Irish Calculators
Loans & mortgages

Mortgage Amount from Monthly Payment Calculator

Calculate the mortgage principal supported by a payment budget after other monthly housing costs. The result uses your rate and term assumptions and is not a lifetime-mortgage quote or lending decision.

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Your numbers

%
Monthly interest is the annual nominal rate divided by 12. This is not a fee-inclusive APR.
Your figures stay on this device.

How the calculation works

Work backwards from a monthly housing budget to a mortgage principal under a chosen rate and term. Other monthly housing costs are removed first, then your deposit is added to principal for an indicative purchase budget.

Principal = available monthly payment × [1 − (1 + monthly rate)^−months] / monthly rate. At zero interest, principal = payment × months.

  1. Find the borrowing payment

    Subtract other housing costs from the monthly budget. A zero remainder supports no mortgage principal; a negative remainder is rejected because other housing costs exceed the budget.

  2. Discount the payments

    Convert the nominal annual rate to a monthly rate. Principal equals the present value of the equal payments; at zero interest it is simply payment multiplied by months.

  3. Add the deposit once

    Add the cash deposit to principal for the indicative purchase budget. This addition does not change the repayment capacity already calculated.

Worked example: Zero-rate budget

Sample inputs demonstrate the method. They are not a forecast or product offer.

The example budget is €1,200 with €200 reserved for other costs. At zero interest, €1,000 available over twelve months supports €12,000 principal. Adding a €2,000 deposit gives a €14,000 indicative purchase budget.

Monthly housing budget
1200
Other monthly housing costs
200
Annual nominal interest rate (%)
0
Remaining term (months)
12
Cash deposit
2000

Expected result

Mortgage principal
€12,000.00
Indicative purchase budget
€14,000.00
Worked example: One payment discounted at 1% monthly

Sample inputs demonstrate the method. They are not a forecast or product offer.

Monthly housing budget
101
Other monthly housing costs
0
Annual nominal interest rate (%)
12
Remaining term (months)
1
Cash deposit
20

Expected result

Mortgage principal
€100.00
Indicative purchase budget
€120.00

Understanding your result

The result is a repayment-based scenario, not a lending offer. Income multiples, credit assessment, deposit requirements and lender policy are not applied by the formula.

A longer term can support more principal for the same payment but may increase total interest. Compare realistic assumptions rather than treating the largest mathematical amount as affordable.

Questions you might have

Does this determine how much a bank will lend?

No. It inverts the repayment formula and does not apply income multiples, credit checks or lender policy.

Choosing the right inputs

Monthly housing budget
Enter the total monthly housing allocation. The dedicated other-costs field is deducted before calculating the payment available for the mortgage itself.
Other monthly housing costs
Include recurring housing expenses reserved within that budget. Do not also subtract these amounts from the total before entering it.
Annual nominal interest rate
Use an annual nominal mortgage rate, not a fee-inclusive APR. The calculation divides this rate by twelve for regular monthly periods.
Cash deposit
Enter cash allocated to the purchase deposit. Separate transaction costs are not automatically deducted from this amount.

Common mistakes to avoid

  • Do not enter a budget already net of housing costs and deduct those costs again.
  • Do not interpret purchase budget as including unentered taxes, legal fees or other acquisition charges.

Assumptions to check

  • The rate and term are scenarios; the deposit is added once. This is not an equity-release or lifetime mortgage calculator.

Sources and method

Read our calculation and testing approach.