Payment Calculator
Monthly payment, bi-weekly interest savings, and affordability — pure financial math for any loan, any currency, anywhere in the world.
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How to Use This Calculator
Tab "Monthly Payment"
Enter your loan amount, annual interest rate, and loan term in years. The calculator instantly shows your monthly, bi-weekly, and weekly payment amounts, plus the total interest you will pay over the life of the loan. Use the "Show payment as" selector to highlight your preferred payment frequency as the primary result.
Tab "Bi-Weekly Impact"
Enter your loan details to see the side-by-side comparison of monthly vs bi-weekly payments. The calculator shows exactly how much interest you save and how many years earlier you pay off the loan by switching to bi-weekly. The math: 26 bi-weekly payments per year equals 13 monthly payments — one extra payment per year that goes entirely to principal.
Tab "Affordability"
Enter your gross annual income and any existing monthly debt payments (car, student loan, credit cards). The calculator applies the 28% housing rule and 36% total debt rule, identifies the binding constraint, and works backward to give you the maximum monthly payment and maximum loan amount you can comfortably afford.
The Formulas
Payment = P × [r(1+r)^n] / [(1+r)^n − 1]
Where: P = principal, r = monthly rate (annual ÷ 12), n = total payments (years × 12)
Bi-weekly payment:
Bi-weekly payment = Monthly payment ÷ 2
Payments per year: 26 (= 52 weeks ÷ 2) vs 12 monthly
Extra annual payment: Monthly payment × 1 (effectively)
Total interest (monthly schedule):
Total interest = (Monthly payment × n) − Principal
28% housing rule:
Max housing payment = Gross monthly income × 0.28
36% total debt rule:
Max total debt payments = Gross monthly income × 0.36
Max new payment = Max total debt − Existing monthly debts
Back-solve for max loan amount:
Max loan = Max payment × [(1+r)^n − 1] / [r(1+r)^n]
These are universal financial formulas with no country-specific data. They apply equally to mortgages, car loans, personal loans, student loans, and any other fixed-rate amortizing debt — in any currency.
Worked Examples
Example 1 — Car loan: $25,000 at 7% for 5 years
A common auto loan scenario. Monthly rate r = 7% ÷ 12 = 0.5833%. Total payments n = 60.
The formula: $25,000 × [0.005833 × (1.005833)^60] / [(1.005833)^60 − 1] = $495.03/month.
Example 2 — Bi-weekly mortgage: $400,000 at 5.5% for 30 years
Switching from monthly to bi-weekly payments on a large mortgage has a dramatic effect over 30 years.
The mechanism: bi-weekly payments produce 26 half-payments per year. That is equivalent to 13 full monthly payments — one extra payment annually. Over 30 years that compounds significantly, eliminating the last 4.5 years of the loan.
Example 3 — Affordability: $75,000 income, 6% rate, 30-year term
With no other debts, the 28% housing rule is the binding constraint for most buyers.
If the same borrower already has $500/month in other debts, the 36% rule limits new payments to $2,250 − $500 = $1,750 — in this case both rules give the same result. With $800/month in other debts, the 36% rule becomes binding: max new payment = $2,250 − $800 = $1,450/month, reducing the max loan to ~$242,000.
Payment Calculator Reference Table
| Loan Amount | Rate | Term | Monthly Payment | Total Interest |
|---|---|---|---|---|
| $10,000 | 5% | 3 years | $299.71 | $789.56 |
| $25,000 | 7% | 5 years | $495.03 | $4,701.80 |
| $50,000 | 6% | 10 years | $555.10 | $16,612 |
| $200,000 | 5.5% | 20 years | $1,375.97 | $130,233 |
| $400,000 | 5.5% | 30 years | $2,271.16 | $418,617 |
| $500,000 | 6.5% | 30 years | $3,160.34 | $637,722 |
All figures are illustrative. Use the calculator above for your exact inputs.
Understanding Loan Amortization
Every loan payment you make is split between interest and principal. In the early months, most of your payment covers interest because the outstanding balance is high. As the balance falls, the interest portion shrinks and more goes to principal — even though the payment stays the same. This is amortization.
In the first month of a $25,000 loan at 7% for 5 years: interest = $25,000 × (7%/12) = $145.83, principal = $495.03 − $145.83 = $349.20. By month 60, interest is under $3 and nearly the entire payment reduces the balance.
The practical implication: extra early payments are extremely powerful. A single extra payment in month 1 saves far more interest than the same extra payment in month 50, because it reduces the principal that interest accrues on for every subsequent month.
Why bi-weekly beats monthly
A year has 52 weeks. Making a payment every 2 weeks means 52 ÷ 2 = 26 payments. Each payment is half your monthly amount. But 26 × (monthly/2) = 13 × monthly. You end up making 13 monthly-equivalent payments per year instead of 12 — one extra payment with zero budgeting effort. That extra payment goes 100% to principal, slashing interest costs and years off the loan.
The 28/36 rule explained
These guidelines come from conventional mortgage underwriting. The 28% rule (also called the front-end ratio) focuses purely on your housing cost relative to income. The 36% rule (back-end ratio or DTI — debt-to-income ratio) looks at all your monthly debt obligations. Most lenders today allow DTI up to 43–50% for certain loan types, but staying at 36% leaves meaningful financial buffer for savings, emergencies, and lifestyle.
Note: the affordability rules use gross income (before tax). After-tax income is always lower, so the actual payment that feels comfortable may be less than the calculated maximum. Many financial planners suggest targeting 25% of net take-home pay for housing to maintain healthy savings rates.