How PriceCrow calculates price per unit
PriceCrow ranks Amazon listings by price per unit. For each search it collects up to 69 Amazon US listings, reads each package's quantity from its title and size, converts it to the one unit you chose, divides the listed price by that quantity, and sorts the results from lowest unit price upward. This page explains every step, the checks along the way, and where the method falls short.
Method reviewed against the code .
1. Where the prices come from
Each search sends your words to Amazon's product API, which searches Amazon.com and returns listings ten at a time. PriceCrow stops at 69 listings or after about 20 seconds, whichever comes first, and drops repeats of the same listing. The price is the listing's current offer price. Listings without a price are not ranked.
That price leaves out clippable coupons, Subscribe & Save, multi-buy promotions, shipping and tax, any of which can change which listing is cheapest at checkout.
2. One unit for the whole search
Every listing in a comparison is priced in the same unit, so the numbers are directly comparable. You can pick from 13 units: count, pounds, ounces, kilograms, grams, milligrams, fluid ounces, gallons, liters, milliliters, cups, quarts and pints. The default, Auto, suggests the unit that fits the product best, and you can change it.
Toilet paper and paper towels compared by count are priced per sheet, because roll sizes are not standard.
3. How each listing's quantity is read
PriceCrow tries these in order and uses the first one that gives a quantity it trusts:
- Sheet rules for toilet paper and paper towels. The number of physical rolls is multiplied by the sheets per roll printed in the title. Marketing equivalents such as "18 Mega Rolls = 72 Regular Rolls" are ignored, commercial formats (jumbo, coreless, centerpull, folded towels) are left out, and so are listings that state no sheet count.
- Amazon's own unit price, checked against the stated size. Many listings carry an Amazon figure such as "$0.18 / count", which Amazon works out from the unit count the seller entered. PriceCrow uses it only when it is the same kind of unit you chose, and compares it with the size printed in the title and size field. If they agree within 10%, the printed size is priced exactly. If they disagree, Amazon's figure is set aside and the printed size is used, but only when every size in the listing agrees.
- Quantities written in the listing. PriceCrow reads counts (count, ct, pods, tablets, rolls, packs, loads and similar), weights and volumes from the title and size field, including multipacks such as "Pack of 4", "6 × 16.9 fl oz" and "2 boxes of 40 pods". When it finds exactly one candidate, that is the quantity. When it finds several, a language model chooses which one is the listing's total, or none of them; its choice is used only when it is at least 65% likely.
- Nothing it trusts. The listing is left out of the ranking instead of being given a guessed number.
4. Unit conversions
Quantities are converted with these factors:
| Unit | Equals |
|---|---|
| 1 ounce | 28.3495 grams |
| 1 pound | 453.592 grams (16 ounces) |
| 1 kilogram | 1,000 grams |
| 1 US fluid ounce | 29.5735 milliliters |
| 1 US cup / pint / quart | 236.588 / 473.176 / 946.353 milliliters |
| 1 US gallon | 3,785.41 milliliters |
| 1 liter | 1,000 milliliters |
Weight, volume and count are never converted into each other. A 64 fl oz bottle has no price per ounce of weight, because how much a fluid ounce weighs depends on what is in the bottle.
5. How listings are ranked
Listings are sorted by price per unit, lowest first. Nothing else affects the order: there are no sponsored placements, and affiliate links do not change it. Three filters are yours to apply. Price keeps listings whose item price, not unit price, falls within the range you set. Prime only keeps listings whose Prime eligibility Amazon confirms; Amazon often leaves it unknown, so this filter can hide eligible listings. Strict match keeps only titles containing all your words, or your exact phrase.
6. How fresh the prices are
Every price on a comparison shows when it was recorded. A search's results are reused for up to 24 hours, the longest Amazon's Associates license allows, and the first visit after that fetches them again. If Amazon returns no prices during a refresh, the previous results stay up with their original timestamp.
