T-Bill vs HISA Yield Comparison

What this tool does

It compares after-tax cash returns between a High-Interest Savings Account (HISA) and a set of T-Bill maturities over a one-year horizon. Enter principal, HISA APR, T-Bill yields (1/3/6/12m), and your marginal tax rate; choose whether you reinvest coupons/maturities.

Mechanics

• HISA: approximate simple interest: principal × APR × 1 year, taxed at your marginal rate.
• T-Bills: priced at discount; the tool approximates interest by tenor and applies tax. If reinvest is on, it chains shorter bills (e.g., 3m × 4) to the one-year point.

Reading the output

You'll see after-tax dollars for HISA and each tenor, plus the winner. The breakeven APR (HISA needed to match the best T-Bill) is your negotiating benchmark.

What changes outcomes

• Your tax rate (interest is fully taxable in many jurisdictions).
• The shape of the T-Bill curve vs posted HISA.
• Reinvestment assumptions and cash availability needs.

Caveats

• Ignores fees, settlement timing, state/provincial quirks, and exact day-count.
• Doesn't compare to GICs, commercial paper, or MMFs (add them separately if needed).
• Not advice; it's a planning tool.

Pro tips

• Run it with a range of HISA rates; use breakeven to push your bank.
• If you have lumpy cash flows, consider laddering T-Bills to match needs.
• Track after-tax not nominal; taxes change the ranking.