Calculate.co.nz NZ Calculators

nz_kiwisaver_calculator

Project a NZ KiwiSaver balance from current age to retirement age.

Month-by-month compound interest projection matching the calculate.co.nz
methodology. Includes employee and employer contributions, voluntary top-ups
at your chosen cadence, the government contribution ($260.72/yr if eligible),
management fees, and PIR tax on investment gains. Returns both the nominal
final balance and the inflation-adjusted real (today's dollars) balance.

Use for ANY KiwiSaver retirement-planning question: 'how much will I have
at 65', 'is $X enough to retire on', 'what happens if I contribute more',
'how much does switching from Conservative to Growth help', 'what's the
impact of fees over 30 years'.

FUND TYPES and expected annual returns:
- Cash: 1.00% (very low risk, low return)
- Conservative: 2.50% (low risk, default)
- Balanced: 4.00% (moderate risk)
- Growth: 7.00% (higher risk, higher expected return)
- Custom: you specify via custom_return_pct

PIR (Prescribed Investor Rate) is the tax rate on your KiwiSaver
investment gains, NOT your income tax rate. Valid values:
- 10.5 (low incomes)
- 17.5 (middle incomes)
- 28 (higher incomes, default)

KEY RULES APPLIED (current as of April 2026):
- Government contribution: full $260.72/yr for eligible members
  (halved from $521.43 on 1 July 2025)
- Not available if annual income exceeds $179,999.99
- Minimum contribution 3.5% from 1 April 2026 (rising to 4% in 2028)

Args:
    current_balance: Starting KiwiSaver balance in NZD.
    current_age: Current age in whole years.
    retirement_age: Projection end age (default 65).
    annual_income: Current gross annual salary (default 100000).
    income_growth_pct: Annual salary increase % (default 1.0).
    employee_rate_pct: 0, 3, 3.5, 4, 6, 8, or 10 (default 3.5).
    employer_rate_pct: Employer contribution % (default 3.5, NZ minimum).
    voluntary_amount: Extra contribution per period in NZD (default 0).
    voluntary_frequency: 'week', 'fortnight', 'month', 'year' (default 'month').
    fund_type: Cash, Conservative (default), Balanced, Growth, or Custom.
    custom_return_pct: Required if fund_type='Custom'.
    pir_rate_pct: PIR as percentage: 10.5, 17.5, or 28 (default 28).
    annual_fee_pct: Management fee % annually (default 0.40).
    inflation_pct: For real-value reporting (default 3.0).
    switch_fund_at_age: Optional age to switch fund (e.g. derisk at 55).
    switch_to_fund: Fund to switch to at that age.
    switch_custom_return_pct: Required if switching to Custom fund.

Input Schema

TableJSON Schema

Name	Required	Default
`fund_type`	No	Conservative
`current_age`	Yes
`pir_rate_pct`	No
`annual_income`	No
`inflation_pct`	No
`annual_fee_pct`	No
`retirement_age`	No
`switch_to_fund`	No
`current_balance`	Yes
`voluntary_amount`	No
`custom_return_pct`	No
`employee_rate_pct`	No
`employer_rate_pct`	No
`income_growth_pct`	No
`switch_fund_at_age`	No
`voluntary_frequency`	No	month
`switch_custom_return_pct`	No

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?

No annotations are provided, so the description carries the full burden, and it delivers: it discloses the month-by-month compounding methodology, all included factors (contributions, government contribution, fees, PIR tax), the output format (nominal and real balance), current legal rules as of April 2026, and clarifies that PIR is not income tax. This is exceptionally transparent.

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?

The description is long but well-organized with clear section headers and bullet lists, and it front-loads the purpose and methodology. The length is justified by the 17-parameter surface and the policy rules; only slight redundancy with the schema's default values keeps it from a perfect score.

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?

Even though there is no output schema, the description explicitly states that both nominal and inflation-adjusted real balances are returned. It covers all parameters, special cases like fund switching, custom return handling, and current contribution/income thresholds. Nothing an agent needs to call the tool correctly is missing.

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%, so the description must compensate, and it does comprehensively. The Args section explains all 17 parameters with defaults, allowed values, and conditional requirements (e.g., custom_return_pct required when fund_type='Custom'), plus fund-type return expectations not present in the schema.

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?

The description opens with a specific verb and resource: 'Project a NZ KiwiSaver balance from current age to retirement age.' The scope is unmistakable and none of the sibling tools cover KiwiSaver, so it is clearly differentiated.

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?

The description explicitly says 'Use for ANY KiwiSaver retirement-planning question' and gives five concrete example queries, which establishes the intended context clearly. However, it does not name alternatives or state when not to use the tool, so it falls just short of the top score.

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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