Ever wondered how Treasury bills actually work and whether they're worth your time? I spent some time breaking down the math, and it's actually simpler than most people think.

So here's the deal with T-bills - they're short-term government debt that mature in less than a year. The interesting part? They don't pay you interest like regular bonds do. Instead, you buy them at a discount. Say a bill has a face value of $1,000 but you grab it for $985. That $15 difference is your return. Prices get quoted on a scale where 100 is full value, so that example would show as 98.5.

Now, if you want to actually compare T-bills to other investments, you need to annualize the return. That's where it gets useful. First, you need three pieces of info: the purchase price, when you bought it, and the maturity date. From there, count the days until it matures.

The calculation itself breaks down like this - subtract the price from 100, divide by the price, then multiply by 100 to get a percentage. That gives you the yield for the holding period. To annualize it, multiply by 365 and divide by the number of days you're holding it. That final number is what you compare against other investments.

Let me walk through a real example. Say you buy a 13-week T-bill (that's 91 days) at 99.0. The yield works out to about 1.01% for that 91-day period. When you annualize it, you're looking at roughly 4.04% annually. That's the number that matters when you're deciding between a T-bill and other fixed-income options.

Why does this matter? Because once you annualize the return, you can actually compare short-term Treasury bills against longer-dated bills, Treasury bonds, corporate bonds, and other instruments. It's the same principle whether you're looking at US Treasury bills or exploring simuladores for letras del tesoro from Argentina or other markets - the math works the same way. Understanding this calculation helps you make real apples-to-apples comparisons across different investment horizons and types.