A new fatigue crack growth formulation at the small time scale is proposed in this paper. This method is different from the traditional cycle-based fatigue formulation and is based on the incremental crack growth at any time instant during a cycle. It can be used for fatigue analysis at various time and length scales and is very convenient for the fatigue analysis under random variable amplitude loading without cycle-counting. The proposed model only requires basic material mechanical properties to develop the shape and magnitude of the traditionally used curve. Stress ratio and near-threshold crack growth behavior are included in the proposed methodology. In the proposed methodology, the reversed plastic zone concept and the crack closure concept is adopted to determine the lower integration limit in a single load cycle, respectively. Both approaches have been investigated and their advantages and disadvantages are discussed in detail. The proposed methodology is validated with extensive experimental observations for various metallic materials. Very good agreements with experimental data are observed. Error analysis and statistical testing are employed to compare the accuracy of the crack closure model and the reversed plastic zone model.