Calculate.co.nz NZ Calculators

nz_mortgage_calculator

Calculate NZ mortgage repayments and lifetime interest totals.

Uses the standard amortising-loan PMT formula with monthly compounding.
Returns monthly, fortnightly, and weekly repayment figures, total
repayments over the life of the loan, total interest paid, and the
principal/interest share as percentages.

Use for ANY mortgage or home loan question: repayment amounts,
affordability checks, comparing loan terms, estimating total interest,
or evaluating refinance scenarios.

For NZ context: typical NZ mortgage terms are 20-30 years, and rates
quoted are annual nominal rates compounded monthly. Most NZ borrowers
pay weekly or fortnightly even though rates are quoted monthly.

Args:
    loan_amount: Principal borrowed in NZD (e.g. 650000).
    annual_interest_rate_pct: Annual rate as a PERCENTAGE not decimal
        (e.g. 6.5 for 6.5%, NOT 0.065).
    term_years: Loan term in whole years (typical NZ: 25 or 30).

Input Schema

TableJSON Schema

Name	Required	Description	Default
`term_years`	Yes
`loan_amount`	Yes
`annual_interest_rate_pct`	Yes

Schema Changelog

Changes observed during successful MCP inspections.

First observedSep 1, 2026

TDQS

A4.7/5.0

Behavior5/5

Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

With no annotations provided, the description carries the full burden and delivers: it discloses the exact formula ('standard amortising-loan PMT formula with monthly compounding'), the complete return inventory (monthly/fortnightly/weekly figures, total repayments, total interest, principal/interest shares), and NZ-specific compounding and payment conventions. No contradictions with annotations (none provided).

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Conciseness4/5

Is the description appropriately sized, front-loaded, and free of redundancy?

Well-structured and front-loaded: the core purpose leads, the Args section is clearly labeled, and the NZ context paragraph earns its place by explaining why weekly/fortnightly payments matter despite monthly compounding. Slightly redundant — the scenario list in paragraph two partially restates the opening purpose — but there is no wasted content.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Completeness5/5

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

With no output schema and no annotations, the description supplies everything needed in one place: parameter semantics, return values, the underlying formula, and domain conventions. An agent can select and invoke this tool correctly without consulting anything else, and the NZ-specific notes make it safe against common misassumptions about rate and term conventions.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Parameters5/5

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema description coverage is 0%, and the description fully compensates: each of the three parameters is documented with units (NZD), format (annual rate as a percentage not decimal, with explicit warning 'NOT 0.065'), and typical values ('whole years', '25 or 30'). The percentage-vs-decimal distinction is exactly the kind of semantic trap that prevents invocation errors.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Purpose5/5

Does the description clearly state what the tool does and how it differs from similar tools?

States a specific verb and resource — 'Calculate NZ mortgage repayments and lifetime interest totals' — and further scopes to 'ANY mortgage or home loan question.' This clearly distinguishes it from sibling calculators like nz_vehicle_loan_calculator or bmi_calculator by tool name and stated domain.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Usage Guidelines4/5

Does the description explain when to use this tool, when not to, or what alternatives exist?

Explicitly opens with 'Use for ANY mortgage or home loan question' and lists concrete scenarios (repayment amounts, affordability checks, comparing loan terms, refinance). It does not, however, name excluded cases or point to alternatives such as nz_vehicle_loan_calculator when the loan is not a mortgage.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.

All Calculate.co.nz NZ Calculators tools Inspect

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