7. Which comparisons become pages
Anyone can run any search, but only some become public pages such as cheapest dishwasher pods per count. A comparison is published for search engines only when its product passes a quality check and it ranks at least three listings. Every published comparison is listed in the A–Z directory and grouped in categories.
8. Cases from our test suite
These listings come from PriceCrow's automated tests. Each is a trap that a simple size parser gets wrong.
| Listing | Compared per | PriceCrow's result |
|---|---|---|
| White Mulberries Dried Organic Bulk 2 Lbs Bag, $21.98; Amazon label "$1.37 / ounce" | Ounce | Amazon's label implies a 16 oz bag. The title and size both say 2 lb, so it is priced as 32 oz: $0.687 per ounce. |
| Finish Powerball Dishwasher Pods, 84ct, $14.99; Amazon label "$0.18 / count" | Count | The label agrees with 84 pods but is rounded to the cent, so the 84 is priced exactly: $0.1785 per pod. |
| Mulberries 16 oz, with a size field of 32 oz | Ounce | The two sizes contradict each other, so the listing is not ranked. |
| Juice 64 fl oz | Ounce (weight) | Not ranked: fluid ounces measure volume. |
| Organic Olive Oil 33.8 fl oz, $10.00 | Liter | 33.8 fl oz is about one liter: about $10 per liter, not 33.8 liters. |
| Presto! 2-Ply Toilet Paper, 24 Mega Rolls = 120 Regular Rolls, 308 Sheets Per Roll | Sheet | 24 physical rolls × 308 sheets = 7,392 sheets. The 120 "regular rolls" are ignored. |
| Amazon Basics 1000 Sheet 1-Ply Bath Tissue, 48 Rolls (4 Packs of 12) | Sheet | 48 rolls × 1,000 sheets = 48,000 sheets. |
| Angel Soft Toilet Paper, 18 Mega Rolls = 72 Regular Rolls | Sheet | No sheet count in the listing, so it is not ranked. |
9. Known limitations
- A comparison covers up to 69 listings from Amazon US, not every product Amazon sells.
- Coupons, Subscribe & Save, shipping and tax are not included, so the cheapest listing here may not be the cheapest at checkout.
- A listing that prints the wrong size is priced wrong, unless Amazon's own label contradicts it.
- Big-brand toilet paper that lists only "mega" or "family" rolls, with no sheet count, is left out of the per-sheet ranking.
- Per sheet ignores sheet size and ply, and per count treats every item as equal, so a small tablet counts the same as a large one.
- Amazon's results for a search can include related products, such as a dispenser in a refill search. Strict match or a more specific search narrows them.
- PriceCrow shows today's price, not price history, so it cannot tell you whether today's price is a good one for that product.
Found a wrong unit price?
Use the Feedback button on any comparison, or contact us, with the search and the listing. To work out a unit price yourself, use the unit price calculator. For a shopper's walkthrough, read how to find the cheapest unit price on Amazon.
Questions
Does PriceCrow use AI to calculate unit prices?
Only to choose between quantities already printed in a listing. When a title holds more than one possible quantity, such as "6 × 16.9 fl oz" and "16.9 fl oz", a language model picks which one is the total, or none. Its pick is used only when it is at least 65% likely. The model never supplies a number; the division and unit conversion happen in PriceCrow's own code.
How often are PriceCrow's Amazon prices updated?
A search's prices are reused for up to 24 hours, the longest Amazon allows, and every price shows when it was recorded. The first visit after that fetches new prices from Amazon. Prices can still change on Amazon between our check and your purchase.
Can a seller pay to rank higher on PriceCrow?
No. There are no sponsored placements. Listings are ordered only by calculated price per unit. Links to Amazon are affiliate links, which never change the order.
Why is a listing missing from the ranking?
Either Amazon did not return it among the first 69 results for the search, it had no price, or PriceCrow could not read a quantity it trusts in the unit you chose. A listing whose stated sizes contradict each other is left out rather than guessed.