Guides
Short notes on the formula, the assumptions, and the official sources a calculator cannot replace.
Fixed vs variable-rate mortgage: how much can your payment move?
Fixed vs variable-rate mortgage: what 0.5–2 point moves do to the payment, when a cheaper start stops winning, and how floating plans reset.
Dollar-cost averaging vs lump sum: what the maths and the evidence say
Dollar-cost averaging vs lump sum: how DCA lowers average cost, why positive returns favour investing sooner, and what Vanguard's 2023 study found.
US take-home pay by state 2026: the same $75,000 salary in 14 states
One $75,000 salary in nine no-wage-tax states plus CA, NY, IL, PA, MA: federal and FICA identical, state tax differs. Net by state, with a $150,000 table.
Savings rate and years to financial independence: why saving 50% beats earning more
How savings rate drives years to financial independence under the 4% rule: 10–70% at 3%, 5% or 7% real return, with a worked example from zero.
15 vs 30 year mortgage: what a shorter term really saves (with 20 and 25 years)
15 vs 30 year mortgage (plus 20 and 25): monthly payment, total interest, investing the difference, and prepaying a 30-year loan to finish in about 20.
MPF fund expense ratio (FER) explained: what it includes and what fees cost your retirement pot
What MPF FER includes and excludes, average/high/low by fund type at 31 August 2026, a 35-year fee-gap example, and what the eMPF cut to 0.29% is worth.
UK take-home pay 2026/27: Income Tax, National Insurance and student loans explained
How a 2026/27 UK salary becomes take-home pay: Personal Allowance taper, England vs Scotland bands, Class 1 NI, student loans. Example: £30,000 in England.
Your FIRE number and the 4% rule: how to calculate it, and where it breaks
How to calculate your FIRE number (spending ÷ withdrawal rate), years to the target, how long money lasts at 3–6% withdrawals, and why return order matters.
US take-home pay 2026: federal tax, FICA and state tax explained
How a 2026 US W-2 salary becomes take-home: federal tax, FICA, and state tax in CA, NY, IL, PA, MA plus nine no-wage-tax states. Example: $60,000 in Texas.
The Rule of 72: how long does it take money to double?
Divide 72 by the annual rate to estimate years to double. How accurate it is from 1% to 30%, why 72 and not 70, the 114 and 144 versions for tripling and quadrupling, and how to use it on inflation and fees.
Korea take-home pay 2026 (연봉 실수령액): pension, health, employment insurance and income tax
How a Korean annual salary becomes monthly take-home pay in 2026: National Pension at 9.5%, health and long-term care insurance, employment insurance, and income tax from the March 2026 withholding table. Worked example: ₩36 million.
MPF tax deduction limits in Hong Kong: HK$18,000 mandatory, HK$60,000 TVC
Hong Kong 2025/26 MPF tax deductions: HK$18,000 mandatory, HK$60,000 TVC/annuity shared cap, where to claim on BIR60, and what a TVC really saves.
Japan take-home pay 2026 (手取り): social insurance, income tax and resident tax explained
Japan take-home pay 2026: social insurance, income tax with the 2026 deduction changes, and resident tax. Worked example: JP¥4 million in Tokyo.
Joint vs separate assessment in Hong Kong: which saves tax in 2025/26?
When a married couple in Hong Kong should elect joint assessment for salaries tax, with IRD's 2025/26 examples worked step by step and the shared HK$3,000 reduction cap.
Hong Kong salaries tax 2025/26: rates, allowances and the HK$3,000 reduction
Hong Kong salaries tax rates 2025/26: progressive bands (2%–17%), two-tier standard rate (15%/16%), IRD allowances, deduction caps, and the HK$3,000 reduction.
Currency conversion cost: mid-market rate, spread, fees and DCC explained
What the mid-market rate is, how the exchange rate spread and fees add up, why 'no fee' conversions can still be expensive, and what card dynamic currency conversion (DCC) means. Worked example: HK$10,000 to US dollars.
How MPF contributions are calculated: the 5% rule, HK$7,100 and HK$30,000, plus a retirement projection
How Hong Kong MPF mandatory contributions work: 5% each from employer and employee, no employee contribution below HK$7,100 a month, a HK$1,500 cap above HK$30,000. Self-employed rules and a 30-year balance projection.
US dividend withholding tax for non-US investors: why 30%, treaty rates and W-8BEN
Why US dividends paid to non-US investors are taxed at 30% at source, how tax treaties cut it to 15% or 10%, what Form W-8BEN does, and how much withholding costs over 30 years. Figures from the IRS.
How compound interest works
What compound interest is, the formula CalcIsland uses, and why a fixed rate is not an MPF return or a forecast of the US stock market.
How mortgage payments are calculated (with a worked example)
Amortising mortgage payment formula, assumptions, and a worked example you can verify in the calculator — plus why early payments are mostly interest and how extra payments cut principal.