Five calculators for the figures independent workers most often get wrong, each one built on rates read from irs.gov and each one showing its arithmetic rather than a single answer.
Self-employment tax is 15.3% on 92.35% of your profit, and it is charged whether or not you owe a dollar of income tax. This shows the whole calculation, including the half you deduct back.
The penalty is figured separately for each payment period, so the only number worth knowing is what to pay by each date. Last year's tax is the one figure you can know in advance.
72.5 cents through 30 June and 76 cents from 1 July. Split your log at that date, or the total is wrong in one direction or the other.
Below the threshold everyone gets 20%. Above it, a specified service business gets nothing and everyone else meets a wage limit. This shows which side you are on.
An employer pays half your Social Security and Medicare. Working for yourself, you pay both halves. This is the gap, and the hourly rate that closes it.