Methodology
Transparency is a core principle of Calk-AU. This page explains exactly how each calculator category works, what data sources we use, and how we keep everything accurate. Built by Konstantin Iakovlev.
Client-Side Calculations
All Calk-AU calculators perform computations entirely in your browser using JavaScript. No data is sent to any server. This means your financial information stays on your device — we never see, store, or process your inputs. This design also ensures instant results with no loading times.
Tax Calculators
Income tax is calculated by applying the ATO's published tax bracketsfor the relevant financial year. The formula applies each bracket's rate to the income within that range:
| Taxable Income | Rate | Tax on This Bracket |
|---|---|---|
| $0 – $18,200 | 0% | Nil (tax-free threshold) |
| $18,201 – $45,000 | 16% | 16c per $1 over $18,200 |
| $45,001 – $135,000 | 30% | $4,288 + 30c per $1 over $45,000 |
| $135,001 – $190,000 | 37% | $31,288 + 37c per $1 over $135,000 |
| $190,001+ | 45% | $51,638 + 45c per $1 over $190,000 |
The Low Income Tax Offset (LITO) of up to $700 is applied automatically for eligible earners. Medicare levy (2%) is added, with the low-income shade-in range from $27,222 to $34,027. GST is a flat 10% — to add GST, multiply by 1.1; to remove, divide by 1.1.
HECS-HELP Calculator
From 2025-26, HECS-HELP uses a marginal repayment system (similar to income tax brackets) rather than flat rates applied to total income. Rates are sourced from the ATO's study and training loan page. The compulsory repayment threshold is $67,000.
Superannuation Calculators
Super guarantee (SG) is calculated at 12% of ordinary time earnings. The retirement projection calculator uses compound growth with annual SG contributions. We source super rules from ATO super guidelines. Key thresholds: concessional cap $30,000/year, non-concessional cap $120,000/year, Division 293 threshold $250,000.
Mortgage & Property
Mortgage repayments use the standard amortisation formula: M = P × [r(1+r)^n] / [(1+r)^n – 1], where P is the principal, r is the monthly interest rate, and n is the total number of payments. This is the same formula used by Australian banks and lenders.
Stamp duty rates are sourced from each state's revenue office. We maintain rate tables for all 8 states and territories, including first home buyer exemptions, concessions, and foreign buyer surcharges. The borrowing power calculator uses a 3% serviceability buffer above the actual rate, matching standard bank assessment practices.
Pay & Employment Calculators
Pay calculations are based on the standard Australian 38-hour work week per the National Employment Standards (NES). Overtime rates (time-and-a-half, double-time) follow common award conditions. Redundancy pay uses the Fair Work Act minimum scale. The national minimum wage ($24.95/hour from 1 July 2025) is sourced from Fair Work Australia.
Health Calculators
The BMI calculator uses the standard formula: weight (kg) ÷ height (m)². The calorie calculator uses the Mifflin-St Jeor equation, which research shows to be the most accurate for estimating Basal Metabolic Rate. Pregnancy due datesuse Naegele's rule: LMP + 280 days.
Inflation Calculator
Historical inflation data is sourced from the Australian Bureau of Statistics (ABS) Consumer Price Index. We use annual CPI figures from 2000 to 2025. Each year's rate is applied sequentially to compute cumulative inflation.
Update Process
We follow a structured update cycle to ensure accuracy:
- 1 July annually: All calculators updated for the new financial year — tax brackets, super rates, HECS thresholds, stamp duty rates, and levy amounts.
- Post-Budget: Any changes announced in the Federal Budget are implemented as soon as they are legislated.
- Ongoing: State-level changes (stamp duty, land tax, payroll tax) are monitored via state revenue office publications.
Validation
We cross-check key calculator outputs against:
- ATO's own calculators (income tax, PAYG, HECS)
- ASIC MoneySmart tools (mortgage, super, retirement)
- Published ATO tax tables for PAYG withholding
Rounding
Tax calculations are rounded to the nearest dollar, consistent with ATO rounding rules. Currency amounts display to two decimal places. Percentages display to one or two decimal places depending on context.
Limitations
Our calculators use standard rates and thresholds. They may not account for all individual circumstances, such as private tax rulings, specific deductions, non-standard employment arrangements, or state-specific concessions not covered. Results are for general guidance only. Always verify with a qualified professional — see our Terms of Use and Privacy Policy.