Do I have to withhold taxes from my nanny's pay?
No, you can choose to pay your nanny's social security and Medicare taxes on her behalf. And your nanny can choose whether or not to have you withhold income tax.
How much will I owe?
FICA and Medicare taxes total 15.3% of your nanny's pay. You can withhold 1/2 of this from your nanny's pay, or you can choose to pay the entire amount. You will also pay federal unemployment tax of up to $56 per year per employee. Important update: the FICA tax has been temporarily reduced to 7.65 (your share) + 5.65 (nanny's share) = 13.3%.
California payroll tax is usually about 3.5%. The state will assign a rate to you, which will change each year.
How do I pay what I owe to the government?
It's easy. Just one extra page, Schedule H, with your federal tax return. Schedule H calculates how much social security, Medicare, and federal unemployment insurance you owe. The amount is added to your regular taxes and paid all together. If you think that you will be underpaid at the end of the year, you should increase your quarterly tax payments or salary withholdings to avoid penalties.
California also has an annual payroll report, DE 3HW, which is mailed separately from your state tax return, together with a check for the state payroll taxes.