Compare diameter, not the number alone
Needle numbering systems are not interchangeable or linear. Metric labels describe diameter in millimeters. A number such as 8 can identify different diameters in US and Japanese systems.
A concrete example
The included reference lists US 8 knitting needles as 5.0 mm. Clover Japanese size 8 is 4.5 mm, size 0 is 2.1 mm, and size 15 is 6.6 mm. Japanese patterns can use numbered sizes; they do not exclusively use metric labels.
Check manufacturer conventions
Some legacy UK and hook-size conventions differ or have uncertain equivalents. FiberTools leaves unsupported cells empty rather than guessing. Check the actual manufacturer label and diameter instead of treating every row as a universal standard.
Use the converter
Choose knitting needles or crochet hooks and search a supplied label or a metric value such as 5mm. Read the complete matching row and its source limitations. A search match is a table entry, not a recommendation for the project.
Make a representative swatch
Begin with the pattern and exact yarn guidance, then measure a swatch in the intended stitch pattern, construction, and finishing. The same nominal diameter can produce different fabric with different tools, yarn, or tension. Matching the label alone does not guarantee gauge or fit.