Practical calculator guides
These guides separate checkable arithmetic from the facts you must establish: measured quantities, current tariffs, transaction eligibility and professional design. Each new guide includes a synthetic worked example, dated primary references and links to relevant calculators. Neither a guide nor a calculator is evidence of a professional review.
Home, energy and insurance planning
Solar savings: value used electricity separately from exported electricity
A checkable solar bill-saving example separating self-consumption, exports, tariffs and fixed charges.
Solar payback: compare net cost, yearly savings and timing
Understand simple payback, discounted value and replacement costs without treating a planning result as an installation quote.
Roofing quantities: roof planes, squares and whole bundles
Convert measured roof area to purchase quantities while keeping slope, waste and product coverage explicit.
Heat pump sizing: a floor-area estimate is not a design load
Understand heat-loss screening, design conditions and why equipment output must be matched to a proper load calculation.
Insulation estimates: resistance, thickness and climate are different inputs
Use R-values, material coverage and local requirements without mistaking a volume estimate for a compliant retrofit specification.
Window replacement: use whole-window U-values and separate the cost case
A transparent degree-day example and the limitations of treating window heat-loss reduction as a bill forecast.
Flooring estimates: net area, allowance and whole packs
Avoid under-ordering or double-counting by separating measured area, layout allowance, pack coverage and labor scope.
Rebuild cost is not the sale price of a home
Separate the rebuilding scope, contents and policy terms before using a planning estimate as an insurance discussion aid.
Life insurance needs: model the funding gap instead of an income multiple
Build an explicit needs-and-resources scenario and keep coverage amount, duration and policy eligibility separate.
EV charging costs: battery energy is not always billed energy
Calculate charge energy, wall losses, per-distance costs and charging time with explicit assumptions.
Country tax, borrowing and payroll components
US federal income tax in 2026: marginal bands are not a flat rate
Work through a single-filer ordinary-income example and see what a federal-only calculator leaves out.
Canadian sales tax: compare ordinary rates, not every transaction
A dated GST/HST/PST comparison and examples of extracting tax from an inclusive price.
Canadian income tax: federal bands are only one layer
Understand a 2026 federal band example and why provincial tax, credits and payroll contributions must remain visible.
UK Personal Allowance taper: why the headline band is not the whole marginal effect
Work through an England, Wales or Northern Ireland 2026/27 ordinary-income scenario and separate Scottish and payroll rules.
UK home-purchase taxes: England and Northern Ireland, Scotland and Wales
Compare a £300,000 ordinary residential purchase across SDLT, LBTT and LTT without mixing their thresholds or reliefs.
Irish income tax and USC: two separate calculations
Check a 2026 single-person example and understand why the result still excludes PRSI and other payroll deductions.
Irish CAT: aggregate relevant benefits before using the threshold
A worked current-benefit calculation showing prior gifts, relationship groups and the risk of taxing prior benefits twice.
Canadian mortgage stress testing: qualifying rate versus contract payment
Distinguish the OSFI uninsured-mortgage rate floor from a lender’s complete affordability assessment.
Irish mortgage limits and Help to Buy: keep three constraints separate
A first-time-buyer scenario separating loan-to-income, loan-to-value and conditional tax-refund support.
UK student-loan repayments in 2026/27: identify the plan first
Compare annual repayment thresholds and distinguish current deductions from future interest and write-off projections.