Because we work with organizations of various sizes with different track records, we do not calculate a grant writing "success" rate. What we do is help you to avoid common mistakes and to put your best case forward. We know for certain that your success rate will increase when you heed the advice of a seasoned professional! Over time, you should achieve a 33% to 50% success rate, depending on how much you stretch your organizational capacity. The fact is the more grants you develop, the better your chances. Before you balk at the cost of a grant writer consider that the return on investment is usually worth it.
Do not write proposals for your colleagues or a city council, instead of the grant funder. Once a fire chief refused to allow us to tell the federal government that their fire station was built by volunteers who had dropped beer bottles between the cinder blocks in lieu of rebar. He was afraid of how their department would be perceived by the public. They didn't get the seismic retrofit grant. He was fired shortly thereafter.