Smarter money planning
How to estimate compound interest, loan payments, mortgage costs, and SIP growth
Compare savings and borrowing scenarios with calculators that show the formula, the assumptions, and worked examples.
How do you calculate compound interest?
Project how a lump sum grows with a fixed rate and the compounding frequency you choose.
Why this formula works
Formula: Future value = initial deposit * (1 + annualRate / 100 / compoundsPerYear)^(years * compoundsPerYear).
We convert the annual rate into a rate per compounding period, then apply that growth rate across the total number of periods. Interest earned is the future value minus the original deposit.
Quick steps
- Divide the annual rate by 100 and by the number of compounding periods each year.
- Multiply the years by the compounding frequency to get the total number of periods.
- Apply the compounded growth formula and subtract the initial deposit to find the interest earned.
Worked examples
| Inputs | Future value | Interest earned |
|---|---|---|
| 1000 at 5% for 10 years, compounded yearly | 1628.89 | 628.89 |
| 5000 at 4% for 5 years, compounded quarterly | 6100.95 | 1100.95 |
What is the monthly payment (EMI) on a loan?
Estimate the monthly payment (EMI), total repayment, and total interest for a standard amortizing loan.
Why this formula works
Formula: Payment = (principal * monthlyRate * (1 + monthlyRate)^months) / ((1 + monthlyRate)^months - 1).
The loan payment formula spreads the principal and the interest into equal monthly installments. When the interest rate is 0, the payment is simply the principal divided by the number of months.
Quick steps
- Convert the annual rate into a monthly rate by dividing by 100 and then by 12.
- Use the amortized payment formula across the full number of months.
- Multiply the monthly payment by the term to find total repayment and subtract the loan amount to find total interest.
Worked examples
| Inputs | Monthly payment | Total repayment | Total interest |
|---|---|---|---|
| 10000 at 6% for 36 months | 304.22 | 10951.90 | 951.90 |
| 25000 at 4.5% for 60 months | 466.08 | 27964.53 | 2964.53 |
How do you estimate a mortgage payment with the French method?
Estimate a fixed-rate mortgage using the financed amount, the standard amortized payment formula, and loan-to-value.
Why this formula works
Formula: Financed amount = property price - down payment. LTV = (financed amount / property price) * 100. Monthly payment uses the amortized loan formula over a term of years × 12 months.
We first calculate how much you actually borrow, then apply the monthly loan payment formula to that financed amount. Loan-to-value shows how large the loan is relative to the property price.
Quick steps
- Subtract the down payment from the property price to get the financed amount.
- Convert the mortgage term into months and calculate the monthly payment with the amortized loan formula.
- Divide the financed amount by the property price to calculate the loan-to-value ratio.
Worked examples
| Inputs | Financed amount | Monthly payment | Total interest | LTV |
|---|---|---|---|---|
| 300000 price, 60000 down, 3.5% for 25 years | 240000 | 1201.50 | 120448.97 | 80% |
| 250000 price, 50000 down, 4% for 20 years | 200000 | 1211.96 | 90870.56 | 80% |
How much can a SIP or regular investment grow over time?
Project a monthly investment plan with an initial amount, recurring contributions, and an expected annual return.
Why this formula works
Formula: Projected value = initialInvestment * (1 + monthlyRate)^months + monthlyContribution * (((1 + monthlyRate)^months - 1) / monthlyRate).
The final value comes from two sources of growth: the initial investment compounding for the full term and the monthly contributions compounding from the end of each month. When the return is 0, the future value is simply the total contributed amount.
Quick steps
- Convert the annual return into a monthly rate and the years into total months.
- Compound the initial investment across the full term.
- Add the future value of the monthly contributions and compare it with the total invested amount to find the growth.
Worked examples
| Inputs | Projected value | Total invested | Growth |
|---|---|---|---|
| 1000 initial, 200 monthly, 7% for 10 years | 36626.62 | 25000 | 11626.62 |
| 5000 initial, 150 monthly, 5% for 15 years | 50661.86 | 32000 | 18661.86 |
Personal finance formulas at a glance
Each calculator uses these core formulass.
| Calculator | Formula summary | Main outputs |
|---|---|---|
| Compound interest | Future value = deposit * (1 + rate / periods)^totalPeriods | Future value and interest earned |
| Loan / EMI | Amortized monthly payment formula | Monthly payment, total repayment, total interest |
| Mortgage | Loan payment formula plus financed amount and LTV | Financed amount, payment, interest, LTV |
| SIP / regular investment | Initial balance growth plus compounded monthly contributions | Projected value, total invested, growth |
What these estimates assume
Each calculator uses these assumptions.
- Compound interest assumes a fixed annual rate and the compounding frequency you choose.
- Loan EMI and mortgage results use equal monthly payments. Mortgage estimates follow the French amortization method commonly used across Europe, including Italy.
- SIP and regular investment growth assume monthly contributions added at the end of each month.
- These estimates do not include taxes, fees, insurance, or rate changes over time.