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Calculate compound interest on savings or investments. See how your money grows over time with monthly or yearly compounding.
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- Skriv inn Principal (kr)
- Skriv inn Annual Rate (%)
- Skriv inn Years
- Skriv inn Compounding
- Klikk på Beregn-knappen
- Les resultatet som vises under kalkulatoren
The Compound Interest Formula Explained
Compound interest is interest calculated on both the initial principal and all previously accumulated interest. The formula is:
A = P × (1 + r/n)^(n×t)
Where:
- A = Final amount (principal + interest)
- P = Principal (initial investment)
- r = Annual interest rate as a decimal (e.g., 0.07 for 7%)
- n = Compounding frequency per year (1=annually, 12=monthly, 365=daily)
- t = Time in years
Worked example: kr105,000 invested at 7% annually for 20 years, compounded monthly:
A = 10,000 × (1 + 0.07/12)^(12×20) = 10,000 × (1.005833)^240 = kr420,672
Compare to simple interest over the same period: kr105,000 + (kr105,000 × 0.07 × 20) = kr252,000. Compounding adds an extra kr168,672 in this example.
Compounding Frequency: Does It Matter?
The more frequently interest compounds, the more you earn — but the differences diminish at higher frequencies:
| Compounding Frequency | kr105,000 at 8% for 10 years | Total Interest Earned |
|---|---|---|
| Annually (n=1) | kr226,684 | kr121,684 |
| Quarterly (n=4) | kr231,840 | kr126,840 |
| Monthly (n=12) | kr233,058 | kr128,058 |
| Weekly (n=52) | kr233,362 | kr128,362 |
| Daily (n=365) | kr233,656 | kr128,656 |
| Continuous | kr233,678 | kr128,678 |
The difference between monthly and daily compounding is less than kr630 on a kr105,000 investment over 10 years. The frequency matters far less than the interest rate and the time horizon.
Continuous compounding uses the formula A = P × e^(r×t), where e ≈ 2.71828. This is the theoretical maximum and is used in financial modeling, though no real product compounds continuously.
The Power of Starting Early: Time vs. Amount
Time is the single most powerful variable in compound interest. This example illustrates why starting early matters more than investing more:
| Investor A (Early) | Investor B (Late) | |
|---|---|---|
| Start investing age | 25 | 35 |
| Stop investing age | 35 | 65 |
| Years of contribution | 10 years | 30 years |
| Annual contribution | kr52,500/year | kr52,500/year |
| Total contributed | kr525,000 | kr1,575,000 |
| Value at age 65 (7% return) | kr6,321,735 | kr5,677,780 |
Investor A contributed three times less money but ends up with kr640,500 more — purely because of the extra 10 years of compounding. This is the most important financial lesson of compounding: time in the market beats amount invested.
The Rule of 72 and Other Mental Math Shortcuts
The Rule of 72 estimates how long it takes to double your money: divide 72 by the annual interest rate.
- At 6%: 72 / 6 = 12 years to double
- At 8%: 72 / 8 = 9 years to double
- At 10%: 72 / 10 = 7.2 years to double
- At 1% (savings account): 72 / 1 = 72 years to double
The Rule of 114 estimates triple: 114 / rate = years to triple.
The Rule of 144 estimates quadruple: 144 / rate = years to quadruple.
Inflation version: The Rule of 72 works in reverse too. At 3% inflation, your purchasing power halves in 72 / 3 = 24 years. This is why leaving money in a 0.5% savings account during a 3% inflation environment is effectively losing 2.5% purchasing power per year.
Compound Interest in Real Life: Savings, Loans, and Inflation
Compounding works for you in savings accounts and investments — and against you in debt. Understanding both sides is critical:
Savings and investments (compounding works FOR you):
- S&P 500 historical average return: ~10% nominal, ~7% after inflation. kr10,500 invested in 1990 would be worth over kr220,500 in 2024.
- High-yield savings accounts (currently 4.5–5%): kr105,000 grows to kr163,065 in 10 years — significantly better than traditional 0.01% accounts.
