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Ex: I'm looking at the 3 month (13-week) Treasury bill here, maturing 11/16/2023. The evaluated price is 98.617417.

So is the interest rate: (100 - 98.617417)/98.617417 * (365/(13 * 7)) * 100 = 5.62327?

I've seen some formulas use 366 days instead of 365. How do I know which to use?

Or is there a more accurate way of calculating the interest rate here?

Schwab defines "Evaluated price" as "The evaluation-pricing model is a model calculated estimate determined from alternative information in addition to recent trade data. The price may differ from actual market transactions and should be used for approximate account valuation purposes only. Contact us for additional quote information about a particular holding."

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    Does this help?
    – AKdemy
    Aug 13, 2023 at 18:20

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I've seen some formulas use 366 days instead of 365. How do I know which to use?

You can use either, or use 360, which is common for annualizing yield based purely on a discounted purchase price - as long as you're consistent when comparing rates with other investments. T-bills don't have an "actual" published interest rate - they have a yield based on the difference between the (discounted) purchase price and par price (100). So in this case the T-bill earns an un-annualized yield of 1.403% over 13 weeks. When you annualize that, e.g. to compare it with other investments that have a different holding period, then you should be consistent when annualizing, and you can use 365 for non-leap years and 366 for leap years, or always use 365, so long as you're consistent.

Note that the # of days you use has absolutely no bearing on the actual return you get on the investment.

The "Evaluated price" blurb simply means that you may not be able to actually buy bills at that price - it means that it's a model price based on actual market a trades and other market data. What you can actually buy bills for will depend on what price you can get at the initial auction, or what price someone will sell them to you for on the secondary market.

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