 2 and 'Shelf price' to the bundle total. For example, '2 for $5' with 500g per pack: packageCount=2, shelfPrice=5, amountPerPackage=500.
How do I represent a BOGO (buy-one-get-one-free) deal?
Set 'Number of packages' to 2 and 'Shelf price' to the price of one package. The calculator will divide that price across the total quantity of both packages.
What does 'usable percentage' mean?
The share of the package you realistically use before spoilage, waste or expiry. Leave it at 100% for non-perishables. For fresh produce in a small household it might be 60-80%; for a bulk pack you cannot finish it could be lower still.
What is the comparison basis?
The reference quantity for reporting a per-basis unit price (for example 'per 100g' or 'per fl oz'). It only affects how the unit price is displayed; it does not change which offer wins. The winner is decided by the underlying per-single-unit price.
Can I compare grams and kilograms?
Yes. Mass units (mg, g, kg, oz, lb) are all interconvertible on the gram base, so any combination compares fairly. The same applies to volume units on the millilitre base.
Can I compare grams and millilitres?
No. Mass and volume are incompatible dimensions without a specific product density, which the calculator does not carry. The tool will refuse the comparison and flag the offending offer rather than invent a conversion.
Does this tool certify that a retailer's unit price is legally compliant?
No. The verify mode is an arithmetic check: it recomputes the expected unit price from the entered pack details and reports whether the displayed value matches within a small tolerance. It is not a legal audit and does not certify compliance with any state or federal unit-pricing regulation.
What is shrinkflation?
Shrinkflation is a reduction in the quantity delivered per package while the shelf price stays constant (or does not rise proportionally). The calculator quantifies it as the percentage change in unit price between an old and new pack size on the same base unit.
What does 'matches within rounding tolerance' mean in the price verification?
The verifier flags a match when the difference between the displayed and recalculated unit price is at most max(0.005, 0.5% of the recalculated price). Small deviations at that level are consistent with retailer rounding of the fourth decimal; larger deviations are reported as 'displayed lower' or 'displayed higher'.
Evidence, sources and editorial review
Compare Products, Verify Unit Prices, and Detect Shrinkflation groups its evidence and methodology review here so sources, assumptions and responsibility can be checked together.
Methodology
Every offer is normalised the same way: effective price = max(0, shelf price minus fixed discount) times (1 minus percentage discount / 100) times (1 plus tax percent / 100) plus extra fees; nominal quantity = package count times amount per package converted to the common comparison unit; usable quantity = nominal quantity times usable percent / 100; usable unit price = effective price divided by usable quantity. Mass units (mg, g, kg, oz, lb) convert to a gram base and volume units (ml, cl, L, US fl oz, US quart, US gallon) convert to a millilitre base using published NIST factors; counted-item units (item, sheet, roll, dose, serving) never cross subtypes. The comparison mode picks the lowest usable unit price as the winner (stable tie: lower index wins) and flags a tie when the winner and runner-up are within max(0.005, 0.5% of the winner's unit price). The verify mode recomputes the expected per-basis unit price from the same effective-price formula and reports whether the retailer's displayed value matches within the same tolerance. The shrinkflation mode compares two packs on a shared base unit and reports the quantity change, price change, unit price change, and the extra amount paid today for the previous quantity.
- Model version:
unit-price-truth-check-3; share-link schema version:3. - Mass and volume conversion factors follow NIST Handbook 44.
- Counted-item units never cross subtypes: an item is not a sheet, a sheet is not a roll.
- The verify mode is an arithmetic check, not a legal compliance audit.
- Shrinkflation measurement compares two packs on a shared base unit; the tool does not judge whether the change is deliberate.
Reviewed according to the CalcDomain Editorial Policy & Calculator Methodology. We document formulas, edge cases, sources, update dates, and correction paths for calculator pages.
Updated
References & Authoritative Sources
- U.S. National Institute of Standards and Technology (NIST) — Office of Weights and Measures — Unit Pricing — state law overview · consulted July 5, 2026 · Authoritative state-by-state coverage of unit-pricing display requirements.
- U.S. National Institute of Standards and Technology (NIST) — NIST Handbook 44 — Specifications, Tolerances, and Other Technical Requirements for Weighing and Measuring Devices · consulted July 27, 2026 · Reference for the mass and volume conversion factors used by this calculator.
- New York State Department of Agriculture and Markets — New York unit-pricing law (Agriculture and Markets Law Article 17) · consulted July 5, 2026 · Example of a mandatory-disclosure state — requires unit price and total price to consumers.
- Rutgers Cooperative Extension (New Jersey Agricultural Experiment Station) — Unit Pricing: Comparing Product Value · consulted July 5, 2026 · Consumer shopping guidance — same-unit comparison, spoilage/storage caveats.
Related Calculators
Update history
| Date | Version | Change |
|---|---|---|
| 2026-07-27 | 3 | Rebuilt as the Unit Price Truth Check: comparison across 2-6 offers with unit conversion, retailer-displayed price verification, and shrinkflation detection. Bespoke renderer with tabbed interface, receipt, embed configurator and shareable state. |
Methodology & Review
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