The IRS has a pay as you go system and requires businesses to submit estimated quarterly taxes that are finalized in their annual return. When employees declare dependencies in their W-4, taxes are calculated and paid for them by the employer. Employees only need to submit an annual return with their W-2, along with any other info, and often get a return amount.
Freelancers are self-employed businesses, so you need to pay quarterly taxes. The easiest way to calculate it is by dividing your income on the previous year's return by 4 and paying by the deadline via the IRS payment system.
What the IRS calls a quarter is actually groups of months. Take careful note, because the amount of days are very different.
- Q1 Jan - March
- Q2 April - May
- Q3 June - August
- Q4 September - December
- https://www.irs.gov/payments/pay-as-you-go-so-you-wont-owe-a-guide-to-withholding-estimated-taxes-and-ways-to-avoid-the-estimated-tax-penalty
- https://www.irs.gov/payments
- https://www.reddit.com/r/tax/comments/m36oyb/quarterly_estimated_taxes_not_paid_on_the_quarter/