A monthly mortgage payment looks like one number, but it is the answer to a formula with three inputs, and each input moves your total cost by tens of thousands. This guide works the formula by hand on one example loan, shows where the money actually goes year by year, compares 15, 20 and 30-year terms, prices extra payments, and then does something most mortgage guides skip: it shows what happens to a floating-rate home loan in Pakistan or India when the bank resets the rate, and whether a prepayment should shorten your loan or lower your EMI. Every figure below was calculated, not rounded from memory, and every rate is an example rate, not a quote.
The Mortgage Formula, Worked to the Cent
Every fixed-payment home loan, whether a US mortgage, a Pakistani home finance plan or an Indian home loan EMI, uses the same amortization formula:
M = P × r × (1 + r)n / ((1 + r)n − 1)
- M is the monthly payment of principal and interest.
- P is the amount you borrow, not the price of the house. Your down payment is already gone from it.
- r is the monthly rate: the annual rate divided by 100, then by 12.
- n is the number of monthly payments: years × 12.
Take a loan of $300,000 at an example rate of 6.5% for 30 years.
- Step 1, the monthly rate. r = 6.5 ÷ 100 ÷ 12 = 0.0054166667.
- Step 2, the number of payments. n = 30 × 12 = 360.
- Step 3, the growth factor. (1 + r)360 = 6.9918.
- Step 4, the payment. M = 300,000 × 0.0054166667 × 6.9918 / (6.9918 − 1) = $1,896.20 a month.
Now look at the very first payment. Interest for month one is the balance times the monthly rate: 300,000 × 0.0054166667 = $1,625.00. Only the remaining $271.20 reduces what you owe. That split is the single most important thing to understand about a long mortgage, and the next section shows how slowly it changes.
If you would rather not do this by hand, the IO Snack Mortgage Calculator uses exactly this formula. It shows a $ sign, but the maths does not care about currency, so a rupee amount typed in gives a correct rupee answer.
Where Your Payments Go: a 30-Year Amortization Table
Here is the same $300,000 loan at 6.5%, summarised by year. Each row adds up twelve payments of $1,896.20.
| Year | Principal paid | Interest paid | Balance left |
|---|---|---|---|
| 1 | $3,353 | $19,401 | $296,647 |
| 2 | $3,578 | $19,177 | $293,069 |
| 5 | $4,346 | $18,409 | $280,833 |
| 10 | $6,009 | $16,745 | $254,328 |
| 15 | $8,310 | $14,445 | $217,677 |
| 20 | $11,491 | $11,263 | $166,996 |
| 25 | $15,890 | $6,864 | $96,912 |
| 30 | $21,973 | $781 | $0 |
Three things jump out of this table.
- Year one is 85% interest. Of the $22,754 you pay in the first year, $19,401 is interest. After ten years of payments you still owe $254,328.
- Principal only overtakes interest in year 20. Before that, most of every payment is the cost of borrowing.
- You owe half the loan only after month 257, about 21 years and 5 months in. That is why selling or refinancing in the first few years recovers so little equity from payments alone.
On a long loan, the rate and the term decide your total cost far more than the price you negotiated for the house.
15, 20 or 30 Years: What the Term Really Costs
Same $300,000, same 6.5% example rate, three terms. The calculator's term buttons offer exactly these three: 15 yr, 20 yr and 30 yr.
| Term | Monthly payment | Total interest | Total repaid |
|---|---|---|---|
| 15 years | $2,613.32 | $170,398 | $470,398 |
| 20 years | $2,236.72 | $236,813 | $536,813 |
| 30 years | $1,896.20 | $382,633 | $682,633 |
Going from 30 to 15 years costs $717.12 more a month and saves $212,235 in interest. Going from 30 to 20 years costs $340.52 more a month and saves $145,821. The 20-year term is the one people forget, and it is often the better fit: it captures about two-thirds of the 15-year loan's interest saving ($145,821 of $212,235) for less than half its monthly jump.
