You are expecting way too much precision for floats with values so small. Take a look at the constants epic defines for use in their math functions to get a ballpark idea of the ranges of precision you should be working within. Precision up to 1e-8 is probably as precise as you should attempt to keep rounding error relatively in check. Even that will not always be acceptable under many circumstances. Depending on what you are doing, you could probably even use 1e-4 precision and still have more than enough accuracy for your requirements.