Interest Calculator

An interest calculator projects how an investment or savings balance grows over time given an initial deposit, regular contributions, an annual interest rate, and a compounding frequency. It also models the effect of taxes and inflation, so you can see not just what your future balance will be — but what that balance will actually be worth in today's purchasing power.

What this calculator estimates

• Final balance — total value of the account after contributions and compounding over the chosen period
• Total contributions — the sum of your initial deposit plus all recurring deposits made over the period
• Total interest earned — the growth generated by compounding, separate from your own contributions
• After-tax balance — balance after applying a simplified flat tax rate to the interest earned
• Real value (inflation-adjusted) — the after-tax balance expressed in today's purchasing power, accounting for the eroding effect of inflation over time

Simple Interest vs Compound Interest

Simple interest

Simple interest is calculated only on the original principal — it does not earn interest on previously earned interest. The formula is:

Interest = Principal × Rate × Time

For example, $10,000 at 5% simple interest for 10 years earns exactly $5,000 in interest, for a final balance of $15,000. Simple interest is used in some short-term loans, savings bonds, and certain fixed-term products.

Compound interest

Compound interest is calculated on both the original principal and on the interest already added to the account. Previously earned interest begins earning interest of its own — which is the core mechanism behind long-term wealth building. The formula is:

A = P × (1 + r/n)^(n×t)

Where A is the final amount, P is the principal, r is the annual interest rate (decimal), n is the number of compounding periods per year, and t is time in years. The same $10,000 at 5% compounded annually for 10 years grows to $16,289 — $1,289 more than simple interest on the same terms.

The power of compounding over time

Compounding's real advantage appears over long time horizons. At 7% annual compounding with no extra contributions:

• $10,000 after 10 years → $19,672 (gain: $9,672)
• $10,000 after 20 years → $38,697 (gain: $28,697)
• $10,000 after 30 years → $76,123 (gain: $66,123)

The growth in the third decade ($37,426) is nearly four times the growth in the first decade ($9,672). This acceleration is why starting to save and invest early matters so much — each additional year of compounding produces more growth than the previous one.

Compounding Frequency

More frequent compounding increases the effective annual yield above the stated annual rate. At 6% APR, here is how the effective annual rate differs by compounding frequency:

• Annually: Effective rate = 6.000%
• Quarterly: Effective rate = 6.136%
• Monthly: Effective rate = 6.168%
• Daily: Effective rate = 6.183%

For short time horizons or small balances, the difference between annual and monthly compounding is minimal. Over decades and with large balances, even a 0.168% difference in effective rate compounds into meaningful additional growth. Most savings accounts and CDs compound daily or monthly; most retirement accounts compound based on the underlying investments.

The Rule of 72

The Rule of 72 is a quick mental shortcut for estimating how long it takes an investment to double at a given interest rate:

Years to double ≈ 72 ÷ Annual Interest Rate (%)

• At 3%: doubles in ≈ 24 years
• At 6%: doubles in ≈ 12 years
• At 9%: doubles in ≈ 8 years
• At 12%: doubles in ≈ 6 years

The Rule of 72 also works in reverse: to double your money in 10 years, you need an annual return of approximately 7.2%. Use it to quickly sanity-check projections, compare investment scenarios, or understand the real cost of inflation — at 3% inflation, the purchasing power of $1 halves in about 24 years.

The Impact of Regular Contributions

Adding regular contributions dramatically amplifies long-term growth. Consider two people both earning 7% annually:

• Person A invests $10,000 as a one-time lump sum and never adds more. After 30 years: $76,123.
• Person B starts with nothing but saves $200 per month. After 30 years: $243,994.

Person B contributed $72,000 of their own money over 30 years. The remaining $172,000 is pure compounding. A consistent monthly savings habit produced more than three times the wealth of a $10,000 one-time lump sum — illustrating that contribution rate often matters as much as the interest rate, especially over long horizons.

