This part of ISO 6336 specifies the fundamental formulæ for use in the determination of the surface load capacity of cylindrical gears with involute external or internal teeth. It includes formulæ for all influences on surface durability for which quantitative assessments can be made. It applies primarily to oil-lubricated transmissions, but can also be used to obtain approximate values for (slow-running) grease-lubricated transmissions, as long as sufficient lubricant is present in the mesh at all times. The given formulæ are valid for cylindrical gears with tooth profiles in accordance with the basic rack standardized in ISO 53. They may also be used for teeth conjugate to other basic racks where the actual transverse contact ratio is less than εαn = 2,5. The results are in good agreement with other methods for the range, as indicated in the scope of ISO 6336-1. These formulæ cannot be directly applied for the assessment of types of gear tooth surface damage such as plastic yielding, scratching, scuffing or any other than that described in Clause 4. The load capacity determined by way of the permissible contact stress is called the “surface load capacity” or “surface durability”.