Compare HYSA, CD, and Mortgage scenarios
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HYSA, CD, and Mortgage on one page

This tool puts HYSA / CD / MMA savings and Fixed / ARM mortgage scenarios on one page. Savings balances and mortgage payments answer different questions; do not treat an APY and a mortgage payment as directly comparable returns. Calculations run in your browser. Inputs also appear in the shareable URL, which can be included in server requests or logs when that URL is opened.

APY, interest rate, and APR are different inputs

For savings, enter the advertised APY (Annual Percentage Yield). It already includes compounding, so this calculator does not add another daily or monthly compounding adjustment. For an illustrative mortgage principal-and-interest payment, enter the interest rate from the Loan Estimate, not its APR. A mortgage APR also reflects certain upfront charges and is a separate comparison measure. See the CFPB APY definition and CFPB mortgage rate/APR explanation.

Illustrative after-tax interest

The optional tax toggle applies the entered federal plus state percentages to the illustration's interest. For example, 24% + 9.3% entered on $500 of interest shows about $166.50 of modeled tax. This simple estimate does not calculate your full tax liability, account-specific tax treatment, or deductions.

A checkable example: $10,000 at 5% APY for 12 months, no contributions, earns $500 before tax and has a $10,500 balance. Entering 22% federal and 0% state shows $110 of modeled tax, $390 of net interest, and $10,390.00 including the principal. The tax estimate is applied once at the end; it does not model taxes paid or withdrawn each year. Schedule and CSV balances remain before modeled tax.

Contributions are added at the end of each month. At a constant 4.5% APY, $10,000 initially plus $500 monthly for 24 months gives $23,441.31 before modeled tax, of which $22,000 is deposited principal. These are mathematical examples, not current account offers.

CD terms and early withdrawal

The CD balance shown assumes you keep the deposit for the full entered term at the same APY. This tool does not model an early withdrawal, its penalty, or a change in rate. The monthly-contribution field is a hypothetical savings input; some CDs do not permit additional deposits. Check your bank's actual rate, access terms, and penalty schedule before acting.

Fixed vs ARM scenario mechanics

A Fixed scenario holds the entered interest rate constant. Monthly P&I uses M = P × r / (1 - (1+r)-n), where r is the entered annual percentage divided by 100 and then 12 and n is the selected term in months (360 for 30 years).

The one-reset ARM example takes one assumed interest-rate change after the initial period, applies a symmetrical rate-change limit above and below the starting rate, and holds that rate for the remaining term. This is not a lender's contractual lifetime cap, initial cap, periodic cap, or rate floor. It does not forecast an index, margin, or every future reset. For a real ARM, check the index and margin in your contract.

Try a $400,000 principal, 6% starting interest, 30 years, a 5-year initial period, a 12% assumed reset, and a 1-percentage-point limit. The reset is limited to 7% in this example: initial P&I is $2,398.20, reset P&I is $2,630.76, and row-summed interest is $533,118.75. A fixed 6% comparison keeps its payment at $2,398.20 instead. Payments are monthly, internal balances are not rounded between steps, and displayed rows round to cents. Summing rounded rows can differ from a lender's payment rounding and final-payment adjustment.

New share links store the reset period, assumed rate, and rate-change limit for both A and B. Older ARM links did not preserve all settings. They do not show a result or comparison until you explicitly choose the missing assumptions. A guessed five-point limit is not silently treated as your original choice.

In an ARM-to-ARM comparison, initial and after-reset monthly P&I differences are shown separately. The after-reset difference compares each scenario's assumed reset, not the same calendar month; the chosen initial periods can end in different years. A zero initial difference does not mean later payments match.

What the mortgage comparison does not calculate

The comparison panel shows payment and interest differences for the entered scenarios. It does not calculate refinancing closing costs or a break-even month. A smaller payment alone does not prove a refinance saves money; compare written Loan Estimates, including points, fees, payment, and cash to close.

PMI rule of thumb

Enter a PMI amount from your offer if it applies. The form does not ask for a home value and therefore cannot determine your LTV or your lender's actual PMI charge automatically.

Three interest-calculation pitfalls people miss

1. When compounding frequency does — and does not — change a quote

If two hypothetical savings accounts both start with $10,000 and keep a fixed 5.25% APY for one year, each earns about $525 before tax, regardless of whether its internal compounding is monthly or daily. Holding a nominal interest rate fixed instead can produce different APYs as the compounding frequency changes; that is a different comparison. The English savings form accepts APY and does not offer a compounding-frequency switch. Check the offered APY and account conditions rather than adding compounding again.

2. A tax assumption is not your tax return

An APY is a before-tax savings yield. This form's after-tax number is a simplified illustration based on the percentages you enter. It does not determine whether a particular account or security is taxable, and it does not model municipal bonds, retirement accounts, or your complete tax return.

3. Compare real loan offers, not just their payments

Two scenarios can have different payments because of rate, term, principal, taxes, insurance, or PMI. Use the comparison to understand which assumption moved the model, then check the actual Loan Estimates. The CFPB's loan-offer comparison guide explains why upfront costs and the five-year cost matter alongside the monthly payment.

Review and correction

2026-10-10 Correction: ARM links previously lost the entered rate-change limit and reopened with five points instead. New links retain the limit, including zero; incomplete older links require an explicit choice. The interface now calls this a one-reset example rather than a complete ARM contract. Entry descriptions no longer advertise a refinancing break-even or after-tax APY calculation that this form does not provide.

Examples were checked with independent monthly calculations and automated tests. The linked CFPB definitions, ARM explanation and loan-comparison guidance were reviewed on 2026-10-10 using AI assistance; this is not professional financial review. No live rate, lender quote, personal tax liability, or future return was verified.Recalculate the simple-versus-compound example and its APY conversion for a separate look at how the chosen input changes the model.

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