Loan principal supported by your monthly payment
The principal is the mathematical amount your entered payment can amortize. It is not a lending approval or a recommended borrowing limit. Total interest is the difference between all scheduled payments and that principal. Monthly principal-and-interest payment used is the direct input or total budget less reserved costs. Reserved costs are not interest and are not included in total loan payments. The table compares the same payment and term at the entered rate and one percentage point lower/higher within the supported rate range.
To compare a proposed payment with gross income and other monthly debts, use the Debt-to-Income Calculator as a separate ratio check alongside your spending budget.
The formula
r is the nominal annual interest rate divided by 1,200 and n is the number of monthly payments. Each payment occurs at the end of a month. The calculation solves the present value of that payment stream without rounding each installment to cents. For total-budget entry, loan payment = total budget − reserved monthly costs. Reserved costs cannot exceed the budget. The rate comparison repeats the same present-value calculation at unique rates clamped to 0–60%, holding the loan payment and term unchanged.
A budget without interest
A payment budget of 1,000 per month over twelve months supports principal of 12,000 at zero interest. With a positive rate, part of those same payments pays interest, so the supported principal is lower.
How to use this calculator
- Choose a principal-and-interest-only budget or a total budget including other monthly costs. Enter the amount and, for a total budget, what must be reserved outside the loan.
- Enter the annual interest rate and a whole repayment term in months.
- Review the live supported principal, loan payment used and total interest. Expand the nearby-rate comparison using the same payment and term.
Define the payment budget carefully
For a home loan, property taxes, insurance, association charges, and maintenance can be separate from principal and interest. Subtract the costs you need to reserve before entering the loan payment budget. Otherwise the resulting principal may exceed what your broader monthly budget can support.
A longer term can support a larger loan at the same monthly payment, but commits you to more payments and can increase total interest. The calculation does not inspect your income stability, other debts, emergency savings, or future obligations. Those factors matter when deciding what payment is manageable.
Reserve monthly costs before solving the loan amount
A total housing or vehicle budget can include costs that do not repay principal. In total-budget mode, enter those costs as one monthly reserve. The calculator subtracts that amount before solving the loan principal. It does not look up taxes, insurance or maintenance costs.
For a total monthly budget of 1,000 with 250 reserved for other costs, the loan payment is 750. At zero interest over twelve months, that supports 9,000 principal and 9,000 loan payments. The 250 monthly reserve stays separate from loan interest and repayments.
Assumptions & limitations
What this calculation assumes
- The rate and monthly payment remain constant.
- The loan fully amortizes with no balloon payment.
- No fees are deducted from the principal or added to payments.
What to keep in mind
- Lender underwriting, credit checks, income multiples, and collateral restrictions are not modeled.
- Actual daily interest and rounding may change a lender quote.
Common questions
Is this the maximum a lender will approve?
No. It is an equation based on the payment you entered. A lender applies its own eligibility and affordability checks.
Should I enter a total housing budget?
Choose Loan payment and other costs, enter the total budget and reserve amounts such as tax, insurance or maintenance. Only the remainder supports loan principal and interest. Reserved costs are entered assumptions, not looked-up expenses.
What does the nearby-rate comparison change?
Only the annual rate. The loan payment and repayment term stay the same. It shows how a rate one percentage point higher or lower changes supported principal and interest; the comparison is bounded to 0–60% and is not a lender quote.
Sources & further reading
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