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Literary agent Chip MacGregor has a simple formula that he uses when authors ask him about what it takes to be able to write as a fulltime job. He explains it on his blog at:
Using the simple form below, you can apply his formula to your own work. Keep in mind that you may have additional expenses, such as health insurance, if you quit your day job to spend your days writing.
NOTE: This tool is intended only as a guide and should not be taken as financial advice.