💰 Interest Calculator
This compound interest calculator helps estimate the growth of investments by calculating accumulated interest and final balances for both an initial principal and any additional recurring contributions. It also allows optional considerations such as taxes on interest earnings and the effects of inflation.
| Initial Investment ($): | |
| Annual Contribution ($): | |
| Monthly Contribution ($): | |
| Interest Rate (% per year): | |
| Compound Frequency: | |
| Years to Grow: | |
| Contribution Timing: | |
| Tax Rate on Interest (%): | |
| Inflation Rate (% per year): |
Interest Overview
Interest is the amount paid by a borrower to a lender for the use of money, expressed either as a percentage or a fixed amount. It is a fundamental concept underlying most financial systems and instruments.
There are two primary types of interest: simple interest and compound interest.
Simple Interest
Simple interest is calculated only on the original principal. For example, if Derek borrows $100 at a 10% annual interest rate, he pays $10 in interest after one year, totaling $110.
If the loan extends to two years with annual interest, he pays $10 each year, resulting in $120 total repayment.
The basic formula is: interest = principal × rate × time
For more frequent calculations (e.g., monthly), the formula adjusts by dividing time by the frequency. However, simple interest is rarely used in practice; most real-world scenarios involve compounding.
Compound Interest
Compound interest builds on both the principal and previously earned interest. Using the same example, Derek's first year interest is $10, bringing the total to $110. In the second year, interest is calculated on $110, resulting in $11. The final repayment becomes $121 instead of $120 under simple interest.
More frequent compounding leads to higher returns over time. Continuous compounding yields the highest possible return due to its mathematical limit.
Rule of 72
The Rule of 72 is a quick way to estimate how long it takes for money to double. Divide 72 by the interest rate to find the approximate number of years. For example, at 8%, it takes about 9 years for $1,000 to double.
Fixed vs. Floating Rates
Interest rates can be fixed (unchanging) or floating (variable). Floating rates are tied to benchmarks like the Federal Reserve rate or LIBOR, with banks adding a margin for profit. This calculator uses fixed rates.
Contributions
The calculator supports periodic contributions, useful for regular saving. Contributions made at the end of a period earn slightly less interest than those made at the beginning.
Taxes
Interest income may be taxed depending on the source. Taxes can significantly reduce final returns. For instance, savings growing to $320.71 without taxes may drop to $239.78 with a 25% tax rate.
Inflation
Inflation reduces purchasing power over time. Historically averaging around 3% in the U.S., it can significantly impact real returns. To maintain value, investments must outpace both inflation and taxes, which can be challenging.
Overall, taxes and inflation together make it difficult to grow real wealth, requiring consistent and sufficiently high returns.