Personal Loan Calculator: Monthly Payment, Total Interest, and What the Rate Actually Costs
Personal loans use the same French amortization mathematics as mortgages, but with substantially shorter terms (12–84 months in most markets) and materially higher interest rates. Understanding how to use the calculator correctly — and how to read the total cost figures, not just the monthly payment — is what separates good borrowing decisions from expensive ones.
The Formula Behind the Monthly Payment
All personal loan monthly payment calculations use the French amortization formula:
Monthly payment = P × [r(1+r)^n] / [(1+r)^n − 1]
Where:
- P = principal (the amount borrowed)
- r = monthly interest rate = annual rate ÷ 12
- n = total number of monthly payments
Worked example: MXN 50,000 loan at 18% annual TIN, 24 months
- r = 0.18 ÷ 12 = 0.015
- n = 24
- Monthly payment = 50,000 × [0.015 × (1.015)^24] / [(1.015)^24 − 1] = 50,000 × 0.04994 = MXN 2,497/month
- Total repaid: MXN 2,497 × 24 = MXN 59,928
- Total interest: MXN 9,928 (19.9% of the original loan)
Notice: at 18% TIN over 24 months, you pay back nearly 20% of the principal in interest. At 36% TIN (common in Mexico) over the same term, total interest would be approximately MXN 19,500 — nearly 40% of principal.
TIN vs TAE/CAT: The Rate That Includes Everything
Spain — TAE (Tasa Anual Equivalente / APR): Includes TIN plus all mandatory fees (comisión de apertura, insurance premiums if required). Required by law to be disclosed on all Spanish loan documentation.
Mexico — CAT (Costo Anual Total): Mexico’s equivalent of APR. Mandated by the CNBV (Comisión Nacional Bancaria y de Valores) to be displayed prominently in all loan advertising and documentation. The CAT includes the interest rate plus all commissions, fees, and mandatory insurance. It is the correct comparison metric.
Always compare CAT or TAE, not TIN. A loan advertised at 18% TIN but with a 3% opening fee plus mandatory loan-protection insurance may have a CAT above 28%.
2026 Market Rates by Country and Lender Type
Mexico
| Lender type | TIN range | CAT range | Notes |
|---|---|---|---|
| Major bank (BBVA, Santander, Banorte) | 18%–36% | 25%–48% | Stricter eligibility; lower rates for existing customers |
| Digital lender (Kueski, Konfio) | 24%–60% | 35%–90% | Faster approval; shorter terms; higher CAT |
| Department store credit (Coppel, Liverpool) | 30%–80% | 50%–120% | Revolving; highest cost; no collateral |
| Credit union (caja popular) | 15%–28% | 20%–35% | Members only; better rates for solid history |
Spain
| Lender type | TIN range | TAE range | Notes |
|---|---|---|---|
| Major bank (CaixaBank, BBVA, Santander) | 7%–12% | 8%–15% | 12–84 month terms; stricter income verification |
| Digital lender (Cofidis, Cetelem, WiZink) | 10%–18% | 12%–22% | Pre-approved offers common; online process |
| Fintech (Younited Credit, Creditea) | 8%–20% | 10%–24% | Fast disbursement; risk-based pricing |
Colombia
| Lender type | TIN range (approx.) | Notes |
|---|---|---|
| Major bank (Bancolombia, Davivienda) | 22%–30% | Consumer loan; tasa de usura ceiling enforced by Superfinanciera |
| Fintech (Addi, Bold) | 25%–35% | Buy-now-pay-later; short terms |
Personal Loan vs Credit Card vs Revolving Line: Which Costs Less?
