Tax for Plumbers

Self-Employment Tax and the 92.35% Multiplier

Self-employment tax is the single largest tax hit for most self-employed plumbers, often exceeding federal income tax itself. For 2026, the combined SE tax rate is 15.3% — comprising 12.4% for Social Security (up to the $184,500 wage base) and 2.9% for Medicare (uncapped). An additional 0.9% Medicare surtax applies to SE earnings above $200,000 for single filers ($250,000 MFJ).

Before calculating SE tax, net self-employment earnings are multiplied by 92.35% (i.e., 0.9235). This adjustment, codified in IRC Section 1402(a), approximates the fact that employees only pay FICA on their gross wages — the employer half is not included in the employee's taxable compensation. For a plumber with $95,000 Schedule C net profit, the SE tax base is $95,000 x 0.9235 = $87,732.50, and the SE tax is $87,732.50 x 0.153 = $13,423.07.

Critically, the deductible portion of SE tax (50% of the total, per IRC Section 164(f)) reduces adjusted gross income. This means a plumber paying $13,423 in SE tax gets a $6,711 above-the-line deduction, which in turn reduces both income tax and the QBI calculation. Many plumbers overlook this cascading benefit when estimating quarterly payments, leading to systematic overpayment of estimated taxes.

One structural escape from the full 15.3% burden is electing S-corporation status. An S-corp plumber pays FICA only on the 'reasonable salary' portion of income; the remainder flows through as a distribution not subject to FICA. The IRS scrutinizes unreasonably low salaries (see Revenue Ruling 74-44), but for a plumber netting $95,000, a reasonable salary of $55,000–$65,000 is generally defensible, saving roughly $4,500–$6,000 annually in FICA taxes. The trade-off is additional compliance cost: S-corps require payroll processing, quarterly 941 filings, W-2 issuance, and a separate Form 1120-S.

Self-employed individuals pay both the employer and employee share of Social Security and Medicare taxes on 92.35% of net self-employment income. (IRC Section 1401 (SE tax rates), IRC Section 1402(a) (92.35% multiplier), IRC Section 164(f) (deductible half of SE tax))

Section 199A Qualified Business Income Deduction for Plumbers

The Section 199A deduction — commonly called the QBI deduction — allows eligible self-employed plumbers to deduct up to 20% of their qualified business income from taxable income. Because plumbing is a non-SSTB, the deduction is available at all income levels without phase-out, making it one of the most valuable provisions in the Tax Cuts and Jobs Act for tradespeople.

The QBI deduction is the lesser of: (a) 20% of QBI, or (b) the greater of 50% of W-2 wages paid by the business, or 25% of W-2 wages plus 2.5% of the unadjusted basis of qualified property (UBIA). For a solo plumber with no employees and modest depreciable property, the W-2 wage / UBIA limitation can become binding above the income threshold ($191,950 single / $383,900 MFJ for 2026, indexed). Below that threshold, the limitation does not apply and the plumber simply takes 20% of QBI.

For a plumber with $95,000 net profit, the QBI deduction is straightforward: QBI equals net profit minus the deductible half of SE tax ($95,000 - $6,711 = $88,289). The QBI deduction is $88,289 x 20% = $17,658. This deduction reduces taxable income but not AGI — it sits on the Form 1040 below the AGI line, similar to the old personal exemption position. It does not reduce SE tax.

Plumbers who hire employees or invest in substantial qualifying property (service vehicles, trenching machines, pipe threading equipment) benefit from the W-2/UBIA alternative. A plumber paying $40,000 in total W-2 wages has a W-2 limitation of $20,000 (50% of wages), and adding 2.5% of, say, $80,000 in qualifying property UBIA yields an alternative limit of $10,000 + $2,000 = $12,000 — so the 50% W-2 test wins at $20,000. If QBI-based 20% exceeds both W-2 alternatives, the W-2/UBIA cap applies above the threshold.

Non-SSTB businesses may deduct up to 20% of qualified business income, subject to W-2 wage and property basis limitations above certain income thresholds. (IRC Section 199A, Treas. Reg. 1.199A-1 through 1.199A-6)

Vehicle Expenses: Mileage Method vs. Actual Cost

For most self-employed plumbers, the service vehicle is simultaneously the largest deduction opportunity and the most common audit trigger. Plumbers typically drive 20,000 to 35,000 business miles annually, making the vehicle deduction worth $14,500 to $25,375 at the 2026 standard mileage rate of 72.5 cents per mile.

