Public Error Log
When we find a calculation error on CalculateTax.uk, we fix it and document it here. This page lists every known error, what was wrong, what the correct value should have been, the severity of the discrepancy, and how quickly it was resolved. We believe transparency about mistakes is better than pretending they do not happen.
Resolved errors
What happened: The CIS (Construction Industry Scheme) take-home pay calculator was using the 2023-24 Class 4 NI rate of 9% instead of the current 2025-26 rate of 6%. The Class 4 rate was cut from 9% to 8% in January 2024, then further reduced to 6% from April 2025. The CIS calculation module was referencing the older constant.
Financial impact: On £50,000 of self-employed profit, the error caused Class 4 NI to be overstated by £1,131 per year. Specifically: the incorrect calculation was (50000 - 12570) × 0.09 = £3,368.70, while the correct figure is (50000 - 12570) × 0.06 = £2,245.80. The £1,122.90 difference meant CIS pages were showing take-home pay that was too low by approximately that amount.
Pages affected: All pages under /cis/ that calculated self-employed take-home pay with Class 4 NI.
Resolution: The Class 4 rate constant was corrected to 0.06 and all CIS pages were regenerated. The fix was verified against HMRC's published self-employed NI rates for 2025-26.
HMRC source: Self-employed NI rates 2025-26
How to report an error
If you believe a calculation on this site is incorrect, please email [email protected] with:
- The URL of the page showing the incorrect figure
- The value you believe is wrong
- The value you believe it should be
- The HMRC source or other official reference supporting the correct value
We investigate every report. If the error is confirmed, we fix it, regenerate the affected pages, and add an entry to this error log within 24 hours. If the discrepancy is due to a different assumption (for example, you are comparing against a different tax code or pension setup), we will explain the difference.
Our approach to errors
Tax calculations affect real financial decisions, so accuracy matters more than speed. Our approach is:
- Verify before publishing. Every rate change is confirmed against the HMRC source, not press summaries or secondary websites.
- Fix immediately. When an error is found, the fix takes priority over new features or content.
- Document publicly. This error log exists because hiding mistakes is worse than admitting them. Every resolved error includes the incorrect value, the correct value, the financial impact, and how long it took to fix.
- Test systematically. The calculation engine includes test cases derived from HMRC worked examples. When a new error is found, a corresponding test case is added to prevent regression.
No calculator is perfect. The goal is to be transparent about imperfections and to correct them quickly when they are found.