Methodology: How ratioDB Calculates Price per TB and the TB Index

Where do the prices come from?

All prices come from amazon.com and amazon.de and are retrieved automatically.

• New: the lowest new price of a product, whether Amazon itself or another seller offers it. If that value is missing, ratioDB uses Amazon's own price.
• Used: the lowest price for a used unit.
• Refurbished: the lowest price for a refurbished unit.
• Shipping: No price includes shipping. Prices on amazon.com also exclude sales tax.
• Timestamp: Every row shows when the price was last retrieved. ratioDB does not show prices older than 14 days.

ratioDB refreshes prices every 2 hours. The last update ran on Oct 28, 2026 at 1:17 AM UTC.

How does ratioDB calculate price per TB?

Price per TB = total price divided by (capacity of one unit times pack count). ratioDB uses 1 TB = 10^12 bytes, the same unit printed on the box.

ratioDB shows RAM, memory cards, USB drives and optical discs per GB because their capacity is usually listed in GB. It uses the capacity printed on the package.

TB vs TiB vs GB: what is the difference?

1 TB (terabyte) is 10^12 bytes, or 1,000 GB. 1 TiB (tebibyte) is 2^40 bytes, about 1.0995 TB. To convert TiB to TB, multiply by 1.0995. To convert TB to TiB, multiply by 0.9095.

Manufacturers list capacities in decimal TB. Windows calculates in TiB but labels it "TB", so a 4 TB drive appears as about 3.64 "TB". macOS uses decimal units and shows the full 4 TB. No storage is missing, only the unit differs.

Switch the unit between TB, TiB and GB under "Filters and view", and ratioDB converts all prices accordingly.

How is the TB index built?

For every day, ratioDB sorts all new listings in a group or form factor by price per TB and takes the median of the 10 cheapest. That is the TB index for that day.

• Why the 10 cheapest: The index shows what you can realistically pay if you shop for a good price. Expensive premium models do not distort it.
• Why the median: A single bait price or outlier barely moves the median.
• New only: Used and refurbished listings never count toward the index.
• Gaps: A day needs at least 3 listings with a price. If data is missing on a day in between, ratioDB carries over the previous day's value.
• 90-day low: the lowest index value of the last 90 days.

Example: the TB index for new hard drives on amazon.com is $35.00 per TB, and its 90-day low is $32.22 (as of Oct 28, 2026).

How does ratioDB calculate percentage changes?

Percentage changes of the TB index only compare listings that had a price on both days. ratioDB takes the 30 cheapest of them and uses the geometric mean of their price changes (the Jevons formula, also used in official price indexes).

• Same listings: New listings with a launch price or listings that disappear do not change the percentage. The figure measures real price movement.
• Selection: The 30 cheapest are ranked by the average of both days, so the selection favors neither winners nor losers.
• Outliers: Each individual price change counts at most as a halving or doubling (factor 0.5 to 2).
• Periods: Below the index value you see 1, 7 and 30 days. In the index tiles you choose 7, 14, 30 or 90 days.

In the list, the trend column shows today's price per TB for every single listing against its price 1, 7, 14, 30 or 90 days ago. If there was no price back then, it shows "n/a". If the price barely moved, it shows "0.0 %". "Biggest drops" and "Biggest rises" compare the price per TB of single new listings the same way, over 7, 14, 30 or 90 days.

What does the "90-day low" badge mean?

A listing gets the "90-day low" badge when its price per TB today is at or below the lowest price that held for at least 2 days in a row during the last 90 days.

The 90-day low always compares the same product in the same condition: new with new, used with used, refurbished with refurbished. The daily price is the cheapest offer from all sellers, Amazon included, without shipping.

An earlier low only counts if the price held for at least 2 days in a row. A listing that was cheaper for just 1 day does not set the low. ratioDB shows the resulting value in the details of every listing as "Lowest 90 days".

To make the badge meaningful, 2 more conditions apply: the listing needs at least 30 days of price history, and its highest price in that period must be at least 5 % above today's price. A price that never moved does not get the badge.

The "90-day low" quick filter above the list shows only the listings that are at their 90-day low right now.

How does ratioDB handle new, used and refurbished?

ratioDB lists every product separately per condition and labels every row. The list shows all conditions together, while the index and all averages use new listings only.

Used and refurbished listings are often cheaper and would pull the index down. Use "Condition" to filter the list to exactly one condition.

Which products does ratioDB track?

ratioDB tracks a verified list of 1,640 products in 6 groups and 37 product types, currently with 2,040 listings on amazon.com and amazon.de.

• Selection: The list is built from the best-selling products in each Amazon category and is being expanded.
• Checks: Capacity and pack count are double-confirmed for every product. Card readers, devices, bundles and listings with contradictory details are removed.
• Mapping: Every product is assigned to a product type, for example internal 3.5" or M.2 NVMe. The mapping is checked per product.
• Hard drive specs: Series, recording (CMR or SMR), spindle speed and cache come from the manufacturers' data sheets, linked in the details of every listing. If there is no data sheet for the exact part number, the field stays empty. Under "Filters and view" you can filter hard drives by CMR or SMR.
• Open product list: All products without prices are available as CSV and JSON under the CC BY 4.0 license, linked in the footer.

What are the limits of the data?

ratioDB shows Amazon prices without shipping as of the last update. The price in your cart can differ.

• 1 store per country: ratioDB compares listings on amazon.com and amazon.de. Other stores are not included yet.
• Time lag: A price can change between the update and your visit. The price on Amazon always counts.
• Warranty: The warranty is not known for every product. In that case the row shows "n/a".
• Not every product: The list covers the best-selling products, but not every listing on Amazon.

Found an error? Report it directly in the details of a listing with "Report an error". Reports go to support@ratiodb.com.

Who runs ratioDB?

ratioDB is run by DTILE GmbH, an SEO and GEO agency from Hamburg, Germany. GEO stands for Generative Engine Optimization, meaning optimization for AI search engines.