What Denver Freelancers and 1099 Workers Owe the State on September 15 and What They Always Get Wrong
Federal, state, and city obligations don't arrive on one bill. Here's how to calculate each layer before the deadline hits.
Federal, state, and city obligations don’t arrive on one bill. Here’s how to calculate each layer before the deadline hits.
September 15 is a hard deadline. It catches Denver’s freelance workforce the same way every year — not because people don’t know it exists, but because they don’t know what they’re actually supposed to pay, to whom, or how to calculate it for Colorado specifically.
The Q3 estimated tax due date applies to anyone earning income without automatic withholding: the UX contractor billing clients from a home office in LoHi, the healthcare consultant working out of a Capitol Hill apartment, the video producer in RiNo collecting five-figure invoices after a busy summer festival run. For most of these workers, Q2 and Q3 are their highest-earning months. Which makes the September 15 payment both the most important quarterly payment and the one most likely to be undercalculated.
Here’s the structural problem: federal and state estimated taxes are due on the same date, but they’re filed with different agencies on different forms through different portals. Layer on Denver’s Occupational Privilege Tax and Colorado’s FAMLI program, and you have four distinct financial obligations that no single national tax resource explains in one place. TurboTax and NerdWallet will walk you through Form 1040-ES. They will not tell you what DR 0104EP is, where to pay it, or that you may owe Denver money you’ve never heard of.
This piece does all of that.
The Q3 Deadline Is September 15 — for Both IRS and Colorado, Separately
For federal purposes, Q3 2026 estimated taxes are due September 15 under the IRS’s installment schedule for Form 1040-ES. Colorado’s Q3 estimated payment, filed on Form DR 0104EP, shares the same date. That’s the one piece of overlap. Everything else — the form, the calculation, the portal, the payment — is separate.
Missing September 15 isn’t catastrophic if you pay shortly after. Underpaying is the worse problem. Filing on time with too little creates a shortfall that persists until you file your annual return, with penalties accruing from the original due date forward. Most freelancers don’t figure this out until they get a notice in April. By then, the math has already gotten worse.
Your Colorado Tax Bill Is Not a Percentage of Your Federal Tax Bill
This is the most common calculation error Denver CPAs report seeing. It makes intuitive sense, which is exactly why it keeps happening. Self-employed workers often have a rough sense of their federal tax burden and assume Colorado’s slice is something like a fraction of that number.
It isn’t. Colorado taxes income at a flat 4.40% rate on Colorado taxable income — a rate set entirely independently of the federal self-employment tax, applied to a separately calculated base. The sequence matters: Colorado starts with your federal adjusted gross income, applies any Colorado-specific additions and subtractions required on the DR 0104, then applies the applicable Colorado deduction to arrive at Colorado taxable income. Multiply that by 4.40%.
The dollar amount you sent the IRS for federal self-employment tax is not an input in that calculation. It has no bearing on what you owe Colorado.
Freelancers who estimate their Colorado payment as “a little less than what I sent the IRS” are almost certainly underpaying. The two numbers are independently derived. A Colorado 4.40% rate applied to Colorado taxable income often yields a smaller dollar figure than the 15.3% combined self-employment tax on the same income — but not always. Income level, deduction eligibility, and whether you’ve hit the Social Security wage base ceiling all shift the relationship.
To pay, go to revenue.colorado.gov, select “Make a Payment,” then choose “Individual Income Tax” and the estimated payment option. First-time users can pay as a guest without creating a full account — but confirm that guest-payment option is still available when you log in, since CDOR reorganized the portal in 2024–2025. Also verify the current Colorado income tax rate for 2026 directly on CDOR’s site before you run the math. The 4.40% rate is subject to TABOR-related adjustment and worth confirming annually.
DR 0104EP — What It Actually Is and How to File It
DR 0104EP is Colorado’s estimated individual income tax payment voucher. If you’ve paid federal estimated taxes before, you know 1040-ES. DR 0104EP is the state equivalent — different portal, different form, no synchronization of any kind. Sending money to the IRS does nothing for your Colorado obligation. Nothing.
The form itself is simple: your Social Security number or ITIN, the tax period, the payment amount. You don’t submit a worksheet with it the way you might with a detailed federal estimate. Colorado reconciles the payment against your actual liability when you file your annual DR 0104.
