Loan affordability calculator: work back from a payment
Estimate the loan principal associated with a payment you choose, then test that payment against your budget and reserve. A mathematical borrowing amount is not a lender approval or an affordability recommendation.
Model checked: 9 November 2026 · Best Finance Calc
Open the free calculatorRelated editable workbook
Start with a payment you can test
In Solve a question, choose the loan-amount question and enter the planned payment, annual rate and number of payments. The input is a scenario to investigate, not a recommended percentage of your income. In Budget, separately enter income, expenses, starting cash and the reserve you want to preserve. Test several payments instead of treating one output as a safe borrowing limit.
Worked example: US$400 for 36 months
At 12% nominal annual interest with monthly payments over 36 months, a US$400 regular payment corresponds to approximately US$12,043.00 of principal before fees and insurance. Total planned payments are US$14,400.00; the modeled interest component is approximately US$2,357.00. This inverse calculation assumes level payments and a constant rate. It does not include existing debt obligations, eligibility checks, taxes or loan charges.
How the inverse payment formula works
For payment M, periodic interest r and payment count n, principal P = M × [1 − (1 + r)^(-n)] / r. At a zero rate, P = M × n. A higher rate reduces the amount supported by the same payment. A longer term can increase that amount while increasing the time you remain in debt. This solver is a mathematical model, not a jurisdiction-specific credit assessment.
Convert principal into usable cash carefully
The simple inverse result is principal before charges. If insurance must fit inside your payment cap, allow for it separately. Financed and deducted fees reduce the usable cash supported by a given financed balance; upfront fees require cash outside the installments. Return to Simulate to enter those charges and inspect the resulting payment schedule. Do not read US$12,043.00 as a guaranteed net payout.
Stress-test cash rather than only the installment
Try a lower-income month, a higher-expense month, a different rate and a smaller opening cash balance. A positive monthly remainder does not automatically preserve your chosen reserve throughout the forecast. The workbench does not decide which bills are essential, forecast future income or apply a bank's debt-to-income thresholds. Currency selection is a display choice, not an exchange-rate conversion.
Questions about this model
Is the calculated loan amount an approved credit limit?
No. It is the mathematical principal supported by the assumptions you entered, before lender eligibility rules.
Are fees already inside the inverse amount?
No. Model fees and insurance separately in Simulate before using the result as a net-cash scenario.
Model and reference material
Educational estimates, not a credit offer or personal financial advice. The examples use stated assumptions and the site’s documented engine.