You could try: %06.*g for width 6 down to 1. I believe this will always find an answer, and the one with the largest number of significant digits. For exa

You could try: %06.*g for width 6 down to 1. I believe this will always find an answer, and the one with the largest number of significant digits. For example: void format(double val, int width, char* buf) { for (int w = width; w > 0; w--) { int count = sprintf(buf, "%06.*g", w, val); if (count == width) return; } } "Working" example (unsafe, exits on failure, etc): https://godbolt.org/z/WiaPkT https://godbolt.org/z/WiaPkT Note that you can't represent negative values equal or below -1e-100 since those take 7 characters.