Taxes and Inflation

How taxes reduce real returns

Interest and investment gains are taxable in most jurisdictions outside of tax-advantaged accounts. A 6% return with a 25% tax rate leaves an after-tax return of 4.5%. Over 30 years, the difference between a 6% and 4.5% return on $10,000 is over $40,000. Tax- advantaged accounts — such as Roth IRA, 401(k), and HSA in the US — are designed to reduce this drag, either by sheltering gains from tax entirely or by deferring tax until retirement when your rate may be lower.

How inflation erodes purchasing power

Inflation means that the same dollar buys less over time. At 3% annual inflation, a $100,000 balance in 20 years has the purchasing power of about $55,000 in today's dollars. A savings balance that looks impressive in nominal terms can be disappointing when expressed in real (inflation-adjusted) terms. The real value figure in this calculator adjusts your after-tax balance for inflation so you can see whether your savings are genuinely growing or merely keeping pace with rising prices.

USA: Investment Accounts and Tax

401(k) and Traditional IRA

Contributions to a traditional 401(k) or IRA reduce your taxable income in the year of contribution, and all growth is tax-deferred. You pay ordinary income tax on withdrawals in retirement. For projections in tax-deferred accounts, the tax rate in this calculator represents your expected marginal rate at withdrawal — often lower than your current working rate.

Roth IRA and Roth 401(k)

Roth accounts accept after-tax contributions, but all qualified withdrawals — including every dollar of growth — are completely tax-free. For Roth projections, set the tax rate to 0% in this calculator. The benefit is greatest when you expect to be in a higher tax bracket in retirement than you are today, or when you want to lock in current rates.

Taxable brokerage accounts

Interest from bonds and savings products is taxed as ordinary income. Qualified dividends and long-term capital gains are taxed at preferential rates (0%, 15%, or 20% depending on total taxable income). A flat rate in this calculator is a simplification; actual tax depends on the type of investment income and your bracket.

Frequently Asked Questions

Is the projected return guaranteed?

No. This calculator models a fixed, constant rate of return for planning and comparison purposes. Real-world investment returns fluctuate year to year and can be negative. For a high-yield savings account or CD with a fixed APY, the projection is reliable as long as the rate does not change. For stock market investments, treat the projection as one scenario in a range — always model several rates to understand the spread of outcomes.

What interest rate should I use?

For bank savings accounts or CDs, use the current APY from your institution. For a broad stock market index fund, historical real returns have averaged 7–10% annually before inflation over long periods, though past performance does not guarantee future results. For a conservative bond-heavy portfolio, 3–5% is a common planning assumption. Always run scenarios at a low, mid, and high rate to understand the range of possible outcomes rather than anchoring to a single number.

What is the difference between APR and APY?

APR (Annual Percentage Rate) is the stated annual rate without accounting for the effect of within-year compounding. APY (Annual Percentage Yield) reflects the actual annual yield after compounding and is always equal to or higher than APR. For savings accounts, banks typically advertise APY because it shows the actual yield you receive. For loans, APR is quoted. When entering a rate in this calculator, use the stated annual rate (APR) and set the compounding frequency to match your account's terms.

How much does starting early really matter?

Starting early is one of the most powerful levers in personal finance. Consider two people who both earn 7% annually. Person A invests $5,000/year from age 25 to 35 (10 years, $50,000 total) then stops completely. Person B invests $5,000/year from age 35 to 65 (30 years, $150,000 total). At age 65, Person A has approximately $602,000 and Person B has approximately $472,000 — despite Person A contributing $100,000 less. Starting 10 years earlier, even for a shorter period, produced greater final wealth entirely due to compounding.

How does inflation affect my savings goal?

If your goal is to have a certain amount available in the future in real (purchasing power) terms, you need to save more than the nominal target suggests. For example, if you want $500,000 of today's purchasing power in 25 years at 3% inflation, you actually need a nominal balance of about $1,046,000. Use the inflation field in this calculator to see what your target nominal balance must be, and plan contributions accordingly.