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.
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.
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 28 July 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.
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.
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, Australia's Medicare levy reductions, Australia's superannuation rules, Singapore CPF rules, Malaysia EPF withdrawals, KiwiSaver first-home and hardship withdrawals, savings suspensions, partial member-credit years and member-specific PIR, 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. These are projections for education — not predictions, and not advice.