Our methodology
Methodology
The value of a calculator depends on its assumptions. This page explains the methods behind Money Harbour’s tools and how changing information is handled.
General standards
- Calculators publish the main assumptions close to the result.
- Changing rates, costs and thresholds include a checked or reviewed date.
- Examples are labelled as illustrations rather than quotes.
- Commercial relationships do not change calculator outputs.
- Known limitations are stated rather than hidden behind false precision.
Mortgage calculator
For repayment mortgages, the calculator uses the standard amortisation formula:
Monthly payment = P × r(1+r)ⁿ ÷ ((1+r)ⁿ−1)
Where P is the mortgage amount, r is the monthly interest rate and n is the number of monthly payments.
The mortgage amount is the property price minus the deposit. Loan-to-value is the mortgage amount divided by the property price. The calculator multiplies the rounded monthly payment across the selected term to illustrate total repayment and total interest.
For interest-only mortgages, the monthly payment is the mortgage amount multiplied by the monthly interest rate. The original capital remains due at the end and is included in the displayed total repayment.
Comparisons and limits
Rate scenarios keep the loan and term fixed while changing the annual interest rate. Term comparisons keep the loan and rate fixed. The tool assumes the selected rate applies throughout the full term and excludes product fees, legal costs, valuations, insurance, broker fees and early-repayment charges. It is not an affordability assessment or mortgage offer.
Mortgage decision calculators
Overpayment calculator
We first calculate the contractual repayment using the amortisation formula above. We simulate the original schedule month by month, then a second schedule after applying any lump sum immediately and adding the monthly overpayment. Interest saved and months saved are the differences between those schedules. The model assumes an unchanged rate and does not apply lender-specific overpayment limits or early repayment charges.
Property transaction tax
Residential SDLT, LBTT and LTT are calculated progressively: only the portion of the price inside each band is multiplied by that band’s rate. The result exposes every band used. Rates were checked against GOV.UK, the Scottish Government 2026–27 tax ready reckoners and GOV.WALES in July 2026. The tool models ordinary single residential purchases, first-time treatment where available, additional-property rates and the England/NI non-resident surcharge; special transactions and other reliefs are excluded.
Mortgage deal comparison
Each deal’s true cost over the selected period is calculated as payments made + upfront fees − cashback + balance remaining − original mortgage. A product fee added to the loan increases the starting balance and therefore captures both the fee and interest charged on it. Rates are held constant and later revert rates are not modelled.
Affordability estimate
The tool shows simple planning references of 4× to 4.5× gross household income, plus a separately labelled 5× stretch reference. The payment at 4.5× is recalculated at the entered rate and at an illustrative rate three percentage points higher. These are not lender criteria, a lender stress test, an Agreement in Principle or a lending decision.
First-time buyer cash needed
Total cash is deposit + applicable first-time-buyer property tax + each user-entered transaction cost. The non-tax example costs are editable illustrations, not quotes or market averages.
Remortgage true-cost calculator
The calculator uses the standard repayment-mortgage amortisation formula:
Monthly payment = P × r(1+r)ⁿ ÷ ((1+r)ⁿ−1)
Where P is the outstanding balance, r is the monthly interest rate and n is the number of monthly payments remaining.
It projects the current mortgage and replacement deal month by month over the selected comparison period. For each route, true cost is payments made + upfront fees − cashback + balance remaining − original mortgage. A product fee added to the new loan increases its starting balance and the interest charged. Break-even is the first month in the selected period when the replacement route’s cumulative true cost is no higher than staying on the current rate.
What it does not include
The result assumes both entered rates stay unchanged during the selected period. Users can include product fees, legal or valuation costs, broker fees and early-repayment charges under the provided fee inputs, but the tool does not determine which charges apply. It excludes lender-specific criteria, future revert rates, changes in payment schedules and transaction-specific advice. Those limits are why the result is an estimate, not a quote or recommendation.
UK salary calculator
The salary calculator estimates one employee’s PAYE deductions for the 2026/27 tax year. It applies the selected tax code and either the England, Wales and Northern Ireland bands or Scottish non-savings, non-dividend bands. The standard £12,570 Personal Allowance is reduced by £1 for each £2 of income over £100,000 and reaches zero at £125,140.
Employee Class 1 National Insurance uses the official weekly or monthly thresholds for the selected pay frequency. Two-weekly and four-weekly thresholds are multiples of the weekly figures. Student and postgraduate loan deductions use HMRC pay-period thresholds and are rounded down to whole pounds. A one-off bonus is placed in one pay period because NI and loan deductions are not annualised by payroll.
Pension treatment
- Salary sacrifice: reduces Income Tax, employee NI and student-loan pay in the estimate.
- Net pay arrangement: reduces Income Tax but not employee NI or student-loan pay.
- Relief at source: 80% of the gross contribution is deducted from cash pay and 20% is shown as provider relief. Separately claimed higher-rate relief is not included in payroll take-home.
Limitations
The result is an annual planning estimate, not HMRC payroll software. It does not model multiple jobs, mid-year starts, year-to-date cumulative tax, benefits in kind, dividends, savings, statutory pay, directors’ annual NI, Child Benefit charges, Self Assessment or employer-specific pensionable-earnings rules. Emergency codes are recognised but estimated on an annual cumulative basis and labelled accordingly.
Rates were checked against HMRC, GOV.UK and Scottish Government publications on 29 July 2026. The calculation engine is tested against official thresholds, HMRC student-loan examples and a Scottish Government comparison case. See the source list and current rates.
Invoice generator
The invoice generator runs in the browser. Line amounts equal quantity multiplied by unit price. Subtotal is the sum of line amounts; optional VAT is calculated at 20%; total due is subtotal plus VAT. The user is responsible for deciding whether VAT applies and for including the legally required business details.
Reviewing changing information
Pages containing rates, tax thresholds or typical market costs display when those figures were checked. A date is not a guarantee that every provider changed at the same moment; links to authoritative guidance should be checked before acting.
Corrections
Methodological or factual corrections can be submitted through the contact page.