HYSA Compound Interest Calculator
Estimate after tax interest for a high yield savings account from effective APY and a periodic tax assumption, then review a fixed five year balance scenario.
Earlier editorial review
- Reviewed
- 2026-09-18
- Methodology
- 1.1
Sources
How the calculation works
Enter the initial deposit, quoted APY, assumed tax rate on interest, and crediting frequency. APY is an effective annual yield. The model derives the equivalent periodic rate and applies the tax assumption to each period’s interest before compounding what remains.
For a 10,000 USD deposit at 5% APY with a 25% assumed tax rate and monthly crediting, the modeled after tax effective annual yield is 3.728965%.
That scenario produces first year net interest of 372.896525 USD. Dividing by 12 gives an average of 31.074710 USD per month. If the same assumptions continue for five years without deposits or withdrawals, the ending balance is 12,008.817037 USD and cumulative net interest is 2,008.817037 USD.
The projection keeps APY and the tax assumption fixed. Change either input to compare scenarios; the table does not forecast future bank rates or actual account payments.
Step by step
- Enter a deposit, effective annual APY, assumed tax rate, and crediting frequency.
- Review the modeled yield, first year net interest, and average monthly amount.
- Read each ending balance together with cumulative interest above the initial deposit.
Worked example
Enter an initial deposit of 10,000 USD, APY of 5%, a 25% assumed tax rate, and monthly crediting. The modeled after tax effective annual yield is 3.728965%.
First year net interest is 372.896525 USD, or an average of 31.074710 USD per month. At the same APY, tax assumption, and frequency for five years with no added or withdrawn money, ending balance is 12,008.817037 USD. Cumulative interest above the initial deposit is 2,008.817037 USD.
When to use this calculator
Use this calculator to examine how an entered APY, periodic crediting schedule, and tax assumption affect a fixed deposit scenario.
Compare the result with current account terms, including the date of the quoted APY, qualifying balance, fees, and access conditions. Those terms are not modeled here.
Common input mistakes
Enter the effective annual APY, not a nominal rate that should be compounded to create APY. APY already reflects the gross annual effect of its compounding schedule. Do not apply the frequency a second time to the quoted APY.
Treat the tax percentage as a modeling assumption for each period’s interest. It does not establish actual withholding or tax owed. Check the account’s rate conditions, fees, and access terms separately.
Formula and conventions
Gross periodic rate = (1 + APY / 100)^(1 / n) − 1
After tax periodic rate = gross periodic rate × (1 − tax rate / 100)
Modeled after tax effective annual yield (%) = 100 × ((1 + after tax periodic rate)^n − 1)
Ending balance after t years = initial deposit × (1 + after tax periodic rate)^(n × t)
Cumulative net interest = ending balance − initial deposit
n is 12 for monthly, 4 for quarterly, or 365 for daily crediting. APY is treated as an effective annual yield. The tax assumption is applied to interest in each period before the remainder compounds. This timing is a model assumption, not an actual tax withholding rule.
Assumptions and limits
The model assumes a constant APY and tax percentage for five years, applies the tax assumption to each period’s interest, and includes no later deposits or withdrawals. It does not model fees, inflation, changing rates, actual tax payment timing, account terms, eligibility, or deposit insurance coverage. Results are calculations under the selected assumptions, not a rate forecast or account recommendation.
Key terms
- APY
- Annual Percentage Yield is an effective annual rate that reflects the gross effect of compounding over one year.
- Crediting frequency
- The selected interval used to derive a periodic rate from APY and apply the tax assumption.
- Cumulative net interest
- Projected interest above the initial deposit after the indicated number of years, with no added or withdrawn funds.
Related comparisons
Compound Interest Comparator can show how the same deposit grows under other rate and contribution assumptions. Real Return Calculator adjusts a future amount for an entered inflation assumption. These tools answer different questions from this after tax APY scenario.
Compare quoted APYs only when they refer to the same date and comparable account terms. Check qualifying balances, fees, access rules, and any rate conditions separately because this model does not include them.
FAQ
What does this savings calculator estimate?
The calculator converts an effective annual APY into a periodic rate, applies your assumed tax percentage to each period’s interest, then compounds the remainder. It reports a modeled yield and balance scenario. It does not determine an account’s actual interest crediting or tax treatment.
How does the calculator use APY?
APY is an effective annual yield, so it already reflects the gross effect of the stated compounding schedule. The calculator derives the equivalent monthly, quarterly, or daily rate from APY, then applies the tax assumption before each period’s remaining interest compounds.
Why can frequency change the modeled after tax yield?
With a zero tax assumption, the effective annual yield remains the entered APY for each frequency. When the assumed tax is positive, the model removes that share of each period’s interest before compounding the remainder, so the resulting annual yield can vary by frequency.
Does the tax input calculate my actual tax?
The tax percentage is a scenario input applied to interest in each compounding period. It is not a tax return calculation or a statement about withholding, reporting, deductions, or the rules for a particular account or location.
What does the five year table show?
Each row shows the projected ending balance and cumulative net interest above the initial deposit after that many years. The projection holds APY and the tax assumption constant and includes no later deposits or withdrawals.
Related tools
Compound Interest Comparator compares fixed rate and contribution scenarios. Real Return Calculator adjusts a future amount for an inflation assumption. Treasury Bill Yield examines a different cash instrument and its stated yield convention.