20, 25 or 30 year mortgages: payments, interest and flexibility
Understand duration while holding principal and rate constant, then connect payments with your budget.
Read guide →Simuily / Learn and calculate
Compound interest
Your scenario, explained
Change the inputs and explore the result over time. Figures are estimates under the assumptions you enter.
How it changes over time
Every year, in view
Year-by-year evolution
What the result can tell you
Understand before deciding
Compound interest with monthly contributions: how to read your results
Compound interest describes what happens when returns remain in the balance and can themselves earn returns in later periods. Reading a projection requires more than looking at its final value. You need to separate your contributions from assumed growth, understand what the annual rate means and check how the time horizon changes the outcome. This guide explains those steps without treating a mathematical assumption as a promised investment return.
What compounding actually means
Start with 1,000 currency units and a constant 5% effective annual return, with no fees, taxes or further contributions. At the end of year one, the balance is 1,050. In year two, the same percentage applies to 1,050, producing 1,102.50. The extra 2.50 compared with two simple increases of 50 comes from earning a return on the previous return.
Without additional contributions, the formula is starting capital multiplied by (1 + the periodic rate) raised to the number of periods. The units must agree. Applying an annual rate directly to a number of months is a serious mistake: it can create large, impressive looking figures that do not describe the intended calculation.
Effective rates and nominal rates
The savings calculator in Simuily asks for an assumed effective annual rate. It obtains the equivalent monthly rate by raising (1 + the annual rate) to the power 1/12 and subtracting 1. Twelve monthly periods then reproduce the same annual growth when there are no intervening contributions. Dividing an effective annual rate by twelve gives a different convention.
A nominal annual rate compounded monthly is handled differently: divide the nominal rate by twelve and compound the monthly result. Neither label should be left implicit. Before comparing a real savings or investment product with a projection, check how its rate is defined, how frequently interest is credited and whether the displayed figure includes costs.
Why contributions need their own line
Saving 200 each month for ten years means adding 24,000. If you started with 2,000, total contributions are 26,000. Projected growth is the final balance minus that amount. A chart that calls the entire final balance profit would confuse your own money with the assumed return.
Contribution timing matters too. A beginning-of-month contribution participates in that month's growth; an end-of-month contribution begins participating in the next period. This calculator uses end-of-month contributions. A comparison with another calculator is only meaningful when the timing, rate convention and number of periods match.
One example, three possibilities
This is a hypothetical case. Compare effort, outcomes and timing without treating rates as forecasts. The figures use the same engine as the interactive calculator.
Scenario A
Starting or outstanding principal: 5,000 · Monthly contribution: 150 · Assumed annual rate (%): 4 · Term in years: 10 · Assumed annual inflation (%): 2
- Final balance: 29,405.61 EUR
- Your contributions: 23,000 EUR
- Cumulative growth: 6,405.61 EUR
- In today’s money: 24,122.84 EUR
Scenario B
Starting or outstanding principal: 5,000 · Monthly contribution: 200 · Assumed annual rate (%): 4 · Term in years: 10 · Assumed annual inflation (%): 2
- Final balance: 36,740.41 EUR
- Your contributions: 29,000 EUR
- Cumulative growth: 7,740.41 EUR
- In today’s money: 30,139.93 EUR
Scenario C
Starting or outstanding principal: 5,000 · Monthly contribution: 150 · Assumed annual rate (%): 4 · Term in years: 15 · Assumed annual inflation (%): 2
- Final balance: 45,703.27 EUR
- Your contributions: 32,000 EUR
- Cumulative growth: 13,703.27 EUR
- In today’s money: 33,958.21 EUR
Questions that help explain your figures
What is starting capital?
It is the amount at the start of the calculation: an assigned savings balance or the stated loan principal, depending on the tool.
What is a recurring contribution?
It is new money added each period, separate from any growth generated by the balance.
How do simple and compound interest differ?
Simple interest does not include past interest in its calculation base. Compound interest allows previous interest to participate in later periods.
What is an effective annual rate?
It represents an equivalent one-year change under the stated convention. Its monthly equivalent is (1 + annual rate) raised to 1/12, minus 1.
Is the entered return guaranteed?
No. It is a model assumption, not a determination that a real product will provide that result.
Can I calculate at zero interest?
