A payment is a stream, and the loan is its present value
An ordinary loan calculator takes an amount and produces a payment. This runs the same relationship the other way: your monthly budget is treated as a stream of payments, and the maximum loan is the present value of that stream discounted at the loan rate over the term.
Max loan = payment x (1 - (1+r)^-n) / r, r = annual rate / 12, n = months.
That inversion is the useful one when you are shopping rather than confirming. It answers the question a borrower actually has — what can I take on — instead of the question a lender asks after the amount has already been chosen.
Stretching the term raises the ceiling and the total bill together
Extending the tenure lets the same monthly budget support a larger loan, because the payments are spread over more periods. The increase is not proportional: each additional year adds less to the borrowing capacity than the one before it, since the later payments are discounted hardest.
The total interest moves the other way and keeps climbing. Comparing a twenty-year and a thirty-year run at the same budget usually shows a modestly larger loan bought with a substantially larger interest bill, which is the trade the tenure decision actually is.
A point on the rate is worth tens of thousands of borrowing power
Because the rate discounts every future payment, a change of one percentage point moves the maximum loan by considerably more than one percent. On a long tenure the effect is largest, as the discounting compounds across the greatest number of periods.
This is worth testing before an offer is accepted rather than after. Running the same budget at the rate you have been quoted and at a point either side shows how much of your apparent capacity is actually a bet on the rate holding.
Affordability to you is not the same as eligibility to them
This calculates what a payment supports arithmetically. A lender adds tests this does not model: a debt-to-income ratio covering your other obligations, a credit assessment, income documentation, and a loan-to-value limit tied to whatever secures the loan.
The practical gap is usually the ratio test. Many lenders cap total debt servicing at around 40% to 50% of gross income, so a budget that is affordable in your own accounting can still exceed what the underwriting will approve.
Frequently Asked Questions
How does this calculator work?
It works backwards from the monthly payment you can afford. Using the loan rate and tenure, it finds the largest loan whose EMI equals your budget, which is the present value of all those payments discounted at the rate.
How does the interest rate change how much I can borrow?
A lower rate means less of each payment goes to interest, so more of your budget supports principal and the maximum loan rises. A higher rate has the opposite effect, shrinking how much you can borrow for the same payment.
What else affects how much I can borrow?
Lenders look at far more than the payment you can afford. Income, credit history, existing debts, the loan purpose, and your debt-to-income ratio all influence the amount they will actually approve.
What is a safe monthly payment to enter?
A common approach is to keep all your debt payments well within a comfortable share of your take-home pay, leaving room for savings, essentials, and unexpected costs. Enter a figure you could still pay during a tight month, not your absolute maximum.
Is the maximum loan the amount I should borrow?
Not necessarily. The result is a ceiling based purely on the payment, not a recommendation. Borrowing the full maximum leaves no margin if your income dips or rates rise, so many people deliberately borrow less.
Does the result include fees or insurance?
No. The calculation covers principal and interest only. Processing fees, insurance, and other charges are separate, so the loan you can comfortably handle may be a little lower than the figure shown.
Sources & References
Figures on this page are checked against primary, authoritative sources. Links open in a new tab.
Lenders also assess income, credit, and other debts.
Does not include fees or insurance.
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Results are estimates based on the figures you enter and standard formulas. Rates, fees, taxes, and lender terms vary and change over time, so confirm important numbers with your lender or a qualified professional. This is educational information, not financial advice.
Published the calculator with its formula, worked example, assumptions, limitations and a bespoke guide, and added an automated formula test suite covering it.
Tested the inverted annuity against the forward loan formula: the maximum loan, fed back as a principal, must reproduce the payment budget entered.
Tested that a longer tenure always raises the maximum loan and the total interest together, and that a zero rate makes the loan the payments summed.
Frequently Asked Questions now show on the page: 6 answers, checked in a page-by-page audit.
What the calculation assumes and where it stops now show on the page.
Show it to your clients, not just tell them
Mortgage brokers, accountants, and advisors embed this to walk clients through the numbers live — nothing they enter ever leaves their browser.