Two approaches are compared for estimating static fatigue lifetime at low cumulative failure probability. Both approaches use equations based on linear elastic fracture mechanics, but one uses the distribution of fiber strength to estimate the lifetime at low failure probability, and the second uses the distribution of time to failure. The two approaches give the same result, within experimental error, even though different sets of data are used in the calculation. Both approaches have advantages. Use of the distribution of time to failure is simpler experimentally because it does not require measurement of inert strength. However, this approach is limited to cases in which the strength distribution is unimodal. Use of the strength distribution is more generally applicable because it does not require that the strength be unimodal. Experimental data are presented for fiber tested in two-point bending at 80°C and 60% relative humidity. Lifetime predictions for a bending application are made using both approaches of extrapolation to low failure probability. The uncertainty in the calculations is estimated and the results compared. Two techniques are used to estimate the uncertainty in the lifetime calculation, the propagation of error technique and Monte Carlo simulation. A sensitivity analysis is presented that shows the sensitivity of the lifetime calculation to parameters such as fiber diameter, bend radius, and strength of the fiber.