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Question: On the Treasury website for a T-bill I wish to purchase, I see this information:

Term Issue 91-DAY
Discount Rate % 4.975
Investment Rate % 5.108
PricePer $100 98.742431


I would like to understand how these numbers are calculated, i.e. how they relate to each other.

Answer:
(100-98.742431)*(360/91)=4.97499 ~ 4.975
The price is subtracted from 100, and then, using a 360 day 'year', gives you the 'discount rate'.

((100/98.742431)-1)*(365/91) = .05108336 ~ 5.108%
Now, the price is divided into a hundred, but now uses a 365 day year to calculate the actual investment rate.

Note: this does not take into account compounding which would give you a yield closer to 5.2%