Is it 20mm or 20 mm

The correct formatting is "20 mm" with a space between the number and the unit symbol, according to the International System of Units (SI) standards and most international style guides. This spacing convention applies to all SI units including millimeters, meters, kilograms, and seconds. The space visually separates the numerical value from the unit identifier, improving readability and maintaining consistency with scientific and technical documentation standards.

However, in casual writing, product specifications, and some regional contexts, you'll frequently encounter "20mm" written without a space, and this remains generally understood despite being technically non-standard. The spacing rule exists to prevent confusion with other notations and maintain clarity in technical communication—for instance, distinguishing "20 mm" from potential abbreviations or model numbers. In professional engineering drawings, scientific papers, technical manuals, and quality documentation, always use the spaced format "20 mm" to comply with international standards and maintain credibility. Digital typography and CAD software typically enforce this spacing automatically, though you should verify dimension formatting in your specific industry or organizational style guide, as some companies maintain internal standards that may vary slightly from SI recommendations.