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Hired with W-9, will I owe estimated tax quarterly?
You must pay your taxes at the quarterly intervals. For most people the withholding done by their employer satisfies this requirement. However, if your income does not have any withholding (or sufficient), then you must file quarterly estimated tax payments.  Note that if you have a second job that does withhold, then you can adjust your W4 to request further withholding there and possibly reduce the need for estimated payments.  Estimated tax payments also come into play with large investment earnings. The amount that you need to prepay the IRS is impacted by the safe harbor rule, which I am sure others will provide the exact details on.