| Product | Typical TIN | Structure | Best for |
|---|---|---|---|
| Personal loan | 8%–36% | Fixed term, fixed payment, amortizing | Planned one-time expense with defined payoff |
| Credit card (revolving) | 18%–60% | Minimum payment + revolving balance | Short-term convenience; dangerous if revolved |
| Revolving credit line | 15%–45% | Draw/repay as needed; interest on balance | Irregular cash needs; used responsibly |
| Buy-now-pay-later | 0% (promotional) to 35% | Short installments (3–12 months) | Small purchases; watch for deferred interest |
A personal loan is cheaper than a revolving credit card for any expense you cannot pay off in 1–2 billing cycles. If you owe MXN 20,000 on a credit card at 36% TIN and transfer it to a personal loan at 18% TIN/24 months, your monthly payment rises (from minimum payment to amortizing) but total interest falls by approximately MXN 5,000–MXN 7,000.
Impact of Term Length on Total Cost
Loan: MXN 50,000 at 24% TIN
| Term | Monthly payment | Total interest | Total repaid |
|---|---|---|---|
| 12 months | MXN 4,734 | MXN 6,808 | MXN 56,808 |
| 24 months | MXN 2,643 | MXN 13,432 | MXN 63,432 |
| 36 months | MXN 1,975 | MXN 21,100 | MXN 71,100 |
| 48 months | MXN 1,659 | MXN 29,632 | MXN 79,632 |
Going from 12 to 48 months cuts your monthly payment by 65% but increases total interest from MXN 6,808 to MXN 29,632 — a 335% increase in interest cost. Choose the longest term that keeps the monthly payment affordable; but do not choose it automatically.
Hidden Fees to Account For
Comisión de apertura (opening/origination fee): Charged upfront at disbursement, typically 1%–5% of the loan amount in Spain and Latin America. On MXN 50,000, a 3% opening fee means you receive MXN 48,500 but repay MXN 50,000 (plus interest). Some lenders add this to the loan principal rather than deducting it from disbursement — clarify before signing.
Mandatory loan-protection insurance: Some lenders bundle payment protection insurance that covers your instalments if you lose your job or become ill. This is not inherently bad, but the premium (often 0.5%–1.5% of outstanding balance per year) must be included in your cost comparison.
Early repayment penalty (comisión por pago anticipado): If you want to repay before the term ends, many lenders charge 1%–3% of the prepaid amount. In Spain, consumer credit law caps this at 0.5%–1% depending on time remaining. In Mexico, verify in your contract — some lenders waive it after a minimum number of payments.
Late payment fees: Typically 1%–3% per month on the overdue amount, compounding rapidly. On a MXN 2,500 monthly payment overdue for 60 days at 2% per month: approximately MXN 100 in late fees plus the standard interest on the overdue principal.
Red Flags: What to Avoid
- TAE/CAT above 50% in Spain, above 100% in Mexico: These rates are predatory for any term above 6 months. At 60% CAT on MXN 30,000 over 18 months, total interest exceeds the principal.
- Balloon payments. A loan structured as low monthly payments plus a large final "balloon" payment is designed to obscure total cost. Demand a complete amortization schedule before signing.
- "No credit check" offers. These invariably come with CAT above 80% and aggressive collection practices. Legitimate lenders check credit.
- Unlicensed lenders. In Mexico, verify lenders are registered with the CNBV or CONDUSEF. In Spain, check the Banco de España’s register of financial entities. Unlicensed lenders have no regulatory constraints on rates, fees, or collection tactics.
- Undisclosed default penalties. Beyond normal late fees, some contracts include clauses that trigger a full balance acceleration on first default. Read the full contract, including default provisions.
How to Use the Calculator Effectively
- Enter the loan amount, annual TIN, and term in months.
- Note the monthly payment — verify it fits within 30%–35% of your net monthly income.
- Note the total interest figure — this is the full cost of borrowing.
- Run the same parameters with a shorter term (e.g., 12 months instead of 24) to see the interest savings.
- Add the opening fee to the total interest to get the complete borrowing cost.
- Compare across lenders using CAT (Mexico) or TAE (Spain) — not TIN alone.