The IRS provides two methods for deducting vehicle expenses. The standard mileage rate method (72.5 cents/mile for 2026) includes depreciation, insurance, fuel, maintenance, and repairs in a single per-mile rate. The actual expense method requires tracking every cost — fuel, insurance, repairs, tires, registration, depreciation — and applying the business-use percentage. A plumber using a $55,000 service van 90% for business would compute actual costs and multiply by 90%.

The standard mileage rate is simpler and often more advantageous for plumbers driving older, paid-off vehicles. The actual expense method tends to win for plumbers with newer, expensive vehicles (especially trucks with GVW over 6,000 lbs that qualify for accelerated depreciation) or those with high fuel costs. Once actual expenses are elected in the first year, the plumber may switch to standard mileage in a later year only if straight-line depreciation was used; MACRS depreciation locks out the mileage method permanently for that vehicle.

Regardless of method, the IRS requires contemporaneous mileage logs. 'Contemporaneous' means recorded at or near the time of travel — not reconstructed at year-end. Rev. Proc. 2019-46 and IRC Section 274(d) impose strict substantiation rules. A mileage log must include: date of trip, destination, business purpose, and miles driven. Digital apps (MileIQ, Hurdlr, QuickBooks mileage tracker) satisfy the requirement, but the plumber must ensure the app is running consistently. Estimated or round-number entries (e.g., '20 miles every day') are treated as inadequate substantiation and frequently disallowed on audit.

Plumbers operating vehicles with a gross vehicle weight rating (GVWR) over 6,000 pounds — common for Ford F-250s, Ram 2500s, and Chevy 2500 series — may be eligible for the full Section 179 deduction on the vehicle in the year placed in service, bypassing the luxury auto depreciation caps that apply to lighter vehicles under IRC Section 280F. This can produce a first-year deduction exceeding $50,000 on a qualifying truck or van.

Business vehicle expenses may be deducted using either the standard mileage rate or actual expenses, with strict contemporaneous log requirements. (IRC Section 162(a) (business expenses), IRC Section 274(d) (substantiation), Rev. Proc. 2019-46 (standard mileage rate methodology), IRC Section 280F (luxury auto limits))

Subcontractor Classification and 1099-NEC Reporting Obligations

Plumbing businesses frequently engage helpers, apprentices, and specialty subcontractors (drain cleaners, pipe fitters, fire sprinkler installers). The classification of these workers as independent contractors versus employees is one of the most consequential tax decisions a plumbing business owner makes, carrying penalties that can exceed the underlying tax liability.

The IRS applies a multi-factor common law test organized around three categories: behavioral control, financial control, and the type of relationship. If the plumbing business controls when, where, and how the worker performs tasks — provides tools, sets hours, requires attendance at specific jobs, dictates methods — the worker is likely an employee regardless of what the contract states. Conversely, a subcontractor who uses their own tools, serves multiple clients, invoices per job, carries their own insurance, and controls their own schedule is more likely a true independent contractor.

For every independent contractor paid $600 or more during the tax year, the plumbing business must file Form 1099-NEC by January 31 of the following year. Failure to file carries penalties of $60 per form (filed within 30 days late), $130 per form (31 days to August 1), and $330 per form (after August 1 or not filed), up to statutory maximums. Intentional disregard raises the penalty to $660 per form with no cap.

Misclassification triggers cascading liability under IRC Section 3509. If the IRS reclassifies a 1099 subcontractor as an employee, the plumbing business owes the employer's share of FICA (7.65%), federal income tax withholding (assessed at 1.5% of wages if no 1099 was filed, or 3% if no W-2 was issued), plus penalties and interest. Section 530 relief — which provides a safe harbor for businesses that had a reasonable basis for treating workers as contractors — requires consistent filing of 1099s. A plumber who fails to issue 1099s loses access to Section 530 relief entirely, making the classification fight significantly harder to win.

Many states impose independent additional tests. California's ABC test (Dynamex / AB 5) presumes all workers are employees unless the hiring entity proves (A) freedom from control, (B) work outside the usual course of business, and (C) an independently established trade. Prong B is particularly difficult for plumbers using helpers on their own plumbing jobs. Massachusetts, New Jersey, and Illinois apply similarly strict tests.

Businesses must correctly classify workers and file 1099-NEC forms for independent contractors paid $600 or more, with significant penalties for misclassification. (IRC Section 3509 (employer liability on reclassification), IRC Section 6721/6722 (information return penalties), Section 530 of the Revenue Act of 1978 (safe harbor))