Pile load test is commonly conducted during both design and construction stages. The objective of pile load test during design stage is to obtain the actual soil parameters and ultimate load in-situ. On the other hand, for the test conducted during construction stage the objective is to prove that the actual pile capacity conforms to the design loads. This paper presents probabilistic interpretation of proof pile test to obtain the ultimate pile capacity. As the first step, the “actual” field parameters are back calculated using ultimate pile capacity from proof pile load tests. The probabilistic inverse method is used for back calculation of parameters. Soil parameters obtained from back calculation are then sampled using Monte Carlo simulation technique to generate histogram of ultimate pile capacity. From the synthetic histogram, other statistical metrics such as mean, standard deviation and cumulative probability density of ultimate pile capacity can be obtained.