How We Calculate
Full methodology, data sources, and limitations for every RooLoans calculator.
Data Sources Overview
| Data Point | Source | Update Frequency |
|---|---|---|
| Interest rates | Consumer Data Right (Open Banking) APIs | Weekly (CDR scraper) |
| Comparison rates | CDR API — includes all fees per NCCP Act | Weekly |
| RBA cash rate | rba.gov.au statistics | Each RBA board meeting |
| Stamp duty rates | State revenue offices (SRO VIC, Revenue NSW, QRO QLD, etc.) | Annually (after state budgets) |
| First Home Owner Grant | State treasury / housing departments | Annually (after state budgets) |
| LMI premiums | Helia (formerly Genworth) and QBE LMI published rate guides | When insurers update tables |
| Drawdown stages | AS 4000-2024 General Conditions of Contract + HIA standard building contract | Updated with standard revisions |
| Serviceability buffer | APRA Prudential Practice Guide APG 223 | When APRA changes guidance |
| HEM benchmarks | Melbourne Institute Household Expenditure Measure | Quarterly |
Repayment Calculator
Uses the standard amortisation formula (Australian standard, monthly compounding):
where: P = principal, r = monthly rate (annual÷12), n = payments (years×12)
Default rate is the weighted average variable construction loan rate from CDR API data. Users can adjust this to any rate.
Interest During Build Calculator
Interest is calculated on the drawn balance at each stage using simple interest per month:
Default drawdown % (HIA standard contract):
Slab: 12% | Frame: 17% | Lock-up: 25% | Fit-out: 23% | Completion: 23%
Source: AS 4000-2024 General Conditions of Contract and HIA Lump Sum Building Contract. Users may override with custom percentages (must sum to 100%).
Stamp Duty Calculator
Progressive bracket calculation (similar to income tax). Each state has its own rate schedule. We use the most recent published rates from each state revenue office.
Note: For construction loans, stamp duty applies to the land component only (not the build). This is different from buying an established home where stamp duty applies to the full purchase price.
LMI Calculator
LMI premiums are estimated from published rate tables from Helia (formerly Genworth) and QBE — Australia's two LMI providers. Premiums are expressed as a percentage of the loan amount.
LMI applies when LVR exceeds 80%. Premiums vary by: LVR band (80-85%, 85-90%, 90-95%), loan amount, and insurer.
Borrowing Power Calculator
We use the APRA-mandated serviceability buffer: all lenders must add 3% to the contract rate when assessing repayment capacity (APG 223).
Max loan = PV(available_for_repayment, assessment_rate/12, 360)
This is a simplified estimate. Actual lending capacity depends on your credit history, existing debts, property type, and the individual lender's policy.