Calculator Accuracy Report: Cross-Validation

When you are making financial decisions—whether negotiating a salary in the Netherlands or planning your Box 3 wealth allocation—you need numbers you can trust.

To support our calculation methodology, we regularly test the calculation engines (rulingEngine, box3Engine, and refundEngine) and keep tax-year parameters centralized for review.

Below is the public summary of our 2026 calculator validation process.

Audit Background

We validate calculators at two levels: parameter review and engine regression tests. Parameter review checks that yearly values in src/data/taxParams.ts match the official-source values used by the site. Engine tests check that calculations remain stable when code changes.

The audit scope covered:

• Box 1 tax brackets and social security premium rates.
• 30% Ruling parameters (thresholds, WNT caps, and percentage step-downs).
• Heffingskortingen (Algemene Heffingskorting and Arbeidskorting segments).
• Box 3 fictitious return parameters (validated directly against the Belastingdienst).

Validation Results

The 2026 calculator set uses a single parameter source and automated tests for every calculation engine. The current test suite covers the main tax calculators and historical tax-parameter checks.

2026 Key Parameter Matches

Parameter	Our Engine Value	Reference / Official Value	Status
Box 1 Bracket 1 Limit	€38,883	€38,883	Matched in parameter file
Box 1 Bracket 1 Rate	35.75% (combined)	8.10% + 27.65% = 35.75%	Matched in parameter file
Ruling Threshold (Normal)	€48,013	€48,013	Matched in parameter file
WNT Salary Cap	€262,000	€262,000	Matched in parameter file
Arbeidskorting Max	€5,685	€5,685	Matched in parameter file

Calculation Logic Verification

Beyond static parameters, the mathematical execution of the tax code must be flawless. We audited the following logic pathways:

1. Box 1 progressive calculation: The engine applies the combined first-bracket rate used by the site and separates result lines where useful for users.
2. Arbeidskorting segment accumulation: The employment tax credit builds up over segments and phases out over a later segment; tests guard those thresholds.
3. WNT Cap Enforcement: Our engine correctly limits the 30% tax-free allowance to €78,600 (30% of the €262,000 cap for 2026).

Box 3 Parameters (Belastingdienst)

Because the reference repository does not calculate Box 3 wealth tax, we validated our box3Engine directly against the Belastingdienst 2026 Provisional Data.

Parameter (2026)	Our Engine Value	Official Source
Base Exemption (Single)	€59,357	Belastingdienst parameter source
Base Exemption (Partner)	€118,714	Belastingdienst parameter source
Bank Savings Fictitious Return	1.28%	Provisional 2026 parameter
Investment Fictitious Return	6.00%	2026 parameter
Flat Tax Rate	36%	Belastingdienst parameter source

Maintenance Process

Tax parameters change every year, and some values are provisional until later publication cycles. We maintain the site with a conservative process:

• Update src/data/taxParams.ts when official yearly parameters change.
• Keep provisional values clearly labelled in pages and calculator explanations.
• Run engine tests after every calculation or parameter change.
• Document known exclusions on calculator pages instead of implying full tax-return coverage.