Home purchase budget calculator

This calculator answers the reverse budget question for buying a home: if you want to spend a set amount each month on mortgage repayments, what loan and what purchase price does that imply? It uses a transparent, clearly defined model: a loan that is fully repaid at a fixed rate over a term you choose. The result is a model budget under your own assumptions, not a statement of what a bank will actually lend or what you can afford. Everything runs in your browser; nothing is stored or sent.

Deposit	–
Loan	–
Ancillary costs	–
Total project cost (price + ancillary costs)	–
Term in months	–

This calculator needs JavaScript because the calculation runs entirely in your browser – so your inputs never leave your device. Two fully worked examples are shown further down this page.

What your result means

The two headline figures are the modelled maximum loan and the modelled maximum purchase price under your assumptions. The breakdown table shows how the total funds split across deposit, loan, ancillary costs and total project cost, plus the term in months. None of these figures is an approval. A lender assesses each case individually, looking at affordability (your income and outgoings), loan-to-value, the fixed-rate period and refinancing, and this model deliberately does none of that.

How this calculator works

The maximum loan is the present value of a level monthly payment over the whole term. From the nominal annual interest rate the monthly rate is j = rate / 100 / 12, and from the term the number of months is n = years · 12. When the rate is above 0: loan = payment · (1 − (1 + j)^(−n)) / j. When the rate is exactly 0, this simplifies to loan = payment · n.

Total funds are your deposit plus this loan. Because buying also involves ancillary costs, the budget splits between the purchase price and those costs: with the freely chosen ancillary-cost rate k = per cent / 100, price = total funds / (1 + k) and the ancillary costs are price · k. Price plus ancillary costs come back to exactly the total funds.

The loan is fully amortised over the entered term at a constant rate, with no balance left at the end. This is a deliberately simple model: real mortgages often have a fixed-rate period shorter than the full term, followed by refinancing at an unknown rate. The ancillary-cost rate is a free input (the ancillary-cost calculator breaks it down by item). Taxes beyond that rate, subsidies, maintenance reserves and running costs are not modelled. Internally the calculation is unrounded and only rounded for display.

Formula and variables

j = p / 100 / 12n = Y · 12L = P · (1 − (1 + j)^(−n)) / j (if j = 0: L = P · n)F = D + LV = F / (1 + k)AC = V · k

P: monthly loan payment (payment budget)
D: deposit available for this model
p: nominal annual interest rate, in per cent
j: monthly interest rate
Y: term in whole years
n: number of months
k: ancillary-cost rate as a factor
L: modelled maximum loan
F: total funds (deposit + loan)
V: modelled maximum purchase price
AC: ancillary purchase costs

Worked examples

Example 1: €1,200 payment, €50,000 deposit, 3.5%, 30 years, 10% ancillary costs

Example inputs
Monthly loan payment	€1,200.00
Deposit	€50,000.00
Interest rate p.a.	3.5%
Term	30 years
Ancillary-cost rate	10%

Example results
Modelled maximum loan	€267,233.98
Total funds	€317,233.98
Modelled maximum purchase price	€288,394.53
Ancillary costs	€28,839.45

With j = 3.5 / 100 / 12 and n = 360, the present value of €1,200.00 a month is about €267,233.98 of loan. Adding a €50,000.00 deposit gives €317,233.98 in total funds. At 10% ancillary costs that is a modelled price of €288,394.53 plus €28,839.45 in ancillary costs. It is a model budget under your assumptions, not a bank approval.

Example 2: €800 payment, €100,000 deposit, 2%, 20 years, 12% ancillary costs

Example inputs
Monthly loan payment	€800.00
Deposit	€100,000.00
Interest rate p.a.	2%
Term	20 years
Ancillary-cost rate	12%

Example results
Modelled maximum loan	€158,139.23
Total funds	€258,139.23
Modelled maximum purchase price	€230,481.45
Ancillary costs	€27,657.77

€800.00 a month over 240 months at 2% is about €158,139.23 of loan. With a €100,000.00 deposit that is €258,139.23 in total funds, so at 12% ancillary costs a modelled price of €230,481.45 and €27,657.77 in ancillary costs. A shorter term and a larger deposit shift the split noticeably.

Assumptions

The loan is fully repaid over the entered term at a constant rate (no balance left).
The monthly payment is the loan repayment only; running housing costs are not included.
The ancillary-cost rate is a freely chosen input; price and ancillary costs split the total funds exactly.

Limitations of this calculator

No creditworthiness, affordability or credit check, and no statement about what a bank will actually lend or what a "safe" payment is.
Lenders assess each case individually. Their affordability assessment of your income and outgoings, the loan-to-value, a fixed-rate period shorter than the term and refinancing are not modelled.
No live interest rates and no income or debt-to-income approval rules; the interest rate is your own assumption.
No taxes beyond the ancillary-cost rate you enter, no subsidies, no maintenance reserves or running costs; your inputs are not stored.

Common misconceptions

Reading the result as an approval: The maximum purchase price is a model figure under your assumptions, not a loan approval and not a statement about affordability or creditworthiness.
Confusing the payment with housing costs: The payment you enter is only the loan repayment. Electricity, maintenance, service charges and reserves come on top and are not modelled.
Equating the fixed-rate period with the term: The model repays at a fixed rate over the full term. In practice the fixed-rate period is often shorter, followed by refinancing at an unknown rate.

Frequently asked questions

Does the calculator say what I can afford?

No. It shows a transparent model budget under your own assumptions. Whether a bank lends to you, and on what terms, depends on the lender's own affordability assessment of your income and outgoings, the loan-to-value and other factors that are deliberately not assessed here.

Why is the loan repaid over the full term?

The model is a full repayment: the loan is paid off by the end of the entered term, with no balance left. In practice the fixed-rate period is often shorter than the full term; after it you refinance at a rate that no one knows in advance.

Which interest rate should I enter?

The rate you want to work with. The calculator uses no live market rates. Try several scenarios to see how strongly the rate moves the model budget.

What goes into the ancillary-cost rate?

Typically transfer tax, notary and land-registry fees and, where applicable, an agent's commission. The rate is freely editable because it varies by region and situation. For a breakdown by item, use the ancillary-cost calculator.

Is the payment my whole monthly outlay?

No. The payment is only the loan repayment. Running costs such as electricity, maintenance, service charges and reserves come on top and are not modelled here.

Sources and further reading

Official and independent sources on this topic. The links open each website in a new tab; no content is loaded from them into this page.

Present value of an annuityWikipediaExplains the present value of a level payment stream, the mathematical basis of the model loan.
MortgagesUK Money Helper (MaPS)Independent guidance on deposits, fixed-rate periods and what lenders assess: context for why a model is not financing advice.

Spotted an error in the calculation or the text?

If you notice something that is wrong or unclear: let us know via the contact page. We review every report.

Last reviewed: 23/09/2026 · All calculations run in your browser – inputs are not stored. ·How we check our calculators