What cash really earns after tax and inflation.

• Every rate is an example you can change, not a current rate. Interest is paid once at the end of the year and taxed as ordinary income at the federal bracket and state rate you enter; Treasury bill interest is exempt from state and local income tax.
• After-inflation return = (1 + after-tax return) / (1 + inflation) - 1. Not modeled: CD early-withdrawal penalties, deposit insurance limits, compounding within the year and how T-bill yields are quoted.
• Every number is hypothetical and depends entirely on what you enter. Small changes in assumptions can change results a lot.
• Returns are smooth and constant in the single-number projection; real returns vary every year, and real markets can do better or worse than anything shown.
• Inflation is a single constant rate you choose (except in historical replays, which use the actual CPI of each year).
• Growth compounds monthly (or at the contribution frequency) and contributions are made at the end of each period.
• No taxes unless you turn on the simplified tax drag (all positive growth taxed each year at your rate, which overstates tax for buy-and-hold). No fees unless you set one.
• Inputs stay in your browser. Saving uses this device's local storage; a share link contains the numbers you entered, so anyone with the link can see them.

Coteau may be wrong. Check every number yourself, and talk to a licensed professional before acting on anything.