The rate matters just as much. Keeping the 30-year term and moving the example rate one point either way:
| Example rate | Monthly payment | Total interest over 30 years |
|---|---|---|
| 5.5% | $1,703.37 | $313,212 |
| 6.5% | $1,896.20 | $382,633 |
| 7.5% | $2,097.64 | $455,152 |
One percentage point is worth roughly $200 a month and about $70,000 over the life of this loan. When two lenders quote you different rates, that is the size of the gap you are choosing between.
Extra Payments: How Much Each Amount Saves
Any money paid on top of the scheduled payment goes straight to principal, so every later month charges interest on a smaller balance. The calculator's Extra Payments box has quick buttons for +$100, +$250, +$500 and +$1,000. Here is what each does to the $300,000, 30-year loan at 6.5%:
| Extra each month | Paid off in | Time saved | Interest saved |
|---|---|---|---|
| $100 | 26 years | 4 years | $60,995 |
| $250 | 21 yrs 10 mo | 8 yrs 2 mo | $120,337 |
| $500 | 17 yrs 6 mo | 12 yrs 6 mo | $179,759 |
| $1,000 | 12 yrs 9 mo | 17 yrs 3 mo | $241,162 |
Before you do this, check two things with your lender. First, that extra payments are applied to principal and not held as an advance on next month's payment. Second, whether the loan agreement charges a fee for early repayment. If it does, subtract that fee from the savings above.
What the Calculator Adds on Top: Tax, Insurance and PMI
In the US, lenders usually collect property tax and home insurance with the mortgage, so the bill you see is principal, interest, tax and insurance, often shortened to PITI. The IO Snack calculator has fields for both, entered as annual amounts and divided by 12.
With the calculator's starting values (a $350,000 home, $70,000 down, 6.5%, 30 years, $3,600 a year of property tax and $1,200 of insurance), the loan is $280,000 and the payment breaks down like this:
- Principal and interest: $1,769.79
- Property tax: $300.00 (3,600 ÷ 12)
- Insurance: $100.00 (1,200 ÷ 12)
- Total: $2,169.79, which the calculator displays rounded as $2,170
If the down payment is below 20% of the price, the calculator adds private mortgage insurance (PMI) at an assumed 0.5% of the loan a year. Put $35,000 down on the same house and the loan becomes $315,000: principal and interest rise to $1,991.01 and PMI adds $131.25, for a total of $2,522.26 with tax and insurance. Real PMI pricing varies by lender and borrower, so treat 0.5% as a placeholder. Under US rules summarised by the Consumer Financial Protection Bureau, you can ask for PMI to be removed once your balance is scheduled to reach 80% of the home's original value, and it must end automatically at 78% if your payments are current.
Outside the US, leave the PMI question aside and use the tax and insurance fields for whatever your bank bundles into the monthly instalment, such as property or life cover on the loan.
Floating-Rate Home Loans in Pakistan and India: What a Reset Does to Your EMI
Most US examples assume a fixed rate for 30 years. Many home loans in Pakistan and India are floating instead: the rate is tied to a benchmark and repriced during the loan. HBL's Islamic Home Finance page, for example, describes its pricing as "variable to be repriced annually", quotes it as 1-year KIBOR plus a margin, and offers terms of 3 to 25 years (HBL, read 28 September 2026). Check your own bank's terms for the benchmark, the margin and how often it resets.
Here is what a reset does, using example rates only. Borrow PKR 1 crore (PKR 10,000,000) for 20 years at 12%:
- EMI: PKR 110,109 a month.
- Total interest if the rate never changes: PKR 16,426,067, which is more than one and a half times the amount borrowed.
- Balance after two years (24 EMIs): PKR 9,727,336. Two years of payments have cleared less than 3% of the loan.