At revenue.colorado.gov, select “Make a Payment,” choose “Estimated Payment — Individual Income Tax,” enter your tax year, identifying information, and payment amount. ACH bank debit is available; other payment methods vary.
One practical warning: CDOR’s portal has shifted navigation and URL structure in recent years. Before September 15, walk the full payment path yourself rather than trusting a bookmarked URL from a prior year. I’ve heard from more than one filer who arrived at a dead link on deadline day. Paper filers can mail DR 0104EP with a check to the Department of Revenue, but paying online gives you immediate confirmation — worth a lot when a deadline is involved.
The Tax Denver Home-Based Freelancers Don’t Know They Owe
Denver levies an Occupational Privilege Tax, and if you’re self-employed and working from a home office within Denver city limits, you owe it. The key rule: OPT is triggered by where work is performed, not where your clients are. A freelance data analyst working from an apartment in Baker and billing clients in Chicago, Austin, and New York owes Denver’s OPT. A developer doing the same work from Englewood does not. Same metro area, different obligation.
For self-employed individuals, the math is simple enough but catches people off guard. You owe both the employee-side and the employer-side of the OPT — $5.75 per month and $4.00 per month respectively, combined $9.75 in any month where you earn $500 or more within Denver. That’s up to $117 per year. Confirm current 2026 rates with Denver Treasury before filing.
The ZIP code issue is worth flagging specifically. Parts of 80236 (near Fort Logan and Bow Mar) and 80239 (near Montbello, which borders Aurora and Commerce City) sit at or near Denver’s incorporated boundary. If you’re in one of these codes, call 311 or Denver Treasury’s business licensing staff to confirm whether your address falls inside city limits. Don’t assume based on ZIP code — it’s a five-minute phone call that could save you a surprise bill.
OPT is administered through Denver Treasury at denvergov.org, not through CDOR and not through Colorado Revenue Online. Verify the current direct payment URL with Denver Treasury before you file; the city’s eBiz Tax Services platform has been updated. Self-employed workers file quarterly or annually depending on preference. This payment is entirely separate from both your 1040-ES and your DR 0104EP.
FAMLI — What It Isn’t, What It Is, and Why Freelancers Get It Wrong
Colorado’s Family and Medical Leave Insurance program is not part of your quarterly estimated tax payment. It’s not collected through CDOR. It doesn’t automatically apply to self-employed workers. Many freelancers assume they’ve covered some FAMLI obligation through their quarterly filings. They haven’t.
Self-employed individuals in Colorado can participate in FAMLI, but it requires a voluntary opt-in through FAMLI’s own online portal — separate from Colorado Revenue Online. Without that opt-in, you have no FAMLI coverage and no FAMLI contribution obligation.
If you have opted in, here’s where it touches Q3 planning. The self-employed contribution rate for 2025 was approximately 0.45% of net self-employment income. Verify the 2026 rate at the FAMLI Division’s portal before filing — the rate adjusts annually. At $80,000 in net income, 0.45% is $360 a year. Spread across four quarters, that’s less than $100 per estimate. Real money, but not the thing that should be keeping you up at night.
The murkier question is whether those FAMLI contributions are deductible against Colorado taxable income — which would reduce what you owe on your DR 0104EP. Honest answer: the guidance here isn’t definitively settled, automated filing tools don’t handle it correctly, and it’s worth a call to your CPA before you adjust your estimate based on it.
What FAMLI is not: a substitute for your quarterly income tax payment. Freelancers who’ve opted in and are making premium payments sometimes believe they’ve covered a state tax obligation. They haven’t. Separate programs. Separate portals. Separate math.
Safe Harbor — The Rules That Protect You From Penalties
Colorado’s underpayment penalty applies when you haven’t paid enough throughout the year — not just when you pay late. Safe harbor eliminates that penalty risk entirely if you follow it.
Pay the lesser of 100% of your prior year’s Colorado tax liability or 90% of the current year’s actual liability, and you won’t owe an underpayment penalty at filing even if you still owe a balance. Verify Colorado’s specific thresholds at CDOR before relying on these figures; Colorado doesn’t always mirror federal safe-harbor rules and applies different rules at higher income levels.
Here’s what that looks like in practice: if you owed $3,200 in Colorado income tax for 2025, paying $800 per quarter in 2026 shields you from any underpayment penalty no matter what you actually earn this year. If a large contract triples your summer revenue, you’ll owe more at filing — but no penalty. The 90% option is useful when this year looks lighter than last: project your 2026 liability, take 90% of it, split it four ways.
