How we calculate — methodology & sources

A financial calculator is only as trustworthy as its assumptions are visible. This page documents every convention and every tax rule these tools use, so you — or your accountant — can verify any number we show.

The projection math

Contributions and withdrawals happen at the start of each month, and balances compound monthly at the annual rate divided by twelve — the same convention mainstream SIP calculators use, so results cross-check. Annual step-ups apply at the start of each plan year. The fund expense ratio you enter is subtracted from the return before compounding. The inflation view divides the final value by (1 + inflation)^years. The same compiled code runs on our servers and in your browser, so the two can never disagree, and the engine is covered by automated tests against hand-verified values.

The global compound-interest calculator additionally supports daily, monthly, quarterly and annual compounding. It first converts the selected nominal rate into its exact effective annual rate, then derives the equivalent monthly rate so each recurring deposit receives the correct amount of time in the projection. Beginning-of-month deposits receive one more growth period than end-of-month deposits. Its target contribution is solved by repeatedly running the same projection until the requested future value is reached.

Investment fee comparison: both options receive the same gross annual return, end-of-month contributions and annual contribution increases. The engine converts gross return and each option's combined ongoing percentage fee to equivalent monthly rates. It applies percentage costs to the grown opening balance, adds the contribution after any entry charge, deducts one-twelfth of the fixed annual fee and applies an exit charge only at the selected horizon. Direct fees are tracked separately from the return those deductions could have earned. The break-even solver repeatedly runs the higher-cost option to find the gross return required to match the other ending balance.

Investment fee benchmark: the published dataset runs the same fee engine across three fixed scenarios and an annual ongoing-fee ladder of 0%, 0.05%, 0.10%, 0.25%, 0.50%, 1%, 1.50% and 2%. Within each scenario, starting money, month-end contributions, horizon and gross return remain unchanged; only the fee varies. Direct fees are reconciled separately from missed compound growth, and their sum equals the no-fee reference minus the after-fee ending value. The page, interactive chart and downloadable CSV are generated from this shared engine.

Ireland pension contribution relief (2026): eligible earnings are capped at €115,000 and multiplied by Revenue's age-related percentage: 15%, 20%, 25%, 30%, 35% or 40%. Existing personal or employee pension contributions reduce the remaining room. The Income Tax estimate compares the same earnings before and after the eligible additional contribution across the entered 20% standard-rate band and the 40% balance. Personal credits, USC and PRSI are not presented as pension relief.

Swiss Pillar 3a (2026): a person affiliated with a pension fund receives the CHF 7,258 annual limit. A person without a pension fund receives 20% of net earned income, capped at CHF 36,288. Already-paid contributions reduce remaining room. Because federal, cantonal and municipal circumstances determine the deduction's value, the tax saving multiplies the eligible contribution by a visitor-entered marginal rate. Canton-dependent withdrawal tax is deliberately excluded.

Hong Kong TVC (2026/27): TVC is applied first inside the HK$60,000 combined TVC and QDAP deduction cap, with only the remainder available to qualifying annuity premiums. Salaries tax is calculated both at progressive rates on net chargeable income and at the two-tier standard rates on net income before allowances; the lower result is used. Temporary rebates and provisional-tax timing are excluded.

Coast FIRE: desired annual retirement spending and reliable retirement income are both entered in today's money. We subtract the latter, inflate the portfolio-funded remainder to retirement, and divide it by the selected withdrawal rate to estimate the retirement target. Today's Coast number discounts that target over every remaining month using the nominal return after entered fees and optional tax drag. The modeled Coast age is the first month the planned balance reaches the moving threshold. Required monthly contributions are solved against the same engine with an 80-iteration binary search. The model is deterministic: it does not claim that returns arrive smoothly or that any withdrawal rate is safe.

Emergency fund: the global calculator adds essential monthly housing, utilities, food, transport, insurance and health, minimum debt, care and other unavoidable costs. It subtracts only the continuing monthly income entered by the visitor, multiplies the remaining cash gap by the selected number of months and adds a separate one-time reserve. The 3, 6, 9 and 12-month columns are comparisons, not personalised recommendations. Savings growth converts the entered APY to its exact equivalent monthly rate; interest is applied before each end-of-month contribution. Taxes on savings interest and changing account rates are not modelled.

US life insurance needs: the calculator multiplies the visitor's annual household income contribution by the share survivors would still need and the selected support period. It adds mortgage, other debts, final expenses, education and other identified obligations, then subtracts existing life insurance, liquid assets and other entered resources. The gap cannot fall below zero. We do not discount income using an assumed investment return or inflate it using an assumed inflation rate, so the arithmetic remains visible. The result is not a quote, underwriting decision, policy-type recommendation or premium estimate.