Now suppose the bank reprices at the start of year three. Recalculating the EMI on the remaining balance over the remaining 18 years:
| New example rate | New EMI | Change per month |
|---|---|---|
| 10% | PKR 97,258 | − PKR 12,850 |
| 12% (no change) | PKR 110,109 | 0 |
| 14% | PKR 123,574 | + PKR 13,466 |
| 16% | PKR 137,569 | + PKR 27,460 |
If the 14% rate then held for the rest of the loan, total interest would rise to PKR 19,334,683, about PKR 29 lakh more than the original schedule.
There is a trap hidden in the 14% row. At 14%, one month's interest on PKR 9,727,336 is PKR 113,486, which is more than the old EMI of PKR 110,109. If the payment stayed the same, the balance would grow every month and the loan would never be repaid, however far the tenure was extended. So when rates rise sharply, a longer tenure alone cannot absorb the change, and the EMI has to go up. Budget for that before you sign: work out the EMI at your rate plus two or three points and make sure you could still pay it.
The same logic applies in India. An INR 50 lakh loan over 20 years costs INR 43,391 a month at an example 8.5% and INR 46,607 at 9.5%. That one point adds INR 3,215 a month and INR 7,71,695 of interest over the full term.
The Mortgage Calculator only offers 15, 20 and 30-year terms. For a 25-year home loan, or a term in months, use the Loan Calculator instead: its EMI Calculator tab has a Term Unit switch for years or months, and its Loan Comparison tab puts two rates or terms side by side.
Prepaying: Shorter Loan or Smaller EMI?
When you make a lump-sum prepayment on a home loan, banks in Pakistan and India commonly ask whether you want to keep the EMI and shorten the tenure, or keep the tenure and lower the EMI. The maths strongly favours one of them.
Take the PKR 1 crore loan at 12% and prepay PKR 10 lakh after three years, when the balance is PKR 9,564,552:
| Option | What changes | Interest saved |
|---|---|---|
| Keep the EMI, shorten the tenure | EMI stays PKR 110,109. The loan ends 52 months (4 years 4 months) early. | PKR 4,815,735 |
| Keep the tenure, lower the EMI | EMI drops by PKR 11,512 to PKR 98,596. The end date does not move. | PKR 1,348,480 |
The same PKR 10 lakh saves more than three and a half times as much interest when you keep paying the old EMI. Choose the lower EMI only if the monthly cash flow genuinely matters more to you, for example because a rate reset has already stretched your budget. Before prepaying, ask whether your bank charges a prepayment fee and whether there is a minimum amount.
Checklist Before You Sign
- Compare total cost, not the monthly figure. A lower payment over a longer term usually costs more in total. Ask each lender for the full repayment schedule.
- Stress-test a floating rate. Recalculate the EMI at two or three points above the starting rate. If that number would break your budget, borrow less or put more down.
- Ask how often the rate resets and against what. Annually or every six months, and which benchmark tenor.
- Get every fee in writing. Processing fees, valuation and legal charges, insurance premiums bundled into the instalment, and early-repayment charges.
- Confirm how extra payments are applied. To principal, immediately, with or without a fee.
- Keep an emergency buffer. A home loan runs for decades. Several months of instalments in savings protects you from a missed payment when income dips.
- Check the lender's figures against your own. If your calculation and the bank's schedule differ by more than a few units, ask why. Usually it is a fee or a different rounding convention, and you should know which.
Run Your Own Numbers
Start with the Mortgage Calculator: enter the home price, down payment, rate and term, add tax and insurance if your lender collects them, then press Show Schedule under Amortization Schedule to see your own version of the year-by-year table above and try the extra-payment buttons. For a 25-year term, a rate reset scenario or two offers side by side, switch to the Loan Calculator. If you are weighing a shorter or longer term on any loan, our guide to choosing the best EMI term walks through that decision, and the Compound Interest Calculator helps you compare prepaying against investing the same money.
All figures in this article are estimates built on example rates. They are not financial advice, and they do not replace your lender's official schedule.
