Calculator Accuracy Report: Cross-Validation
Read our comprehensive cross-validation audit report. See how our 30% ruling and Box 3 engines perform against industry-standard reference data.
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 prove our commitment to cent-level accuracy, we regularly subject our calculation engines (rulingEngine, box3Engine, and refundEngine) to rigorous cross-validation audits.
Below is the public summary of our March 2026 Cross-Validation Audit.
Audit Background
We cross-verified our internal engines against the highly respected open-source repository stevermeister/dutch-tax-income-calculator-npm, which serves as the backend for several prominent Dutch salary calculators.
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 (34/34 Parameters)
Our internal database (taxParams.ts) successfully validated 34 out of 34 critical data points across the 2025 and 2026 tax years.
2026 Key Parameter Matches
| Parameter | Our Engine Value | Reference / Official Value | Status |
|---|---|---|---|
| Box 1 Bracket 1 Limit | €38,883 | €38,883 | ✅ Match |
| Box 1 Bracket 1 Rate | 35.75% (combined) | 8.10% + 27.65% = 35.75% | ✅ Match |
| Ruling Threshold (Normal) | €48,013 | €48,013 | ✅ Match |
| WNT Salary Cap | €262,000 | €262,000 | ✅ Match |
| Arbeidskorting Max | €5,685 | €5,685 | ✅ Match |
Calculation Logic Verification
Beyond static parameters, the mathematical execution of the tax code must be flawless. We audited the following logic pathways:
- Box 1 Progressive Calculation: Our combined approach (merging Income Tax and Social Security into a single 35.75% Bracket 1 rate) mathematically equates perfectly to the separated payroll tax perspective.
- Arbeidskorting Segment Accumulation: The Dutch employment tax credit builds up over three segments and phases out over a fourth. Our engine perfectly replicated the piecewise linear accumulation model.
- 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.nl |
| Base Exemption (Partner) | €118,714 | ✅ Belastingdienst.nl |
| Bank Savings Fictitious Return | 1.28% | ✅ Provisional 2026 Target |
| Investment Fictitious Return | 6.00% | ✅ Provisional 2026 Target |
| Flat Tax Rate | 36% | ✅ Belastingdienst.nl |
Open-Source Discrepancy Correction
During the audit, we identified a minor but distinct error in the widespread open-source reference data regarding the 2026 Arbeidskorting phase-out endpoint.
The reference data listed the terminal point as €132,948. Through rigorous cross-referencing with Belastingdienst official documents and leading accounting firms (like Salure.nl and AMS-Accountants), we confirmed the correct termination point is €132,920.
Our internal engines strictly utilize the verified €132,920 figure, ensuring that high-income earners receive the legally accurate (zeroed-out) credit amount without computational leakage.