Two different problems get confused here. Underpayment — too little, regardless of when — is assessed at filing using Form DR 0204, calculated from the due date of each quarterly installment. It compounds. If you were underpaid in Q1, Q2, and Q3, you’re not looking at a small number by April. Late payment is different: it’s daily interest on the amount owed, not a fixed penalty. CDOR publishes the annual rate; it has historically run around 7–8%. Check the current 2026 figure at revenue.colorado.gov.
If you have to choose between a payment made on time that’s slightly short and a correct payment made a few days late, the math is closer than people assume. But in most cases, paying the safe-harbor amount on time is the right call.
What Actually Happens If You Miss September 15
Colorado doesn’t hit you with a fixed dollar penalty for a missed quarterly payment. It charges daily interest on the amount that should have been paid. At 8% annual interest, a $2,000 Q3 payment made one week late costs you about $3. A $20,000 payment made one week late costs about $30. Annoying, but survivable.
The more serious scenario is paying on time with a number that’s too low. Send Colorado $500 in Q3 when the safe-harbor amount was $800, and you’ve locked in a per-quarter shortfall assessed at filing via DR 0204 — calculated as if that underpayment accrued from September 15 through filing day in April. That’s five or six months of accumulation, not seven days. Across multiple quarters of consistent underpayment, the penalty charges add up to hundreds of dollars. That’s the scenario that lands people in a CPA’s office in March looking confused.
Colorado and the IRS don’t coordinate. A missed Colorado payment doesn’t alert the IRS, and vice versa. Each agency runs its own clock. If you’ve missed September 15, pay Colorado today at revenue.colorado.gov, then make the federal payment through IRS Direct Pay or EFTPS. The interest stops accruing when payment is received.
Before September 15 — What to File, Where to Pay, and What to Verify
IRS — Form 1040-ES, Q3
Due September 15. Pay through IRS Direct Pay or EFTPS; confirm current URLs at IRS.gov. Select “1040-ES Estimated Tax” and tax year 2026. Nothing about this satisfies your Colorado obligation.
Colorado Department of Revenue — DR 0104EP
Due September 15. Go to revenue.colorado.gov → Make a Payment → Estimated Payment, Individual Income Tax. The rate is 4.40% flat on Colorado taxable income — verify the TABOR-adjusted 2026 rate at CDOR before you pay. Guest payment is available without a full account, but confirm this at the portal since CDOR reorganized Revenue Online in 2024–2025.
Denver Treasury — Occupational Privilege Tax
Applies if you perform work from a home office within Denver city limits in any month you earn $500 or more in Denver. The combined self-employed rate is $9.75 per month ($5.75 employee-side, $4.00 employer-side). Confirm current 2026 rates and the $500 threshold with Denver Treasury before filing. Pay at denvergov.org through the Treasury division — not through CDOR, not through Revenue Online. If your address is in a boundary ZIP code, call 311 first.
FAMLI Division — only if you have opted in
The 2025 contribution rate was approximately 0.45% of net self-employment income. Verify the 2026 rate at the FAMLI Division’s portal. This does not satisfy any quarterly Colorado income tax obligation. Whether your contributions reduce Colorado taxable income: confirm current CDOR guidance with a CPA before adjusting your DR 0104EP calculation.
Before you pay anything, verify:
The current Colorado income tax rate for 2026 at revenue.colorado.gov. The current Denver OPT rate and monthly earnings threshold at Denver Treasury. The FAMLI self-employed contribution rate for 2026 at the FAMLI Division portal. Whether your home address is inside Denver’s incorporated city limits — call 311 if there’s any question.
If September 15 is close and you haven’t made a Q3 payment yet, pay the safe-harbor amount — last year’s Colorado tax liability divided by four — to Colorado Revenue Online today. Make the federal payment through IRS Direct Pay or EFTPS. Do both before the deadline.
The layered obligation isn’t complicated once you see it clearly. It just requires running the math on the right forms, for the right agencies, with rates that are specific to Colorado — not whatever TurboTax defaulted to.
CityDesk Denver publishes local business and legal coverage for Denver’s working professional community. This article is for informational purposes only and does not constitute tax, legal, or financial advice. Consult a licensed CPA or enrolled agent for advice specific to your situation.
