Owning it · practical guide
Mortgage payments explained: the real math behind the monthly bill
In one line: What's inside a mortgage payment, the 28/36 rule lenders use, amortization's front-loaded interest, and the levers that move the number — with an in-page calculator.
The 60-second answer. A monthly mortgage payment is almost never one number. It is principal + interest — the part the loan math sets — usually plus property taxes, home insurance, and, if your down payment was under 20% (US), private mortgage insurance. On the week of 10 September 2026 the Freddie Mac 30-year fixed average was 6.76% (15-year: 6.09%) — at that rate, a $350,000 loan over 30 years costs $2,272/month in principal and interest, and quietly pays back $468,071 in interest on top of the loan over the term.
And lenders size what you may borrow with a ratio, not a feeling: the standard guideline is that housing costs stay under 28% of gross monthly income and total debt under 36% — the “28/36 rule”. Approval letters often stretch far past it. The rest of this page is about knowing the difference.
Not a reader? Section 4’s calculator shows every number’s source line. Budgeting the move itself? Pair it with the emergency repair fund — the cost category lenders don’t count.
Section 1 · What’s actually inside the payment
| Component | What it is | Who sets it |
|---|---|---|
| Principal | The slice of each payment that pays down the loan balance itself. | Your loan amount, term, and rate. |
| Interest | The lender’s charge — in year 1 of the example below it’s 86 cents of every dollar you pay. | Your rate and remaining balance. |
| Property tax | Local tax, often collected monthly into an escrow account and paid on your behalf. | Your local authority — not the lender. |
| Home insurance | Hazard cover, also commonly escrowed. Flood is never in this line — it’s a separate policy (the water damage guide explains the split). | Your insurer and your risk profile. |
| PMI / extras | Private mortgage insurance when equity is under 20% (US); HOA fees where they apply. | Your lender’s requirements; typically removable once you cross 20% equity. |
The P&I part is fixed for a fixed-rate loan — the other lines move with your tax bill and insurance renewal, which is why the “same” payment creeps up in years when neither the rate nor the house changed.
Section 2 · The math, shown once
Every amortizing mortgage on earth uses one formula — the same one the calculator below runs: M = P × r × (1+r)n / ((1+r)n − 1), where P is the loan, r the monthly rate, n the months. Nothing hides in it. What surprises people is the shape it produces — amortization front-loads interest:
The $350,000 loan at 6.76% over 30 years ($2,272/month):
| Year | Of each payment, interest is | Principal paid that year | Balance at year-end |
|---|---|---|---|
| 1 | 86% | $3,723 | $346,277 |
| 5 | 82% | $4,875 | $328,601 |
| 15 | 65% | $9,567 | $256,636 |
| 29 | 10% | $24,582 | $26,296 |
| 30 | 4% | $26,296 | $0 |
Read that top row again: in year one, barely a seventh of your money buys you anything you keep. This is why extra principal paid in the early years is disproportionately powerful, why refinancing math changes with the years you’ve already paid, and why “I’ve paid 8 years so I’ve paid off a third of it” is never true.
Section 3 · The rule lenders use (and the one you should)
US guidance (CFPB) frames it as two debt-to-income lines: 28% of gross monthly income for housing (front-end), 36% for all debt combined (back-end). Many loan programs approve well beyond 36% — which is exactly why the rule matters more for you than for them:
- Use the back-end number honestly: all loans, cards, car payments and the future mortgage against gross income.
- Approval is a ceiling, not a budget. The lender prices your willingness to repay at their maximum; only you price the part that keeps the house maintained, heated and insured — the ownership ledger the approval ignores (the someday-cost rule is the honest version).
- Stress yourself like Canada stress-tests everyone: before signing, recompute your budget at contract rate + 2%. If it only works at the exact rate, the loan is too big — whatever the letter says.
Section 4 · The monthly payment calculator
The tool runs its math in your browser, mirrors Section 2’s formula exactly, and is general guidance — not a loan offer, not financial advice. US buyers: your lender’s itemized Loan Estimate is the document that governs; UK and Canadian buyers: Sections 6’s local differences apply before any of this.
Section 5 · What actually moves the payment
Same $350,000 loan, same market week — the term is the biggest single lever:
| Term (fixed) | Rate (PMMS, 10 Sep 2026) | Monthly P&I | Total interest |
|---|---|---|---|
| 30 years | 6.76% | $2,272 | $468,071 |
| 20 years | 6.76% | $2,663 | $289,205 |
| 15 years | 6.09% | $2,971 | $184,698 |
- Rate: half a point on this loan is about ±$115–118/month and tens of thousands over the term — which is why shopping multiple lenders is the highest-paid hour available to a borrower (Freddie Mac’s chief economist’s standing advice, same release).
