Quarterlies: the calendar, the vouchers, the safe harbors

How quarterly estimates work

Quarterly estimated taxes are how the self-employed prepay a tax bill that has no employer withholding behind it. Because a solo practice files Schedule C and owes both income tax and the 15.3% self-employment tax on net earnings, the IRS expects those amounts paid in through the year, in four estimated installments, rather than all at once in April 1Ref 1Internal Revenue Service (2026).Estimated taxes.That estimated tax is paid in four quarterly installments (April, June, September, January) with safe harbors of 90% current-year or 100%/110% prior-year to avoid an underpayment penalty.2Ref 2Internal Revenue Service (2026).Self-employed individuals tax center.That a self-employed clinician files Schedule C and owes 15.3% self-employment tax on net earnings, paid in through quarterly estimates.. Miss enough of it and an underpayment penalty is added — which is exactly what a safe harbor is designed to prevent.

The four due dates

The year splits into four estimated periods, with payments due in mid-April, mid-June, mid-September, and the following mid-January 1Ref 1Internal Revenue Service (2026).Estimated taxes.That estimated tax is paid in four quarterly installments (April, June, September, January) with safe harbors of 90% current-year or 100%/110% prior-year to avoid an underpayment penalty.. The dates are not evenly spaced — the second period is short and the last runs long — and when a due date lands on a weekend or holiday it shifts to the next business day, so confirm the exact current-year dates on the IRS estimated-taxes page. You send each payment with a Form 1040-ES voucher or, more simply, through the IRS's electronic payment options 1Ref 1Internal Revenue Service (2026).Estimated taxes.That estimated tax is paid in four quarterly installments (April, June, September, January) with safe harbors of 90% current-year or 100%/110% prior-year to avoid an underpayment penalty..

• Q1: covers Jan–Mar, due mid-April.
• Q2: covers Apr–May, due mid-June.
• Q3: covers Jun–Aug, due mid-September.
• Q4: covers Sep–Dec, due mid-January.

The safe harbors that stop a penalty

A safe harbor is a floor of prepayment that switches off the underpayment penalty even if you still owe more in April. Pay in at least 90% of the current year's tax, or 100% of last year's total tax, and you are protected; that 100% rises to 110% if your prior-year income was above the higher-income line the IRS sets 1Ref 1Internal Revenue Service (2026).Estimated taxes.That estimated tax is paid in four quarterly installments (April, June, September, January) with safe harbors of 90% current-year or 100%/110% prior-year to avoid an underpayment penalty.. The penalty itself is calculated on Form 2210, but if you meet a safe harbor you never reach it.

The higher-of-last-year path is the easy one for a growing practice: you know last year's number exactly, so you can lock in protection before this year's total is even in view.

Sizing each payment off real numbers

Size each payment off your real numbers, not a guess. Start from projected net profit, subtract the deductions you will actually take, and estimate the tax on what's left — then divide across the remaining installments. Deductions change the base: a SEP or 401(k) contribution, Section 179 equipment, the home office, and the QBI deduction for health professionals all shrink what you owe, so build them into the projection 3Ref 3Internal Revenue Service (2026).Qualified Business Income Deduction.That the QBI deduction reduces income tax, so including it in a projection lowers the estimated payment.4Ref 4Internal Revenue Service (2026).Publication 946, How To Depreciate Property.That Section 179 and bonus depreciation expense qualifying equipment in the year placed in service, lowering the income the estimate is based on.5Ref 5Internal Revenue Service (2026).Guide to business expense resources.That ordinary and necessary costs such as mileage and supervision are deductible, reducing the income base for estimates..

• A big equipment purchase mid-year lowers the payments after it 4Ref 4Internal Revenue Service (2026).Publication 946, How To Depreciate Property.That Section 179 and bonus depreciation expense qualifying equipment in the year placed in service, lowering the income the estimate is based on..
• Deductible costs like mileage and supervision reduce the base too 5Ref 5Internal Revenue Service (2026).Guide to business expense resources.That ordinary and necessary costs such as mileage and supervision are deductible, reducing the income base for estimates..
• Recompute after any large swing in income or deductions.

The set-aside habit that prevents surprises

The habit that actually prevents surprises is mechanical: move a fixed percentage of every deposit into a separate tax account the day it arrives. When money comes in, a set share — often something like a third, tuned to your bracket with your CPA — goes straight to the tax account and is never treated as spendable. Then each quarter you pay the estimate from that account, already funded.

The failure mode is spending the gross and scrambling in April; the fix is separating the tax money at the source.

• Open a dedicated account for tax reserves.
• Sweep a set percentage on every deposit, automatically if you can.
• Pay each estimate from that account, not from operating cash.

State estimates and payroll are separate

Two things live outside this federal calendar. First, most states — and some cities — run their own estimated-tax system, and that local layer keeps its own due dates that may not line up with the federal ones; your state's revenue agency sets those, so check them separately. Second, if you have employees, their payroll tax deposits are a completely different obligation from your personal estimates: you withhold and deposit employment taxes on the IRS deposit schedule for the business, and that money is not yours to borrow against 6Ref 6Internal Revenue Service (2026).Understanding employment taxes.That an employer withholds and deposits employees' payroll taxes on the IRS deposit schedule — an obligation separate from the owner's personal estimated taxes.. Keep the two streams — personal estimates and payroll deposits — in separate accounts, mental and actual.