Methodology
Every number on RungRate should be checkable. This page documents where the data comes from, how the core calculations work, and how often each dataset is refreshed.
Data sources
| Dataset | Primary source | Used for |
|---|---|---|
| Treasury bill yields (4–52 week) | U.S. Department of the Treasury — daily Treasury bill rates (treasury.gov) | Ladder builder defaults, current-yields table |
| National deposit rate benchmarks | FDIC — monthly national rates on deposit products (fdic.gov) | CD and savings comparison baselines |
| State income tax rates | State revenue department publications, cross-checked against nonpartisan tax policy summaries | After-tax yield comparison by state |
| Tax treatment of Treasury interest | 31 U.S.C. § 3124; IRS guidance on interest income | State-tax exemption applied in all T-bill calculations |
Tables on this site never copy a competitor's compilation. Figures are drawn from the primary sources above and dated at the moment of the snapshot.
Calculation principles
After-tax yield
The core comparison on RungRate is effective after-tax yield:
after-tax yield = nominal yield × (1 − federal rate − state rate × state_taxable)
- nominal yield — the quoted annualized yield (investment-rate basis for T-bills, APY for CDs/HYSAs), as a decimal.
- federal rate — your federal marginal income tax rate, as a decimal.
- state rate — your state marginal income tax rate, as a decimal.
- state_taxable — 1 for CD and savings interest; 0 for T-bill interest, which is exempt from state and local income tax.
Worked example
A saver in a 24% federal bracket and a 9.3% state bracket compares a 5.00% CD with a 5.00% T-bill:
- CD: 5.00% × (1 − 0.24 − 0.093) = 5.00% × 0.667 = 3.34% after tax
- T-bill: 5.00% × (1 − 0.24) = 5.00% × 0.76 = 3.80% after tax
Identical sticker yields, but the T-bill nets 0.46 percentage points more in this state — the gap RungRate's comparison tools are built to expose. In a state with no income tax, state_taxable is irrelevant and the two net the same.
Simplifications we make (and disclose)
- Marginal rates are applied flat to the interest; we do not model bracket transitions caused by the interest itself.
- T-bill yields are compared on the Treasury's investment-rate basis so they are comparable to APY-quoted products; the two conventions differ slightly in compounding treatment.
- State taxable-income definitions vary; a few states tax interest through mechanisms not captured by a single marginal rate. Tool pages flag these cases where they materially change the outcome.
Update policy
| Dataset | Refresh cadence |
|---|---|
| Treasury yields | Weekly, or faster during sharp rate moves |
| CD / HYSA national benchmarks | Monthly, following FDIC releases |
| State income tax rates | Annually, after each state legislative season |
| Statutory references | Reviewed when the underlying law changes |
Every data table displays its own as-of date next to the figures. If a table's date looks stale relative to this schedule, that is a bug — please report it.
Frequently asked questions
Where does RungRate get its Treasury yield data?
From the U.S. Department of the Treasury's published daily yield data (treasury.gov). Each table on RungRate states the exact series used and the as-of date of the snapshot.
Why is T-bill interest exempt from state income tax?
Federal law (31 U.S.C. § 3124) exempts interest on U.S. government obligations, including Treasury bills, from state and local income taxation. It remains fully taxable at the federal level.
How often is the data on RungRate updated?
Treasury yields are refreshed on a weekly cadence (or faster when rates move sharply), national CD and HYSA benchmark rates monthly, and state income tax rates annually after each legislative season. Every table shows its own as-of date.
How does RungRate calculate after-tax yield?
After-tax yield = nominal yield × (1 − applicable tax rates). For CDs and HYSAs the applicable rate is federal + state marginal rate; for T-bills it is the federal rate only, because Treasury interest is exempt from state income tax. The full formula and a worked example are on this page.