- Down payment: under 20% down, US lenders typically add PMI — commonly quoted around 0.5–1% of the loan per year (industry range; your Loan Estimate states yours). The calculator’s “other” field exists for exactly this line.
- Points: paying upfront to cut the rate only wins if you stay past the break-even month — compute it, don’t vibe it.
Section 6 · Regional differences (US · UK · CA)
| Country | The mechanism | The difference that matters |
|---|---|---|
| United States | PMI below 20% down; standardized Loan Estimate form | Every lender must give the same 3-page form — line-by-line comparable quotes are a legal right; use it. |
| United Kingdom | Deposit culture + lender affordability checks; SDLT land tax | First-time buyers in England & NI pay no stamp duty up to £300,000 (5% only on the portion to £500,000; relief lost above £500k — rules since 1 April 2025). Deposits of 5–10% are common; rates are mostly short-term fixes, not 30-year locks. |
| Canada | The stress test + CMHC insurance | Insured borrowers must qualify at contract rate + 2% or 5.25%, whichever is higher, within GDS ≤39% / TDS ≤44%. Under 20% down, CMHC premiums stack on the loan (4.00% / 3.10% / 2.80% by down-payment band); minimum down is 5% to $500k, 5%+10% beyond; from $1.5M the mortgage can’t be insured (20% down). |
Local rules move — each of these was checked 12 September 2026 against the schemes’ published guidance and current industry summaries; re-check before you rely on a threshold.
Section 7 · If the math went wrong
Signed, and the payment feels bigger than the budget? The order of moves:
- Refinance when rates or your credit genuinely improve — but run it against your remaining term (Section 2’s front-loading means an old loan refinanced late restarts the interest clock).
- Recast (US, where offered): a lump sum against the balance re-amortizes the same rate and term into a smaller payment, cheaply — the underrated move after a windfall.
- Extra principal early — one extra payment a year in the first years beats the same payment in year 25, because it deletes interest before the schedule charges it.
- Struggling? Call the servicer before the first missed payment — hardship and forbearance options exist and are easier to open than to reopen.
FAQ
How is a monthly mortgage payment calculated?
Loan × r × (1+r)n ÷ ((1+r)n − 1), with r the monthly rate and n the months — plus escrowed taxes, insurance and any PMI/HOA on top.
What percentage of income should go to a mortgage?
The standard guideline is 28% of gross income for housing and 36% for all debt (the 28/36 rule). Many approvals exceed it; the guideline is a sanity line, not a law.
How much is PMI?
Industry range commonly quoted around 0.5–1% of the loan per year while equity is under 20% — your Loan Estimate states your actual figure, and it typically comes off once you cross 20% equity.
Should I choose a 15-year or 30-year mortgage?
On the week’s averages, $350k costs $2,272/mo over 30 years but $2,971 over 15 — and saves about $283,000 in interest. The 30-year wins on flexibility; you can always overpay a 30-year toward 15-year speed, never the reverse.
What’s the payment on a $350,000 mortgage?
At 6.76% over 30 years: $2,272/month principal & interest (rate = Freddie Mac weekly average, 10 Sep 2026). Add taxes, insurance and PMI for the real out-the-door number — the calculator above does both halves.
Sources (all checked 12 September 2026)
- Freddie Mac Primary Mortgage Market Survey, 10 September 2026 — 30-year FRM average 6.76%, 15-year 6.09% (freddiemac.com/pmms; release via GlobeNewswire). Rates change weekly — this page stamps the week it checked.
- Consumer Financial Protection Bureau — debt-to-income guidance (28/36 as the standard lender guideline) and PMI requirements below 20% equity; consumerfinance.gov.
- HMRC Stamp Duty Land Tax — first-time buyer relief as re-set from 1 April 2025 (nil rate to £300,000; 5% on the portion to £500,000; relief lost above £500k), England & NI — gov.uk and current industry summaries.
- Canada’s insured-mortgage framework — stress test at contract +2% (5.25% floor), GDS ≤39% / TDS ≤44%, CMHC premium bands 4.00/3.10/2.80%, minimum down payment tiers — CMHC/OSFI rules as summarised by current Canadian mortgage industry guides (2026).
- Worked examples computed from the standard amortization formula at the stated rates — the same math the embedded calculator runs in-browser.
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General information, not professional advice. Homes differ — if a job is beyond your confidence or the guide’s boundary, that is what tradespeople are for.