2026 Quarterly Estimated Tax Estimator

A Python script that turns a simple income config into a quarterly IRS payment schedule — built for anyone earning 1099/freelance income alongside a W-2 job.

Date: 2026-07-21 Type: Utility Theme: Taxes Status: Idea

What it is

A zero-dependency Python script (script.py) that reads a small JSON config describing your income — W-2 wages, freelance earnings, business expenses, and last year's tax bill — and prints a step-by-step federal tax estimate plus a per-quarter payment schedule for all four 2026 deadlines. It computes self-employment tax (Schedule SE), applies 2026 income tax brackets from the Tax Foundation, and shows both safe-harbor options (90% current-year vs. 100% prior-year) side by side so you can pick the one that fits your cash flow.

Who it serves

Anyone with mixed income — a salaried employee with a side consulting practice, a freelance designer or developer, a gig-platform worker receiving 1099-NEC forms — who has never calculated their quarterly tax obligation and wants a clear, number-by-number answer in under a minute.

The money problem it addresses

Most W-2 employees have taxes withheld automatically and never think about estimated payments. Add any meaningful freelance income and you suddenly owe self-employment tax (15.3% on 92.35% of net earnings) plus income tax on top of that — none of it withheld. The IRS expects quarterly estimated payments on April 15, June 16, September 15, and January 15; miss them and you pay 6–7% compounded-daily interest on the shortfall. The most common outcome: a painful surprise on April 15 when a filer discovers they owe $3,000–5,000 they never budgeted for, plus a penalty they could have avoided entirely. This script makes the obligation visible and actionable before it's too late.

How it works

1. Reads a JSON config with your income fields (W-2 wages, withholding, freelance income, business expenses, other income, prior-year tax).
2. Computes SE tax: net self-employment income × 92.35% × 15.3% = self-employment tax; half of that is deducted above-the-line to reach AGI.
3. Computes income tax: applies 2026 marginal brackets (Tax Foundation) to taxable income (AGI minus standard deduction).
4. Derives net tax due from estimated payments: total federal tax minus W-2 withholding already being collected.
5. Builds a schedule: divides the safe-harbor target across all four 2026 quarters, labels each as past / next-up / future relative to today's date.
6. Compares safe harbors: Option A (90% of current-year tax, lower cash outlay) vs. Option B (100%/110% of prior-year tax, zero penalty risk) — auto-selects the lower one.
7. Prints next steps with IRS Direct Pay and EFTPS links.

What's in this folder

• README.md — this file
• cover-image.png — concept illustration for the web app card
• metadata.md — workflow metadata sidecar
• requirements.md — functional requirements and tax computation spec
• script.py — the runnable Python estimator (stdlib only, no pip needed)
• sample-config.json — synthetic filer profile (Jordan Chen, single, W-2 + freelance)
• sample-output.txt — exact terminal output of running python3 script.py against sample-config.json

How to run / read it

# Run against the included sample config
python3 script.py --config sample-config.json

# Specify a safe-harbor method explicitly
python3 script.py --config sample-config.json --safe-harbor prior-100

# See all options
python3 script.py --help

To use with your own numbers: copy sample-config.json to my-config.json, fill in your figures, and point --config at it. No pip installs, no external services, no API keys — everything runs locally with Python 3.9+.

Open sample-output.txt to see the exact output produced by the sample config.

Estimated impact

A W-2 + $28,000 freelance filer like the sample (Jordan Chen) owes ~$3,844.67 in estimated federal tax across 2026. The safe-harbor minimum is $720.05 per quarter under the current-year method. Someone who completely ignores all four payments and pays the lump sum in April faces:

• ~$92–130 in IRS underpayment interest (6–7% annualised, compounded daily, on the missed amounts from each deadline — approximately $37 on Q1, $25 on Q2, $17 on Q3, $13 on Q4).
• A $3,844 cash-flow shock in April when they may not have the funds set aside.

The script surfaces the obligation in real time so the filer can set aside ~$240/month throughout the year and pay $720.05 on each deadline instead — avoiding both the penalty and the surprise.

Good to know

This is an educational estimator covering federal income tax and self-employment tax only — it does not include state income taxes, FICA differences for S-corp owners, the Additional Medicare Tax (0.9% over $200,000 AGI for single filers), or the Net Investment Income Tax. Rates and contribution limits are based on Tax Foundation and IRS sources current as of July 2026. Tax law changes frequently; verify all figures with a CPA or qualified tax professional before making payment decisions. This is not personalized tax, legal, or investment advice.

Sources

• https://taxfoundation.org/data/all/federal/2026-tax-brackets/ — 2026 federal income tax brackets, standard deductions
• https://www.nerdwallet.com/taxes/learn/estimated-quarterly-taxes — quarterly estimated tax overview, due dates, safe-harbor rules, $1,000 threshold
• https://www.irs.gov/faqs/estimated-tax — IRS FAQ: who must pay estimated tax
• https://www.irs.gov/faqs/estimated-tax/individuals/individuals-2 — IRS: when to pay estimated tax
• https://www.kiplinger.com/taxes/tax-deadline/602538/when-estimated-tax-payments-due — 2026 quarterly due date calendar (confirms June 16 for Q2)
• https://www.irs.gov/businesses/small-businesses-self-employed/self-employment-tax-social-security-and-medicare-taxes — SE tax rate (15.3%), Schedule SE multiplier (92.35%)
• https://www.countrytaxcalc.com/tax-guides/usa/quarterly-estimated-tax-guide-2026/ — 2026 guide: deadlines, safe-harbor rules, 100%/110% prior-year threshold
• https://relayfi.com/blog/what-is-the-penalty-for-not-paying-quarterly-taxes/ — underpayment penalty rate (6–7% for 2026 quarters)