Turn interest rates into clear outcomes

Interest can look small as a percentage, but it becomes meaningful across months and years. This calculator helps you compare simple versus compound interest, payment timing, and the effect of an optional tax rate.

Simple and compound interest Monthly or yearly rate period Payment frequency options Charts, schedule, compare, PDF export
Final amount
Principal plus interest
Growth view
Chart and schedule
Options
Type, period, tax

How to use the Interest Calculator

  1. 1

    Enter principal and rate

    Input the principal amount and the interest rate. Choose whether the rate is per month or per year.

  2. 2

    Select interest type

    Pick simple interest or compound interest based on the product you are evaluating.

  3. 3

    Set period and options

    Enter the investment period in years or months. Optionally add a tax rate and choose when interest is paid.

  4. 4

    Calculate and compare

    Review charts and schedule, add scenarios, copy results, and export a PDF report.

Detailed guide and references▶

Overview

The Interest Calculator is a financial tool designed to help you understand how money can grow over time through savings and investments. It supports simple interest and compound interest, and it can apply an optional tax rate.

Golden coins on red background
Interest is small per period but powerful across time, especially with compounding

Understanding interest types

Simple interest

Simple interest is calculated only on the original principal amount.

Simple Interest = Principal × Rate × Time

  • Easier to calculate
  • Growth is linear over time

Compound interest

Compound interest is calculated on the initial principal and also on accumulated interest.

A = P(1 + r/n)^(nt)

Where A is the future value, P is principal, r is annual interest rate, n is compounding frequency, and t is time in years.

  • Growth is exponential over time
  • Earnings can accelerate as interest earns interest

The power of compounding

Compounding frequency

  • Annual compounding: once per year
  • Monthly: 12 times per year
  • Daily: interest calculated daily

More frequent compounding generally increases total interest earned.

Time horizon

Time is a major driver of compound growth. Small differences in start date can produce large differences in the final amount.

Factors affecting interest earnings

Interest rate

Small rate differences can have large long term effects.

Inflation

Real return is nominal return minus inflation.

Taxes

Taxes on interest can reduce net returns. Tax advantaged accounts can help.

Fees

Fees reduce effective returns, so consider the net outcome.

The Rule of 72

Years to double = 72 ÷ interest rate

  • At 6%, money doubles in about 12 years
  • At 8%, it takes about 9 years

FAQs

Are the results exact?▶

They are simplified estimates for education. Real products may use different compounding conventions, fees, and tax rules.

Simple vs compound interest?▶

Simple interest is computed on principal only, while compound interest includes interest on previously earned interest.

What does payment frequency do?▶

It changes when interest is credited or paid. Earlier crediting can increase compound growth.

Can I include tax?▶

Yes. Enter an optional tax rate to estimate how tax reduces your interest.

Key takeaways

  • Compound interest can grow faster because interest earns interest
  • Payment timing and compounding frequency can change outcomes
  • Taxes and fees reduce net returns, so compare net results
  • Use scenarios to compare rates, periods, and interest types quickly

Related calculators

Recommended posts

Calculator

Enter principal, rate, options, and period, then press Calculate

These results are for reference only and were developed for educational and testing purposes.

The results shown are for general reference only and may differ from actual product outcomes.