Global standardization has led to the gradual replacement of the American Society for Testing and Materials standard to the International Standard Organization method. These standards establish useful and accurate baselines for materials manufacturing. ISO has developed over 18,500 standards, as of publication date. There isn't a mathematical formula for converting ISO back to ASTM, but you can refer to published tables to find the equivalent specifications in one system or another.
Identify the product specification linked to the ASTM designation. For example, ASTM D638-94b covers tensile evaluations for plastics, including overall length, radius and width.
Navigate to the online ISO Catalog published by the American National Standards Institute.
Click "keyword" and then type a description for the procedure or product into the "Enter keywords" text box. Click "Go." A list of relevant ISO standards will appear.
Select an ISO standard based on the given description. For example, ISO 527-5:2009 is "Plastics -- Determination of tensile properties -- Part 5: Test conditions for unidirectional fibre-reinforced plastic composites."
Although you don't need to purchase the actual catalog to get the ISO number, you may want to purchase one to be sure the material or procedure is exactly the same. ASTM and ISO aren't always exactly equivalent -- the only way to tell for sure is comparing the descriptions in both naming conventions.