Interest Calculator

How it works

Interest after tax and inflation

To isolate interest on a deposit, set Regular contribution to 0. Enter your assumed tax rate and inflation rate to compare the ending balance with its purchasing power. For example, 1,000 at 5% compounded annually for 1 year earns 50 before tax. With 20% tax on interest and 0% inflation, the ending balance is 1,040. Actual account tax rules can differ.

Keeping rate and contribution timing consistent

Interest growth depends on the starting balance, the rate convention, and when contributions arrive. Use the mode that matches monthly or annual additions instead of combining them into an unexplained total. When comparing rates, check whether they are nominal or effective. The projection follows the entered rate over time; it does not predict future returns or account for omitted charges.

Guides

• Compounding, contributions, and the time between them

  Learn how rates, contribution timing, inflation, and taxes change a savings projection.

Savings growth: formula and assumptions

Each period: balance grows at the equivalent periodic rate, then a contribution is added (or added first for beginning-of-period deposits).

Returns and inflation are assumptions held constant. Annual taxes on positive interest are deducted at year end.

Monthly + annual contributions: formula and assumptions

Ending balance = opening growth + each contribution grown from its event time − modeled interest tax.

The annual stream is additional to twelve monthly contributions. Increases apply separately to each stream. Tax settles at year ends and the final partial year; inflation adjusts purchasing power.

Equivalent nominal rates: formula and assumptions

Equivalent nominal rate = target frequency × [exp(annual log-growth / target frequency) − 1].

Semimonthly means 24 periods and biweekly 26. Continuous compounding uses the logarithm of effective annual growth. This compares rates with equal annual growth.

Worked example

Reference example currency: USD. These fixed examples do not change with your calculator preferences.

Using the Savings growth example:

Starting amount ($)

10000

Regular contribution ($)

200

Contribution frequency

Monthly

Nominal annual return (%)

7

Time horizon (years)

10

Compounding per year

Monthly

Contribution timing

End of each period

Tax rate on annual interest (%)

0

Annual inflation assumption (%)

0

Each period: balance grows at the equivalent periodic rate, then a contribution is added (or added first for beginning-of-period deposits).

Example result

These are illustrative inputs. Change the values to explore a different scenario.

What to keep in mind

Projected returns are hypothetical. Actual returns, fees, tax treatment, and deposit terms may differ.

Formula references

• CFPB: understanding mortgages (opens in a new tab)
• Investor.gov: financial tools and calculators (opens in a new tab)
• OpenStax annuities (opens in a new tab)

References explain the underlying methods; they do not imply endorsement or individual professional review of Finflo.

Common questions