Yes. Savings become starting capital plus contributions, while a repayment loan divides principal by the number of payments.
Are contributions made at the beginning or end of the month?
These tools use contributions at the end of each month. A different timing convention can produce a different result from the same amounts.
What is a value in today’s money?
It adjusts a future balance using assumed inflation. It helps interpret purchasing power and does not automatically deduct taxes or fees.
Does switching currency convert my money?
No. It changes the displayed unit here. Actual conversion needs an exchange rate and its date.
Must language and country match?
No. You can read in English about a Spanish transaction or use Spanish for another market. Location determines local context.
Does the mortgage payment include all housing costs?
No. Principal and interest are separate from recurring costs you enter. Initial purchase expenses form another cost group.
What does repaying principal mean?
It means returning some of the borrowed money. Interest is a separate cost and does not itself reduce principal.
What is an amortisation schedule?
It breaks down payments, interest, principal repaid and remaining balance, under stated rate and timing assumptions.
What is LTV?
It is the loan divided by a reference property value. Simuily’s purchase model uses the price you enter, not a lender’s appraisal.
Can I use APR or TAE as the payment interest rate?
Do not automatically substitute them for the contractual rate. They are locally defined cost measures that can include more than periodic interest.
Is a longer term always better?
With principal and rate held constant, it usually lowers payments and raises total interest. Both effects should be shown.
What happens if the interest rate changes?
A constant-rate schedule no longer describes the whole contract. Model the reset and recalculate from the balance at that date.
Do lower payments and a shorter term save the same interest?
Not necessarily. In a comparable fixed-rate model, keeping payments and shortening the term normally saves more interest; lower payments release monthly cash.
What are closing or purchase costs?
They are expenses associated with completing a purchase or financing in a market. Separate them and check the country, region and transaction.
Are taxes calculated automatically when I select a country?
Not in these tools. Costs are manual. Entered costs should correspond to your local transaction.
Why can two calculators disagree?
Check nominal versus effective rates, contribution timing, payment frequency, rounding and included costs. Match conventions before comparing.
What is a scenario?
It is a set of assumptions. Comparing scenarios shows how results change, not the probability of each outcome.
What does the CSV download contain?
It includes the yearly table, entered values, currency, country context and calculation assumptions. Amounts are exported to two decimal places for review.
Do I need an account?
No. The tools, guides and CSV downloads are public and do not require an account.
Guides to understand each decision
A savings budget: turning income into sustainable contributions
Organise spending, annual bills and monthly headroom to find a contribution you can sustain.
Read guide →Amortization schedules: reading principal, interest and outstanding debt
Check each column and understand why a payment’s principal and interest components change over time.
Read guide →Compound interest with monthly contributions: how to read your results
Understand contributions, compounding conventions, inflation and scenario comparisons before relying on a projected balance.
Read guide →Useful terms on this page
Principal
The starting savings balance or loan amount used by the model.
Contribution
New money added to a balance.
Inflation
A change in the price level; its future value in a model is an assumption.
Payment
A recurring payment whose included components must be stated.
Amortisation
Repayment of loan principal.
Liquidity
The availability of money for use.
Term
The duration of a plan or loan.
Nominal interest
A rate quoted under a frequency and convention that must be stated.
Effective rate
An equivalent rate reflecting compounding over the stated period.
LTV
Loan divided by the specified reference property value.
Down payment
Money paid towards the purchase price, separate from other expenses.
Scenario
A set of assumptions used to compare outcomes.
Emergency fund
Money reserved for unexpected needs or an income interruption. Its size and access should reflect your expenses and circumstances.
Sinking fund
Money allocated to an expected expense with an approximate date, separate from an emergency reserve.
Nominal balance
An amount expressed in the currency and date of the calculation, without adjusting purchasing power for inflation.
Real balance
A balance adjusted by a price factor to express purchasing power at a reference date, not a separate bank account.
Percentage point
The difference between percentages: moving from a 3% rate to 4% is a one percentage point increase.
Fixed charge
A cost expressed as a money amount, distinct from a percentage of assets or a contribution.
Semiannual compounding
A nominal rate convention with compounding twice a year. Its monthly equivalent is (1 + annual rate/2) raised to 1/6, minus 1.
Outstanding balance
Principal still owed on a particular date. It differs from the sum of future payments, which may also contain interest.