Make a qualified guess based on previous experience and realistic numbers. Run it through your peers (specially anyone else involved in delivering it). Listen

Make a qualified guess based on previous experience and realistic numbers. Run it through your peers (specially anyone else involved in delivering it). Listen to the most pessimistic ones, rather than the über optimistic. Multiply that estimation you have now by π. Believe me, you will more often than not correct with such a calculation. Bonus: You will be right by more or less ~10% which you can either give as a discount to your client, or you can charge them 10% extra in the end. Warn them that there is ALWAYS risk when estimating, and all risk should be shared. Win Win FTW!