Inputs
The visitor supplies the information that the model can reasonably use, such as income, balance, rate, term, deposit or property price.
METHODOLOGY
How WorthChex calculates its results.
WorthChex is built around transparent scenario models. The inputs, formulas and limitations are intended to be understandable enough that a visitor can decide whether a result is useful for the situation being considered.
OUR APPROACH
A calculation has three parts.
Model
A defined formula converts those inputs into an estimate. The page explains important assumptions rather than hiding them behind a single number.
Limitations
Real decisions often depend on facts the calculator cannot know. Those boundaries are stated on the relevant page so the estimate is not mistaken for a guaranteed outcome.
AFFORDABILITY & SAVINGS
Planning disposable income and goals.
Affordability
Disposable income = take-home income − entered recurring expensesCash purchases are checked against the entered savings balance and emergency target. Finance scenarios are checked against the monthly payment and remaining disposable income.
Savings
Monthly mode projects the balance forward until the target is reached. Target-date mode works backwards to estimate the contribution required for the selected date.
The growth assumption is optional and should be treated as a scenario. It is not a guaranteed rate.
Compound growth
Future value depends on starting balance, contributions, rate, frequency and timeThe model applies the selected fixed rate at the selected compounding frequency. It separates the amount contributed from the modelled interest or growth.
BORROWING
Amortisation explains why term matters.
For standard repayment borrowing, the monthly payment is based on principal, periodic interest and the number of payments. Each payment contains interest and principal; as the balance falls, the interest part normally falls too.
Illustration: extending a loan term can reduce the monthly payment while increasing the number of months during which interest is charged. That is why WorthChex shows total paid or total interest where the model supports it.
Debt payoff
Monthly interest is calculated from the balance and APR assumption. The payment is then applied. If the payment does not exceed the interest being added, the model flags that the balance will not fall.
Mortgage
Repayment mortgages use monthly amortisation. Interest-only scenarios model the interest payment while leaving the principal outstanding at the end of the selected term.
Personal loan & car finance
The calculators model a fixed APR assumption with optional fees, deposits, overpayments or final payments. They are comparison models, not lender quotations.
UK TAX & PROPERTY RULES
Changeable rules are treated as published inputs, not guesses.
Salary & take-home pay
The 2026/27 tools use published UK Income Tax, employee National Insurance and student-loan thresholds. Scotland uses its separate Income Tax structure.
GOV.UK Income Tax rates
GOV.UK National Insurance
GOV.UK Student Loan tables
Stamp Duty
The residential model covers Stamp Duty Land Tax (England and Northern Ireland), Land and Buildings Transaction Tax with the Additional Dwelling Supplement (Scotland) and Land Transaction Tax (Wales) for the standard scenarios shown on the calculator page. Specialist transactions can require different treatment.
GOV.UK residential SDLT rates · Revenue Scotland LBTT · Welsh Revenue Authority LTT
Loans, credit and car finance
UK lenders quote an APR, which is an annual effective rate. The personal loan, car finance, debt payoff and affordability tools convert the APR into the equivalent monthly rate: monthly rate = (1 + APR)1/12 − 1. Payments are then worked out month by month, so a £10,000 loan over five years at 8.9% APR costs about £205.44 a month.
The mortgage calculator uses the quoted annual interest rate divided by 12, which is how most UK mortgage payments are calculated.
Pensions on the payslip
Salary sacrifice reduces pay before Income Tax, National Insurance and student loan repayments. A net pay arrangement is taken before Income Tax only. Relief at source takes 80% of the contribution from net pay and the provider claims 20% basic-rate relief, so the full amount reaches the pension. Higher-rate relief claimed through Self Assessment is not included.
VAT
The calculator performs VAT arithmetic after you select a rate. It does not determine whether your particular goods or services qualify for a rate, exemption or special treatment.
EDITORIAL & CALCULATION REVIEW
How changes are checked before they become part of the site.
Rule changes
When a published tax, National Insurance, Stamp Duty or VAT rule changes, the relevant source is checked and affected pages are updated with the new assumption or rate.
Maths changes
Calculation logic is documented here and covered by the repository's regression tests. That helps catch accidental changes to established scenarios when calculator code is edited.
Content changes
Guides are written to answer a question directly, include examples and limitations, and link to the calculator or primary source that supports the explanation.
The review process is deliberately transparent: where the model is simplified, that simplification is described; where the rule is changeable, the source is linked; and where the calculator cannot know a fact, the page says so.
LIMITATIONS
What the calculators cannot know.
- future interest rates, investment returns or changes to tax rules
- lender-specific underwriting, fees, promotional offers or contract terms
- your full tax code, benefits, payroll timing or unusual deductions
- irregular spending, future lifestyle changes or personal priorities
- specialist property, VAT or legal circumstances not represented by the simple inputs
For important decisions, compare a WorthChex estimate with the current official guidance, your actual lender or payroll documentation, or professional advice where appropriate.
Methodology is part of the result.
When the assumptions do not match your situation, the right response is to change the scenario or stop and check the authoritative source — not to treat the calculator as definitive.
SOURCE REGISTER — SEPTEMBER 2026
Published rules are treated as inputs, not invented assumptions.
Where a calculator depends on a rule that can change, WorthChex uses the published source available for the relevant tax year or jurisdiction and records the limitation on the page. That means a source can be checked independently of the calculator result.
Salary and payroll
Income Tax and employee National Insurance assumptions for 2026/27 are checked against GOV.UK guidance. The calculator remains an estimate because individual payroll details can vary.
Property taxes
Residential SDLT assumptions are checked against the current GOV.UK residential-rate guidance. Scotland and Wales use different property taxes.
VAT
The VAT calculator applies the rate selected by the visitor. It does not determine whether a transaction is taxable, zero-rated, reduced-rate or exempt.
Income Tax source → · National Insurance source → · SDLT source → · VAT source →
QUALITY CONTROL
Changes are checked in more than one place.
Calculation logic is checked with automated regression tests, while page changes are checked for broken internal links, missing canonical tags, missing assets, inline JavaScript errors and sitemap consistency. Content changes are also reviewed for whether the surrounding explanation actually helps a visitor understand the result.
When an official rule changes, the goal is to update the model, explanation and source together. When a calculation changes, the result is checked against known scenarios before publication.
Limits of a calculator
No browser calculator can see every fact that a lender, HMRC, employer, bank, insurer or investment provider may use. A WorthChex result therefore answers the numerical scenario represented by its inputs. It should not be treated as a quotation, approval, tax ruling or guarantee.