Simple vs Compound Interest: Linear Growth vs Interest on Interest
Compare simple interest and monthly compound interest, learn when each model fits, and use worked examples aligned to the calculators.
The Core Difference
Simple interest grows in a straight line because interest is calculated only on the original principal. Compound interest grows faster because each period starts from a balance that already includes previous interest. That difference is small over short periods and much larger over long periods, high rates, or frequent compounding.
Simple vs compound interest
The math model changes what earns interest next period.
Simple interest
Only the original principal earns interest.
Compound interest
Principal plus accumulated interest earns future interest.
Recurring deposits
The compound calculator also models monthly contributions at month end.
Worked Example: $10,000 at 6% for 3 Years
With simple interest, $10,000 at 6% for 3 years earns $1,800, for a total of $11,800. With monthly compound interest and no monthly contributions, the same starting amount grows to about $11,966.81. The compounding difference is about $166.81 over three years.
Swipe sideways to compare columns.
| Model | Interest earned | Final amount |
|---|---|---|
| Simple interest | $1,800.00 | $11,800.00 |
| Monthly compound interest | $1,966.81 | $11,966.81 |
| Compound advantage | $166.81 | $166.81 higher |
Three-year final balance
Same principal and rate; compounding earns interest on prior interest.
Simple
Monthly compound
Why Time Expands the Gap
At $10,000 and 6% for 10 years, simple interest produces $6,000 of interest and a $16,000 total. Monthly compounding with no deposits produces about $18,193.97. The extra $2,193.97 comes from interest earning additional interest over time.
Swipe sideways to compare columns.
| Model | Total contributions/principal | Interest earned | Final amount |
|---|---|---|---|
| Simple interest | $10,000.00 | $6,000.00 | $16,000.00 |
| Monthly compound interest | $10,000.00 | $8,193.97 | $18,193.97 |
Simple vs compound gap over time
The gap is modest at 3 years and much larger at 10 years.
3 years
10 years
Monthly Contributions Change the Question
The compound-interest calculator can also model monthly contributions. For example, $10,000 starting balance, $300 monthly contribution, 6% annual return, and 10 years produces about $67,357.77. Total contributions are $46,000, and interest earned is about $21,357.77. That example is not a simple-interest comparison; it is a savings-growth projection with cash flows.
Swipe sideways to compare columns.
| Input or output | Value |
|---|---|
| Starting amount | $10,000 |
| Monthly contribution | $300 |
| Annual rate | 6% |
| Time | 10 years / 120 months |
| Total contributions | $46,000.00 |
| Interest earned | $21,357.77 |
| Future value | $67,357.77 |
Which Model Fits Your Situation?
Use simple interest when the agreement explicitly calculates interest only on principal, such as some short-term notes or classroom examples. Use compound interest when interest is periodically credited, reinvested, or added to the balance. Many real loans and accounts have fees, payment schedules, taxes, promotional rules, or APR/APY definitions that are more specific than either simplified model.
Choosing the right interest model
Start with the contract or account terms, not the formula you prefer.
Read the terms
Look for APR, APY, compounding frequency, payment timing, and fees.
If only principal earns interest
Simple interest may match the agreement.
If interest is credited to balance
Compound interest is usually the better model.
If payments or deposits occur
Use a calculator that models cash-flow timing.
Validate before decisions
For real borrowing or investing, confirm figures with the provider or a qualified professional.
Borrowers and Investors Care in Opposite Ways
For savers and investors, compounding can be beneficial because earnings can generate more earnings. For borrowers, compounding can increase cost if unpaid interest is added to the balance. The same math can help you or hurt you depending on which side of the agreement you are on.
Open the Simple Interest CalculatorCalculate linear interest, total interest, and final amount from principal, annual rate, and years.Open the Compound Interest CalculatorModel monthly compounding with starting balance, monthly contributions, annual rate, and time horizon.Open the Interest CalculatorCompare simple and compound interest scenarios side by side when you need both views together.Simple vs Compound Interest FAQ
What is the main difference between simple and compound interest?
Simple interest is calculated only on principal. Compound interest is calculated on principal plus accumulated interest.
What formula does the simple-interest calculator use?
Interest equals principal times annual rate times years, and total amount equals principal plus interest.
How does the compound-interest calculator compound?
It compounds monthly and adds the monthly contribution after interest is applied for each month.
Why is compound interest higher over time?
Because prior interest becomes part of the balance that earns future interest.
Is compound interest always better?
Not always. It helps savers and investors when returns are positive, but it can increase costs for borrowers.
What is APY?
APY reflects annual yield after compounding. It is different from a simple nominal rate.
What is APR?
APR is a quoted annual borrowing rate, but product rules, fees, and compounding details can vary.
Can simple interest apply to loans?
Yes, if the agreement calculates interest only on the original principal or outstanding principal without compounding unpaid interest.
Can compound interest apply to savings?
Yes. Savings and investment accounts often credit interest or returns back into the balance.
Should I use these examples for a real financial decision?
Use them as educational estimates, then verify real account or loan terms with the provider or a qualified professional.
Written by
Do The Calculation Team
Do The Calculation Editorial Board
The Do The Calculation Editorial Board is comprised of software engineers, finance analysts, and technical contributors focused on building clean, accurate, and easy-to-use calculator tools.
Reviewed & Verified By
Dr. Elizabeth Vance, PhD
Senior Editorial Board Member (Finance)
Former investment bank strategist and university lecturer with 15+ years of research in compound growth modeling, asset allocation, and annuity projections. Dr. Vance reviews all core investment and retirement tools to ensure absolute alignment with actuarial standards.