The compounding multiplier
The single most powerful concept in personal finance is compounding — interest earning interest. Small, consistent contributions over decades dwarf larger contributions made later. The Compound Interest Calculator makes that visible: a $500/month contribution earning 7% real return for 40 years finishes at roughly $1.3M, of which less than $250K came from contributions.
Picking a rate to plug in
Banks quote two numbers — a nominal rate and an APY. Use the APY Calculator to convert between them at any compounding frequency, so you're comparing apples to apples across savings accounts, money market funds, and CDs. For real (after-inflation) return, run the same APY through the Inflation Calculator to see whether it actually grows your purchasing power.
Goal-based saving
If you have a number and a deadline — emergency fund, down payment, big purchase — the Savings Goal Calculator answers either question: "how long will it take?" or "how much per month to hit my target by my date?" It's the most direct way to translate a goal into a monthly contribution.
Retirement-specific tools
The 401(k) Calculator models employer match, salary growth, and fees — the things a generic compound interest tool misses. The Retirement Calculator works the other direction: from a target retirement age and desired income, what monthly contribution gets you there. CDs and DRIPs handle fixed-income and dividend-reinvestment scenarios.
Where to park cash: CD, high-yield savings, or Treasurys
These three all promise a safe, known return, and the right pick turns almost entirely on two questions: might you need the money early, and which way are rates heading.
| CD | High-Yield Savings | Treasury Bills | |
|---|---|---|---|
| Rate | Fixed for the term | Variable — moves with the market | Fixed at purchase |
| Access | Penalty for early withdrawal | Fully liquid | Sellable on the secondary market |
| Backed by | FDIC, to $250,000 | FDIC, to $250,000 | US Treasury |
| State income tax | Taxable | Taxable | Exempt |
| Best when | Rates are falling and the money can sit | Rates are rising, or you may need it | You live in a high-tax state |
The state-tax line is the one most comparisons skip, and it can flip the answer. Take $50,000 held for a year. A 4.50% CD earns $2,250 — but a California resident in the 9.3% state bracket keeps about $2,041 after state tax. A 4.40% T-bill earns $2,200 and keeps all of it, because Treasury interest is exempt from state and local tax. The lower advertised rate nets more money. In a state with no income tax, that advantage disappears and the CD wins outright — so the answer genuinely depends on where you live. Check your bracket with the Income Tax Calculator, and run the yields through the CD Calculator and APY Calculator. (Rates above are illustrative, for showing the mechanics — not current market quotes.)
Related categories
For the tax side of retirement (Roth vs. Traditional, capital gains, RMDs), see Tax Calculators. Goal-based saving for shorter horizons (down payment, emergency fund) lives in Personal Finance Calculators.
Frequently Asked Questions
Is a CD better than a high-yield savings account?
It depends on whether you expect rates to rise or fall, and whether you might need the money. A CD locks your rate for the full term, so it wins when rates are falling — your yield is protected while savings rates drop. A high-yield savings account has a variable rate that moves with the market, so it wins when rates are rising, and it stays fully liquid. The practical rule: money you are certain you will not touch can go into a CD for the higher locked rate; money that might be needed belongs in savings, where there is no early-withdrawal penalty.
How do Treasury bills compare to CDs?
Treasury bills are exempt from state and local income tax, which CDs are not. That matters a lot in high-tax states. On $50,000 held for a year, a 4.50% CD earns $2,250 — but a California resident in the 9.3% state bracket keeps about $2,041 of it after state tax. A 4.40% T-bill earning $2,200 keeps the full $2,200, because no state tax applies. The lower headline rate wins. In a state with no income tax, the CD wins outright. Both are effectively default-free: CDs through FDIC insurance up to $250,000, T-bills through the US Treasury.
What happens if I withdraw from a CD early?
You pay an early-withdrawal penalty, typically 3 to 6 months of interest on terms under a year and 6 to 12 months on longer terms. The penalty is charged against interest earned, and if you withdraw before enough interest has accrued it can reduce your principal. No-penalty CDs exist but usually pay 0.25 to 0.50 percentage points less. A CD ladder is the common middle ground — it keeps part of your money maturing every year without giving up the longer-term rate on the rest.
Is CD and savings interest taxed?
Yes. Interest is taxed as ordinary income at your federal marginal rate in the year it is credited, even if you do not withdraw it — so a multi-year CD generates a Form 1099-INT every year, not just at maturity. Most states also tax it, though Treasury interest is state-tax-exempt. Interest earned inside an IRA is not taxed in the year it is credited.
What return should I assume for long-term investing?
For long-horizon projections, 6 to 7% is a common assumption for a diversified stock portfolio after inflation, based on long-run historical averages. That is a planning assumption, not a prediction — actual returns vary enormously year to year and any individual decade can fall well short. For short horizons (under about five years), a guaranteed CD or savings rate is usually the more appropriate comparison, because there is no time to recover from a downturn.