Interest calculator.
Project compound interest, contribution growth, total principal, total interest, and inflation-adjusted buying power in one enriched finance calculator.
Interest Calculator inputs
Model initial investment, scheduled contributions, compounding frequency, tax, inflation, and investment length.
Starting balance and deposits
Enter the principal and optional recurring contributions.
Rate and compounding
Set the annual rate, compounding frequency, tax, and inflation assumptions.
Investment length
Choose the number of years and months to project.
Reference features: Follows the original Interest Calculator features: initial investment, annual/monthly contributions, contribution timing, compounding frequency, investment length, tax rate, inflation rate, results, charts, and schedule.
Visual breakdown
Ending balance, principal, contributions, interest earned, real buying power, and time horizon update live as inputs change.
Accumulation over time
Annual balance path based on contribution schedule, compounding, tax, and inflation assumptions.
Accumulation schedule
First-year monthly detail plus annual summary for the full investment horizon.
About the Interest Calculator
This interest calculator models how a starting balance can grow with compound interest, recurring deposits, contribution timing, taxes on interest income, and inflation. It is designed for savings accounts, certificates of deposit, investment projections, and any fixed-rate planning scenario where you want to compare principal, contributions, interest earned, and real buying power.
The calculator follows the same core ideas as the original interest reference: initial investment, annual contribution, monthly contribution, beginning-or-end contribution timing, compounding frequency, investment length, tax rate, inflation rate, result breakdown, and an accumulation schedule.
Principal and contributions
The starting investment plus scheduled annual and monthly deposits form the principal you put into the account. The result cards separate contributed money from earned interest so growth is easier to understand.
Compound growth
Compounding frequency controls how often interest is credited. Annual, monthly, daily, and continuous compounding can create different effective balances even when the stated annual rate is the same.
Tax and inflation
Taxes can reduce interest retained in the account, while inflation can reduce future purchasing power. The buying-power output estimates what the ending balance may be worth after inflation adjustment.
Simple interest vs. compound interest
Simple interest is calculated as principal × rate × time. It is useful for quick examples, but many real-world savings and investment projections use compound interest because interest is credited repeatedly. Compound interest recalculates each period on the growing balance, which is why interest can begin to earn interest.
Contribution timing
Deposits made at the beginning of a compounding period have more time to grow than deposits made at the end. This calculator lets you model either timing, which is useful when comparing automatic savings plans, annual deposits, or recurring monthly contributions.
Compounding frequency
Use the compounding selector to compare annual, semiannual, quarterly, monthly, semimonthly, biweekly, weekly, daily, or continuous compounding. More frequent compounding usually produces a higher effective balance when all other assumptions are unchanged.
Rule of 72
The Rule of 72 is a quick mental estimate for doubling time. Divide 72 by the annual interest rate percentage to estimate the number of years needed to double a balance. For example, at 8%, 72 ÷ 8 is about 9 years.
How to use this calculator
Enter the initial investment, recurring contributions, annual interest rate, compounding frequency, contribution timing, tax rate, inflation rate, and investment length. Review the ending balance first, then compare total principal, interest earned, contribution totals, and inflation-adjusted buying power in the result cards, charts, and schedule.
Related calculators
Investment Calculator · Compound Interest Calculator · Average Return Calculator · ROI Calculator
⚠ Disclaimer: This calculator is for education and planning only. Actual investment returns, taxes, fees, inflation, account rules, and contribution timing can vary.
Interest Calculator FAQ
Answers about compound interest, simple interest, contribution timing, compounding frequency, taxes, inflation, buying power, and accumulation schedules.
What does an interest calculator estimate?
It estimates ending balance, total principal, total contributions, total interest, and inflation-adjusted buying power from the starting investment, recurring deposits, compounding assumptions, tax rate, and inflation rate.
What is compound interest?
Compound interest means interest is earned on both the original principal and the interest already added to the balance. This creates the interest-on-interest effect that can become powerful over long periods.
How is simple interest different?
Simple interest is calculated only on the original principal. Compound interest recalculates on the growing balance, so it can produce more growth when interest is credited repeatedly.
Why does compounding frequency matter?
Compounding frequency determines how often interest is added to the balance. More frequent compounding can increase the effective return when the stated annual rate stays the same.
Should contributions be made at the beginning or end of a period?
Beginning-of-period contributions have more time to earn interest than end-of-period contributions. The difference can be small in the short run but meaningful over long horizons.
Does this calculator include tax on interest?
Yes. The tax rate input estimates how much interest is lost to taxes before it remains in the projected balance.
Why include inflation?
Inflation reduces purchasing power. The buying-power result discounts the future balance by the inflation assumption so you can compare the ending balance in today's dollars.
What is the Rule of 72?
The Rule of 72 is a quick estimate for doubling time. Divide 72 by the annual interest rate percentage to estimate the years needed to double a balance.
Are the results guaranteed?
No. The results are educational estimates. Actual returns, tax treatment, fees, inflation, compounding rules, and contribution timing can differ.