SelfQuarter: SE Quarterly Tax

SelfQuarter estimates US quarterly self-employment tax payments for a Schedule C filer (single). It computes SE tax (15.3% on 92.35% of net SE), the SE-tax deduction (half), the QBI deduction (20%, phased out above $250k), federal income tax on the result, and an optional state top-rate overlay for 19 of the largest states.

It also shows the prior-year safe-harbor floor (100% of prior tax, or 110% if your prior AGI was above $150,000), so you know the minimum payment to avoid an IRS underpayment penalty.

WHAT IT COMPUTES
- SE Social Security (12.4% to $181k wage base)
- SE Medicare (2.9% all earnings, +0.9% above $200k)
- Total SE tax and the deductible half
- QBI deduction (with phase-out for taxable income above $250k)
- Federal income tax on net SE after deductions
- Optional state top-rate overlay (CA, NY, TX, FL, WA, IL, PA, OH, MA, NJ, GA, NC, CO, AZ, OR, MI, VA, MD, MN, or none)
- Annual total tax and recommended quarterly payment
- Safe-harbor annual (100% or 110% of prior tax) and per-quarter

NOT MODELED
- Itemized deductions, SEP/Solo 401(k), HSA, married filing jointly, dependent credits, NIIT, AMT. v1 is the simple single-filer schedule C path.

SelfQuarter is an estimator. Consult a CPA for tax planning that matters.