UK mortgage overpayment: the scheduled repayment is derived from the entered balance, constant annual mortgage rate and remaining monthly term. We compare two strategies over that original term using the same recurring cash budget: either pay the scheduled mortgage and invest the spare monthly amount, or overpay the mortgage and invest the entire released payment after it is cleared. Mortgage interest uses the entered nominal rate divided by twelve. Investments receive a constant equivalent monthly rate after entered annual fees and tax drag, with contributions at month-end. An entered early repayment charge is carried to the comparison date at that same net investment rate to show its opportunity cost. The break-even gross return is solved by repeatedly running the same projection. Lender interest can be calculated daily, product rates can change and actual investment returns are volatile, so this is a deterministic scenario rather than a forecast.

How withdrawals are split into principal and gain

Each withdrawal is divided using pooled average cost: the taxable-gain share equals the growth share of your portfolio at that moment. This corresponds to HMRC's Section 104 pool after the same-day and 30-day matching rules, and to Canada's adjusted-cost-base approach; it is a planning simplification for the US and Australia. Gains are aggregated per tax year — because allowances and brackets are annual — and taxed as follows.

Country tax and statutory rules (verified through 13 August 2026)

United States (TY2026): long-term capital gains stack on top of your other taxable income across the 0%/15%/20% brackets for your filing status (Rev. Proc. 2025-32), plus the 3.8% net investment income tax above $200,000/$250,000 MAGI. 401(k) withdrawals are ordinary income; qualified Roth withdrawals are tax-free. State tax is out of scope.

United Kingdom (2026/27): gains above the £3,000 annual exempt amount stack on taxable income. We apply 18% within the remaining £37,700 basic-rate band and 24% above it, including years that straddle both rates. Stocks & Shares ISA withdrawals are tax-free; Lifetime ISA withdrawal charges are out of scope.

UK pension drawdown (2026/27): upfront pension commencement cash is capped at 25% of the pot and the remaining £268,275 lump sum allowance. Later flexi-access drawdown is pension income. We calculate the extra annual tax caused by drawdown on top of other income, including the Personal Allowance taper above £100,000 and the separate Scottish non-savings income bands.

Australia (FY2026-27): assets held over 12 months get the 50% CGT discount; the discounted gain is added to your income across the resident brackets (0/15/30/37/45%) plus the 2% Medicare levy. Low-income levy reductions aren't modelled. Superannuation is out of scope.

Australian franking credits (FY2026-27): statement mode preserves the entered franked cash, unfranked cash and attached credit. Estimate mode applies the selected 25% or 30% company rate only to the franked cash portion. The eligible comparison grosses up assessable income by the credit and then applies the same credit as an offset; the not-eligible comparison includes cash income without the denied gross-up or offset. The optional resident-individual estimate uses FY2026-27 brackets, LITO and either the exact ordinary single/no-dependants Medicare threshold and phase-in or no Medicare. It never determines holding-period, at-risk, related-payment or dividend-washing eligibility and never promises a final refund.

Australian novated-lease quote check: the calculator starts from the annual pre-tax and post-tax payroll deductions in a written quote. It recomputes resident tax and the standard Medicare levy, applies the full-year 20% statutory car formula, subtracts the entered employee contribution and keeps employer FBT, GST on that contribution and the lower-grossed-up RFBA as separate audit lines. Eligible electric cars are treated as FBT-exempt only when the visitor explicitly selects a confirmed exemption; the page does not infer eligibility from vehicle price. Total employee cost includes payroll impact and the entered residual, then compares the same resale value and running-cost horizon with a cash purchase.

Canada (2026): 50% of gains are included in income at your combined federal-plus-provincial marginal rate. TFSA withdrawals are tax-free; RRSP withdrawals are fully taxable as income. The proposed two-thirds inclusion rate was cancelled in March 2025 and never became law.

Canadian RRIF and RRSP withdrawals: the dedicated calculator uses the ordinary post-1992 RRIF prescribed factors, recalculated from each year's opening balance and the elected age. It applies no minimum in the setup year, withholds only on RRIF payments above the minimum, and applies the full lump-sum withholding base to ordinary RRSP withdrawals. Quebec mode combines the reduced federal source rate with Revenu Québec's 14% deduction. Every gross payment remains labelled taxable income; withholding is never presented as final tax.

Singapore: no capital gains tax for individual investors. IRAS can tax profits as income where activity amounts to trading; long-term regular investing is not that.

Malaysia EPF / KWSP (rules effective October 2025): Parts A, C and E use the Third Schedule wage ranges through RM20,000, including the published whole-ringgit employee and employer shares. Above RM20,000, and for Part F, the percentage method applies and a combined amount containing sen is rounded up to the next ringgit. The retirement projection uses a smooth monthly equivalent of an editable annual assumption; it does not reproduce KWSP's annual dividend-credit method or predict a future dividend.

