Educational guide · Updated 6 de October de 2026 · 8 min read
An amortization schedule explains how a loan is repaid over time. For each period, it separates payment, interest, principal reduction and remaining balance. Its value extends beyond the final payment date: it shows what would still be owed at an intermediate point, how much of each payment reduces debt and how another assumption changes the loan. Simuily leaves the annual schedule visible to support that reading. Calculations are monthly and the displayed table groups periods, so an annual row summarises several payments rather than representing a single yearly instalment.
Begin with the opening balance
The starting row shows principal before repayments in the schedule. For an existing mortgage, use today's outstanding debt rather than the amount originally borrowed. Past interest cannot be avoided by a decision made now. Also identify the remaining months or years. A schedule suitable for a new loan may be wrong for an existing one if it uses the original duration. Recording the reference date alongside the balance prevents two starting positions being mixed. This matters particularly when comparing an overpayment today with a projection prepared before the latest regular instalment was made.
Follow the sequence of one monthly payment
For a level payment loan using nominal monthly interest, the period's interest is the opening monthly balance multiplied by the annual rate divided by twelve. The payment covers that interest and uses the remainder to reduce principal. The next opening balance is then calculated. Repeating the process creates the schedule. Interest is not charged on the entire original principal throughout the loan's life because the balance falls. Nor is the whole payment deducted from principal, because part pays for borrowing. Reconstructing the first period is a useful way to detect a rate convention mistake.
Distinguish annual flows from cumulative figures
Principal repaid during a year is the sum of its monthly principal components. Cumulative principal repaid includes previous years too. The same distinction applies to interest. Adding a cumulative column row by row counts the same payments repeatedly. To obtain a total, use the final cumulative value or sum only individual year amounts. This is especially important when exporting CSV and building another spreadsheet. Clear column names and units prevent formulas from mixing period flows with balances measured at specific dates. Both types of information are useful, but they cannot be added indiscriminately.
Check straightforward accounting relationships
At each date, opening principal minus cumulative principal repaid should equal outstanding debt, apart from display rounding. Cumulative principal and interest payments should equal their two components combined. At the end of a fully amortizing loan, the balance should reach zero. If it does not, check for a balloon payment, interest only period, another convention or incorrect inputs. A lender may use different dates and rounding, but differences should be explainable. Do not manually adjust numbers simply to hide a discrepancy that has not been understood. Use the identities to locate its source first.
Use intermediate dates in decisions
If a sale is expected in seven years, the balance at that point may be more relevant than thirty year total interest. For an overpayment, use the remaining debt and duration on the payment date. If the rate changes, the later schedule needs recalculation under new conditions. A constant rate curve is a scenario, not a guarantee for a variable rate mortgage. The schedule's value is making these relationships visible and supporting specific contractual questions, rather than presenting every future figure as fixed. It can also show why a relatively long history of payments does not imply a small remaining balance.
Frequently asked questions about schedules
Why is interest greater near the beginning? At a constant rate, the balance used to calculate it is larger. Does the payment automatically fall each year? Not in a level payment model; its internal split changes. Is insurance included as interest? No; entered additional expenses sit outside principal and interest schedules. Can outstanding balances be added to calculate total debt? No, each row describes the same debt at a different date. The workshop reconstructs the first year and complete horizon so payments, columns and closing balances can be checked against one another.
Practical workshop: from the loan to the monthly budget
Consider hypothetical financing of 150,000.00 over 20 years at a constant nominal annual rate of 4%, divided by twelve to calculate monthly interest. Add 100.00 per month for other housing expenses. These are educational inputs, not an available offer. The loan uses monthly principal and interest payments, without an interest only period or a final balloon payment. If your contract uses another convention, review the rate selector and its written conditions before comparing the contractual figures with this example. Matching a rate number alone does not make two repayment models equivalent.
The calculated principal and interest payment is 908.97. Adding the other expenses entered gives a monthly budget of 1,008.97. These figures answer different questions. The first repays the loan, while the second includes costs that do not reduce debt. Accordingly, the loan table does not include those extras as interest or deduct them from the outstanding balance. Keeping categories separate avoids attributing a maintenance expense to financing or assuming that an insurance premium repays part of the principal. It also makes the calculation easier to compare with an itemised lender statement.
Reconstruct the first year
The starting debt is 150,000.00. Under this example's convention, first month interest is 500.00. The rest of the payment reduces principal. During the first twelve months, cumulative interest is approximately 5,909.02 and principal repaid is 4,998.63. At the end of the first year, the outstanding balance is 145,001.37. Small differences from a real contract may arise from rounding or actual payment dates. The model retains internal precision and rounds displayed figures for readability. A lender's statement may instead apply rounding at each monthly step or use a different day counting method.
Check two relationships: starting principal minus principal repaid should equal outstanding debt, and payments made should split into principal and interest. Do not add the entire original loan to the sum of payments when calculating total repayment, because those payments already contain principal. Also avoid describing every unit of principal repaid as a financing expense: reducing an obligation and paying interest have different effects on your wealth. These simple identities help detect copying mistakes when moving the annual schedule into another spreadsheet or comparing it with a manually prepared budget.
Examine the whole horizon without losing the detail
If the rate remained unchanged throughout the term, payments would total 218,152.92 and interest would be 68,152.92. This total excludes purchase taxes, insurance, charges and other expenses outside the loan entered. A financed cost that increases principal belongs in the starting loan amount. A cost paid in cash belongs in the cash budget. Do not include it both ways. Distinguishing initial cash outlay, recurring expenses and principal repayment makes it easier to compare offers with different structures, such as an upfront fee versus a higher ongoing interest rate.
Read several annual rows rather than only the last one. Outstanding debt may matter if you expect to sell, move or refinance before final maturity. A substantial balance may remain at that date even after many regular payments. Interest scheduled after that date would not automatically be part of your ownership period if the loan ends sooner. An early exit analysis also needs the amount required to discharge the outstanding balance and any applicable charges, based on the lender's documents and the specific transaction. The full term interest total answers a different question from a five year ownership budget.
Test sensitivity to another interest rate
As a further exercise, increase the rate by 2 percentage points while keeping principal and duration unchanged. The new payment would be 1,074.65, a monthly difference of 165.68. This scenario does not predict that such an increase will occur. It shows how the budget responds to a different borrowing cost. For a mortgage with future resets, a proper reset calculation would start with the outstanding balance and remaining duration at the reset date. This example instead compares two conditions from the same starting position so the effect of the rate can be isolated.
Add the payment difference to the household budget and examine the margin left after essential commitments. If the result looks too tight, the variables to review may include price, borrowing amount, down payment, duration or purchase date. Extending duration also changes interest and exposure, so a lower payment does not automatically resolve every concern. Record the purpose of each alternative. That distinguishes a choice intended to improve monthly flexibility from one intended to reduce total cost or limit future uncertainty. It also makes a joint decision easier to discuss with another household member.
Before applying the exercise to a real transaction
Choose country and currency separately. Currency labels amounts; it does not convert them using an exchange rate. Country context points toward relevant sources but does not determine your taxes or reproduce every local contract. Confirm the rate, schedule, charges, upfront expenses and reset conditions against the actual transaction documents. Is the result a lending approval? No. Is it a property valuation? No. It is an explanatory calculation that helps you approach the discussion with specific questions, a record of assumptions and a clearer understanding of the payments that need to be checked before you proceed.