T-Bill Rates · Methodology

How the T-Bill Rate Data Is Built

Where the rates on the T-bill rates tracker come from, how the cost and earnings of $10,000 are worked out, and what changed when.

Last changed · Changelog · data.json · latest.json

What the data covers

Every regular Treasury bill auction since January 2000 for the 4, 6, 8, 13, 17, 26 and 52-week bills. Cash management bills, which Treasury sells irregularly for days or weeks, are excluded.

A bill auction is filed by the term it was sold as: a 13-week auction that reopens a bill first sold as a 26-week bill is a 13-week auction. Note, bond and TIPS auctions are on the Treasury auction tracker.

Sources

History since 2000
Treasury's Fiscal Data auctions dataset, read in three date ranges so each response stays small enough to cache. It can take several days to post a result.
Same-day results and announced auctions
TreasuryDirect's feeds of recent and upcoming auctions. Same-day values win where both sources have one, and a blank from either source never erases a number from the other.

Update schedule

Bill auctions close to competitive bids at 11:30 am Eastern. A refresh runs on weekdays at 10 and 40 minutes past each hour from 15:00 to 18:00 UTC, which brackets that close in both summer and winter time, and regenerates the hub, every term page and both JSON files. Between runs the recent history is cached for up to six hours and the TreasuryDirect feeds for up to an hour.

Each weekday at 13:00 UTC an automated check confirms every term page shows the newest result Fiscal Data has for it, and at 19:30 UTC a second check confirms that every bill auctioned that day already shows its result. A miss alerts TFTC the same day.

How the figures are computed

Investment rate
The headline rate: the bond-equivalent yield at the auction's high price, on a 365-day year. It is the number TreasuryDirect shows on holdings and the one to set beside a savings or CD rate.
Discount rate
The same result on Treasury's bank-discount basis: the discount from face value as a share of face value, on a 360-day year. It is always a little lower.
Price per $100
Treasury's auction price, to the six decimals it publishes.
Cost of $10,000
The price per $100 times 100, rounded half-up to the cent. The arithmetic runs in whole millionths of a dollar so floating-point error can never round a true half cent down.
Earnings
$10,000 of face value minus that cost: what the bill pays at maturity over what it cost.
Days
Calendar days from the issue date to the maturity date.
Rate change
The investment rate minus the previous auction of the same term, in basis points, rounded to a tenth.
A year ago
The last auction of the same term on or before the same date a year before that term's latest auction.
Usual auction day
The weekday most of the term's auctions fell on over the last twelve months.
Bidder shares
Indirect and primary dealer awards as a percentage of the competitive amount accepted. Bills carry no demand grade; the bill pages are built for savers comparing rates.

Update dates

Every page and file states one update date, taken from the data: the newest bill auction result it shows, stamped at 17:00 UTC that day, after the 11:30 am Eastern close in both summer and winter time. The same value is the page's structured-data dateModified, its sitemap lastmod and the dateModified in both JSON files, where updatedThrough gives the bare auction date.

Known limitations

Auction rates only
These are the rates each auction set. Bills trade between auctions, and their market yields move every day; this data does not track them.
Cash management bills
Excluded, so a week with a large cash management sale shows only the regular bills.
Tables carry ten years
The term pages' tables stop ten years back to keep the pages light. The chart and the JSON carry the full record since 2000.
Upstream changes
TreasuryDirect renamed its bill fields in October 2026 and the pages stopped at September 10 until the new names were mapped. The daily checks exist to catch the next one quickly.

Corrections

Every figure is re-read from Treasury on each refresh, so a correction Treasury makes shows up on the next run. Changes to sources or arithmetic are dated in the changelog below. If you find a rate that does not match Treasury's official results, tell us through the contact page with the auction date and term.

License and how to cite

Treasury's figures are public domain. TFTC's compilation and the derived cost, earnings and comparisons are free to reuse under CC BY 4.0. Credit:

Source: TFTC (tftc.io/treasury-bills), data through <date>.

The full history is at /treasury-bills/data.json, and the newest result for every term with the announced auctions is at /treasury-bills/latest.json.

Changelog

Dated changes to the data, its sources and the method, newest first.

  1. Same-day check, latest-results file and this page

    A second check each weekday afternoon confirms that bill auctions held that day show on their pages. Added latest.json with the newest result for every term, Dataset markup on every term page, one update date across pages, sitemap and JSON files, and this methodology page.

  2. Daily freshness check

    Each weekday morning an automated check compares every bill page with Treasury's Fiscal Data record and alerts TFTC when a page is missing the newest result.

  3. Missing bill results restored

    TreasuryDirect renamed its bill result fields in October 2026, and every bill auction after September 10, 2026 read as not yet held, so the pages stopped at that date. The renamed fields are now mapped with the old names as fallbacks, checked against Fiscal Data, and the missing results are back.

  4. Tracker launched

    Every regular Treasury bill auction since 2000 for the 4, 6, 8, 13, 17, 26 and 52-week bills, with the investment rate, the discount rate, the price per $100 and what $10,000 of face value cost and earned. Cash management bills are excluded. The full history is open JSON under CC BY 4.0.