Malaysia payroll and PCB (2026): the dedicated salary calculator replays regular remuneration through the selected payroll month, then applies HASiL's resident computerized MTD formula or the non-resident 30% rate. Resident calculations include the RM4,000 EPF relief cap, RM350 PERKESO/EIS relief cap, current TP1 relief, child units, zakat and the separate additional-remuneration calculation for a bonus. EPF uses the current KWSP Third Schedule; SOCSO, EIS and LINDUNG 24 Jam use PERKESO's exact statutory wage bands and RM6,000 ceiling. The result is a payslip-checking estimate, not certified payroll software.

New Zealand KiwiSaver (2026): the dedicated calculator applies the 3.5% employee and compulsory employer minimum from 1 April 2026 and the scheduled 4% minimum from 1 April 2028. Employer money is reduced by the applicable ESCT band. Government money is 25 cents per eligible member dollar up to $260.72, subject to the $180,000 taxable-income limit and age conditions. Projection fund returns, 3.5% pay growth, 2% inflation and the 2.5% post-65 return follow the standardized assumptions published by the FMA; they are not forecasts. The first-home scenario checks only the entered three-year membership timing, leaves $1,000 plus the entered Australian-sourced transfer amount in the account, combines the estimate with projected cash savings and carries the lower balance forward to age 65. The provider remains responsible for the actual eligibility and withdrawal amount.

New Zealand PAYE (2026–27): the regular-pay calculator floors annualised payday income, applies the current individual brackets, adds the 1.75% ACC earners' levy up to its $156,641 earnings ceiling, subtracts IETC only for an eligible ME treatment and converts annual PAYE through Inland Revenue's truncated weekly amount. Main-employment student-loan deductions use the exact weekly, fortnightly, four-weekly or monthly threshold. Employee KiwiSaver, compulsory employer contributions and ESCT use the 1 April 2026 payroll rules.

New Zealand term deposit versus PIE (2026): the comparison applies the entered RWT rate to standard-deposit interest and the entered PIR to the term or cash PIE return. Each annual rate is prorated by credit months; after-tax credits either compound or remain paid-out cash. The break-even solver finds the gross PIE rate that matches the standard deposit under the same term, frequency and treatment. DCS results show opening-principal headroom only and do not determine protected-product or depositor eligibility.

UAE federal private-sector gratuity: the dedicated calculator applies Article 51 of Federal Decree-Law No. 33 of 2021 and Article 30 of Cabinet Resolution No. 1 of 2022. It uses the last basic wage, 21 days per year for the first five years, 30 days thereafter, proportional part-years, exclusion of unpaid absence, the annual-hours ratio for part-time work and the two-year wage cap. Joining and final working dates are inclusive; Article 67 defines a year as 365 days, so adjusted service years equal inclusive service days less unpaid absence, divided by 365. DIFC, ADGM, domestic workers, government employment, UAE-national pension cases and alternative Savings Scheme fund values are outside scope.

Where the data lives

Every rate, threshold, allowance and contribution limit is versioned in D1 or the relevant calculator's tested rule module with its tax year, verification date and a link to the authority's own page. When a rate changes at a Budget or a new tax year, we add a new version and update the tests before the page changes.

What we deliberately don't model

Market volatility (projections use a smooth average return — real sequences differ), US state taxes, UK dividend tax in GIAs, Medicare levy reductions in generic Australian investment projections, Australia's superannuation rules, Singapore CPF rules, Malaysia EPF withdrawals, KiwiSaver hardship withdrawals, savings suspensions, partial member-credit years and member-specific PIR, UAE DIFC or ADGM employment, domestic-worker gratuity, government employment and alternative Savings Scheme fund balances, and India-style exit loads (rare in these five markets). The UK pension calculator additionally excludes UFPLS, phased crystallisation, protected lump sum allowances, temporary emergency PAYE and defined-benefit pensions. The Canadian registered-plan calculator excludes final federal/provincial income tax, benefit clawbacks, legacy RRIF factors, locked-in account maximums and non-resident withholding. Each page notes its own simplifications. The life-insurance calculator does not model underwriting, premiums, policy guarantees, estate inclusion, state law or benefit eligibility. The mortgage comparison does not model remortgaging, changing rates, daily lender interest, investment volatility, sequence risk, loan-to-value pricing, pension tax relief or a personal UK tax calculation. The investment-fee comparison excludes performance fees, tiered schedules, caps, rebates, tax, brokerage per trade, bid-ask spreads and the value of advice or planning. The mortgage calculator's 10% allowance default is editable and is never treated as a lender promise. These are projections for education — not predictions, and not advice.