Skip to content
SoloCalc

How SoloCalc Calculates Estimates

A planning estimate should be explainable. Here is what goes into a SoloCalc result, and what does not.

By SoloCalc · Content updated 23 October 2026

Sources, rules and assumptions are different things

UK tax rates, thresholds and reporting rules are drawn from GOV.UK and HMRC publications. Scottish bands also use Scottish Government material. Mortgage context uses FCA rules and MoneyHelper guidance. Each calculator links to its relevant sources; a link does not imply endorsement.

We keep legal rules separate from planning assumptions. A holiday allowance, income averaging method, optional savings buffer, borrowing multiple or stress rate is not a statutory entitlement or lender requirement. Editable assumptions remain visible.

From inputs to results

  1. The calculator reads the figures and options you enter, with validation appropriate to its scope.
  2. A separate calculation engine applies the selected rule set. Display code formats the results rather than maintaining a second set of tax rules.
  3. Relevant tools share the Income Tax engine, but different domains keep their own treatment: ordinary residential property income does not get self-employed Class 4 NI, and mortgage illustrations do not assess lender eligibility.
  4. The result explains the main components, assumptions and exclusions. All nine calculators are deterministic: the same valid inputs and rule version produce the same result, without AI or model calls.

Tax years and changing rules

Income Tax and NI rules are stored by supported tax year. Choose the year you are estimating or reporting, not simply the latest headline. A verified flag records a check against published sources; it is not HMRC approval. Unconfirmed figures are labelled provisional where used.

VAT's rolling and forward turnover tests are not tax-year tests. MTD depends on gross-income histories, relevant return years and an explicit assessment date. Mortgage repayment assumptions are illustrations rather than a tax-year rule set. Updating wording does not automatically re-verify every tax constant.

Rounding and money

The shared tax engine calculates band amounts and rounds monetary outputs to pennies; combined totals can differ slightly from the sum of individually displayed rounded lines. Some overview tables display whole pounds. The freelance solver searches in pennies and rounds charge rates upwards so display rounding does not reduce its revenue target.

Mortgage repayments use the capital-and-interest amortisation formula with monthly payments and a constant entered annual rate. At zero interest the loan is divided by the number of payments. Actual lender schedules can use different interest timing, fees or rate changes. None of these tools reproduces every rounding or reporting convention on a completed Self Assessment return.

Testing and edge cases

Automated tests sit alongside the calculation engines. They cover representative figures and boundary cases such as exact thresholds, allowance taper steps, regional bands, negative or invalid inputs, VAT date transitions, CIS credits, property relief limits and zero-interest repayments. Interface tests also cover mortgage input switching, validation and reset.

Tests check the implemented assumptions; they are not a substitute for a complete tax, legal or professional review. Cases outside a calculator's stated scope need official guidance or a qualified adviser, not an invented answer.

Worked examples

Calculator example results are generated by calling the same tested engines used by the tools. The inputs are illustrative, not real client outcomes. Stated region, year, income and exclusions matter; examples are not lender quotes or guaranteed refunds.

Why another result might differ

  • Different tax year, residence, income definition or timing of receipts and expenses.
  • Other taxable income, tax-paid credits, losses, reliefs or pension treatment not included or entered differently.
  • Student loans, capital gains, benefits charges, specialist NI or HMRC account balances outside scope.
  • VAT classification, property ownership or company accounting circumstances requiring individual judgement.
  • A lender's evidence, expenditure, credit, property and affordability criteria rather than the income multiples illustrated here.

Do not add totals from separate tools without checking overlapping income and tax assumptions. Use our guide to choosing the correct income figure.

Review and corrections

Read the Editorial & Review Policy for how source checks and material changes are handled, or prepare a calculation issue report. SoloCalc provides general planning information, not tax, accounting, legal or regulated financial advice.