Mortgage Calculator with Overpayment Modelling
Monthly repayments including taxes and insurance, and the effect of overpayments on term and total interest.
Mortgage Calculator
Frequency
Down payment: 20.0% — Loan amount: 4,000
Base Monthly (PITI)
48
P&I Payment
33
Total Interest
4,030
Total Cost
9,030
Monthly Breakdown
The repayment calculation
A repayment mortgage uses the same amortisation formula as any other reducing-balance loan. What differs is the term, and therefore the total interest.
M = P × r × (1 + r)ⁿ ÷ ((1 + r)ⁿ − 1)
- P
- Loan amount — property price minus deposit
- r
- Monthly rate = annual rate ÷ 12
- n
- Total monthly payments = years × 12
Worked example
- Property price
- 300,000
- Deposit (10%)
- 30,000 → P = 270,000
- Rate
- 5% → r = 0.004167
- Term
- 25 years → n = 300
Payment ≈ 1,578 per month · Total paid 473,400 · Total interest 203,400
Interest over the full term is roughly 75% of the amount borrowed at this rate. Extending the term to 30 years lowers the payment to about 1,449 but raises total interest to roughly 251,600 — 48,000 more for a saving of 129 a month.
Costs the repayment figure does not include
A common and expensive mistake is budgeting for the mortgage payment alone. Ongoing costs typically add 30–50% on top:
- Property tax / council tax —Recurs indefinitely and rises. In some US jurisdictions it is a substantial fraction of the mortgage payment itself.
- Buildings insurance —Mandatory with any mortgage. Contents insurance is separate and optional.
- Mortgage insurance —Usually required below a 20% deposit — PMI in the US, and equivalent schemes elsewhere. Often removable once you reach 20% equity, but rarely automatically: you have to ask.
- Service charge and ground rent —For flats and leasehold property. Can rise sharply and is frequently underestimated at purchase.
- Maintenance —A common planning figure is 1% of property value per year, averaged. It arrives unevenly — nothing for three years, then a roof.
- Purchase costs —Stamp duty or transfer tax, legal fees, survey, valuation, and moving. Typically 3–5% of the price and payable upfront, on top of the deposit.
About
The Mortgage Calculator computes your complete monthly housing cost — Principal & Interest (P&I) using the standard mortgage formula, plus optional property tax and home insurance (PITI). Enter the home price, down payment, loan term, and interest rate. A visual pie chart shows the proportion of principal vs interest over the full term, and the amortization table details every month of your loan.
How to use
- 1 Enter the home price and your down payment (amount or percentage).
- 2 Set the loan term (e.g. 30 years) and annual interest rate.
- 3 Optionally add monthly property tax and home insurance amounts.
- 4 Your monthly payment and total loan cost are shown instantly.
- 5 View the amortization schedule to see principal vs interest each month.
- What does PITI mean in a mortgage payment?
- PITI stands for Principal, Interest, Taxes, and Insurance — the four components of a full monthly mortgage payment. Principal and interest pay down your loan. Property taxes are often collected monthly by the lender and held in an escrow account until due. Homeowner's insurance (and PMI if applicable) protects the lender's collateral. This calculator shows both the P&I amount and the full PITI total.
- How much down payment do I need to avoid PMI?
- Private Mortgage Insurance (PMI) is typically required when your down payment is less than 20% of the home price. PMI costs 0.5–1.5% of the loan amount per year and is added to your monthly payment. Putting down at least 20% eliminates PMI entirely and also reduces your loan amount, lowering both your monthly payment and total interest paid.
- Should I choose a 15-year or 30-year mortgage?
- A 15-year mortgage has higher monthly payments but a significantly lower interest rate and pays far less total interest — often half as much as a 30-year mortgage. A 30-year mortgage has lower monthly payments, giving you more cash flow flexibility, but you pay much more in total interest. Use the calculator to compare: enter the same home price with 15 vs 30 year terms to see the exact difference for your situation.
The full guide
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