Self-Employment Tax + Quarterly Estimated Payments

Why this matters

The IRS expects estimated tax payments four times a year from anyone whose income doesn't have tax withheld at source. Service business owners - whether LLC, sole prop, or S-Corp - pay quarterly OR face penalties. The IRS underpayment penalty isn't a slap on the wrist; it compounds + can hit + on a moderately profitable year. Most first-year contractors learn this the hard way in April when their CPA delivers the surprise bill plus penalty.

Self-employment tax fundamentals

If you're a sole prop, LLC, OR partnership: you pay self-employment (SE) tax on your business net income.

SE tax = the employee + employer side of FICA (Social Security + Medicare) that an employee would split with their employer. Self-employed pay BOTH sides because they're both.

Rates (2025):

• Medicare: 2.9% on ALL net earnings (no cap)

Total SE tax: 15.3% on net earnings up to the SS wage base, 2.9% above.

Plus federal + state income tax on the same income.

Why quarterly payments

Employees have tax withheld every paycheck. Self-employed receive gross + must remit themselves. The IRS requires:

• Pay-as-you-earn via estimated quarterly payments
• Use Form 1040-ES
• Federal estimated payments by due date
• State estimated payments separately (if your state has income tax)

2025 federal quarterly due dates:

• Q1: April 15 (for January - March income)
• Q2: June 15 (for April - May income)
• Q3: September 15 (for June - August income)
• Q4: January 15 of next year (for September - December income)

Note: quarters are NOT equal calendar quarters. Q1 is 3 months; Q2 is 2 months; Q3 is 3 months; Q4 is 4 months.

Underpayment penalty (what you're avoiding)

If you don't pay enough quarterly, the IRS charges underpayment interest + penalty:

• Calculated per quarter on the shortfall
• Roughly 8% annual rate (2025 federal short-term rate + 3%)
• Compounds across quarters
• Filed on Form 2210 with annual return

The "safe harbor" rules to avoid penalty:

Pay at least 100% of last year's total tax

OR

Pay at least 90% of THIS year's actual tax

OR

Owe less than at year-end after withholding + estimates

Hit any of these = no penalty.

How to calculate quarterly estimates

Method 1: Safe harbor based on last year (simplest)

• Look at last year's total federal tax owed (line 24 of 1040)
• Divide by 4
• Pay that amount each quarter
• Safe + simple

Method 2: Estimate this year's income + pay 90%

• Project annual income
• Apply tax rates (federal + SE + state)
• Pay 22.5% each quarter (90% / 4)
• Adjust as you go if actual differs

Method 3: Pay as you earn

• Each quarter, calculate actual income that quarter
• Apply tax rate
• Pay that amount
• Requires accurate accounting at end of each quarter

Most contractors use Method 1 in established years + Method 2/3 in growth years.

A working example

Single-member LLC, net business income, single filer 2025:

Total: ~ federal + state per quarter. Significant cash flow; plan for it.

Setting up the discipline

Best practice: open a separate "Tax Reserve" bank account. Transfer 25 - 30% of every payment received into it. Pay quarterlies from there.

Why: when quarterlies are due, the money's there. You don't have to scramble OR borrow.

Without this discipline: most contractors get to Q1 and Q2 OK + then revenue slows OR they overspend + Q3 + Q4 are catastrophic.

Paying quarterly estimates

Federal:

• IRS Direct Pay (free, online): irs.gov/payments
• EFTPS (free, requires enrollment): eftps.gov
• Mail check with Form 1040-ES voucher
• Through tax preparer (some CPAs handle this)

State: each state's revenue department portal.

Receipt + confirmation kept for tax records.

S-Corp owners: different mechanics

If you've elected S-Corp + are taking a salary:

• Your salary has W-2 withholding (federal, SS, Medicare, state)
• Your distributions don't (you're not employed; the corp is the employer)
• Estimated taxes still required on distribution income above withholding

The setup:

• Adjust W-2 withholding rate to cover most of your tax liability
• Pay quarterlies on the gap (typically distributions)
• Or: pay yourself enough salary to fully cover taxes via withholding (loses some of the SE tax savings; trade-off)

S-Corp owners often choose to fully cover taxes through withholding = no quarterlies = simpler. CPA structures this annually.

Common mistakes

• Skipping Q1: penalty starts accruing; pay something even conservatively low
• Underpaying because last year was lower: if revenue grew, switch to Method 2 (90% of this year)
• Paying too much: refund = interest-free loan to government
• Forgetting state quarterlies: state penalties often harsher than federal
• Late payment: even days late = interest; set calendar alarms

Year-end reconciliation

April 15 of the following year:

• File annual return (1040 + Schedule C OR 1120-S for S-Corp)
• Total tax calculated
• Subtract estimates paid + withholding
• Owe the difference OR refund the overage
• Form 2210 if any quarter was underpaid

If you owe a lot at year-end: increase next year's quarterlies.

When to bring in a CPA

If any of these apply:

• Multiple income sources (W-2 spouse, investments, rental)
• S-Corp election
• Multiple states' tax obligations
• Significant equipment / vehicle depreciation
• Plan to make large strategic moves (sell business, big investment)

CPA's annual cost: typically pays for itself in tax savings + peace of mind. Don't go cheap on accounting.

References

• IRS Form 1040-ES (Estimated Tax for Individuals)
• IRS Publication 505 (Tax Withholding + Estimated Tax)
• IRS Direct Pay portal: irs.gov/payments
• EFTPS: eftps.gov
• State revenue department portals (varies)
• Manuall internal: Incorporation Choices for Service Businesses, QuickBooks Setup for Service Business