- 401(k) with employer match: The match is effectively a 50–100% instant return on contributed dollars, amplified by decades of compounding.
Debt (compounding works AGAINST you):
- Credit card at 22% APR: A kr52,500 balance with minimum payments grows to over kr84,000 in 3 years if not paid down aggressively.
- Student loans at 6.5%: kr315,000 over 10 years costs kr214,588 in total interest — 68% extra.
- Payday loans at 400%+ APR: A kr5,250 loan can balloon to kr15,750 in just a few rollovers.
Compound Interest vs Simple Interest: Key Differences
Simple interest is calculated only on the original principal: Interest = P × r × t
Compound interest is calculated on the principal plus accumulated interest each period.
Over short periods, the difference is small. Over long periods, it is dramatic:
| Years | kr105,000 at 7% Simple | kr105,000 at 7% Compound (annual) | Difference |
|---|---|---|---|
| 5 | kr141,750 | kr147,273 | kr5,523 |
| 10 | kr178,500 | kr206,556 | kr28,056 |
| 20 | kr252,000 | kr406,318 | kr154,318 |
| 30 | kr325,500 | kr799,292 | kr473,792 |
| 40 | kr399,000 | kr1,572,322 | kr1,173,322 |
Simple interest is used for short-term loans and some bonds. Compound interest governs savings accounts, mortgages, credit cards, and most investment vehicles.
"Compound interest is one of the most powerful tools in investing. By reinvesting your earnings, your investment can grow exponentially over time — a process that benefits most from starting early and contributing consistently."
💡 Visste du?
- Albert Einstein reportedly called compound interest "the eighth wonder of the world" — though historians cannot verify the quote, the math checks out.
- The concept of compound interest was documented in Babylonian clay tablets from around 1800 BC.
- Benjamin Franklin left $4,444 each to Boston and Philadelphia in his 1790 will, with instructions to accrue at compound interest for 200 years — the fund grew to over $6 million.
Sist oppdatert: March 2026
Frequently Asked Questions
What is the difference between simple and compound interest?
Simple interest is calculated only on the original principal. Compound interest is calculated on the principal plus all previously earned interest. Over time, compound interest grows exponentially while simple interest grows linearly.
How often should interest compound for best results?
More frequent compounding earns slightly more. Daily compounding yields marginally more than monthly, which yields more than annually. However, the differences are small — going from annual to daily compounding on kr105,000 at 8% for 10 years adds only about kr6,972 extra. The interest rate and time horizon matter far more.
What is the Rule of 72?
Divide 72 by the annual interest rate to estimate how many years it takes to double your money. At 6% annual return, your money doubles in approximately 12 years. At 9%, it doubles in about 8 years.
What is a good interest rate for savings?
As of 2024–2025, high-yield savings accounts offer 4.5–5.5% APY. Traditional bank savings accounts offer 0.01–0.5%. For long-term growth, broad market index funds have historically returned about 7% after inflation over multi-decade periods.
How does compound interest affect loans and credit cards?
Compound interest works against you with debt. A credit card balance at 22% APR compounds monthly, meaning unpaid interest gets added to your principal, which then generates more interest. A kr52,500 credit card balance with only minimum payments can take over 20 years to pay off and cost thousands in extra interest.
What is the formula for continuous compounding?
A = P × e^(rt), where e ≈ 2.71828, r is the annual interest rate, and t is time in years. For example, kr105,000 at 5% continuously compounded for 10 years: A = 10,000 × e^(0.05×10) = 10,000 × 1.6487 = kr173,114.
How much do I need to save to become a millionaire?
At 7% annual return: saving kr5,250/month for 30 years accumulates to ~kr5,953,500. Saving kr10,500/month for 30 years reaches ~kr10.13 million. The faster path is starting early — kr2,100/month starting at 22 can reach kr10 million by age 65 at 7% returns.
Does compound interest apply to retirement accounts like 401(k) and IRA?
Yes. Funds within 401(k), IRA, and similar accounts grow through compound returns on investments (stocks, bonds, funds). The tax-deferred or tax-free nature of these accounts amplifies compounding further by preventing annual tax drag on gains.