Know what you owe before the IRS tells you.
The IRS safe-harbor formula, run on your own numbers, plus a calendar file for all four deadlines — so a quarter you forgot existed never turns into a penalty.
Runs entirely in your browser · Secure Stripe checkout · Verified against IRS.gov
A real number, and a reminder you’ll actually see
Know the real number. The IRS safe-harbor formula — 90% of this year, or 100%/110% of last year, whichever is smaller — run on your own figures.
Never miss the two deadlines everyone misses. June 15 and January 15 don't line up with a calendar quarter the way people expect — a downloadable calendar file fixes that for good.
The 110% rule applies itself. If last year's AGI was over $150,000, the calculator switches automatically — no need to remember which rule applies to you.
Read the guide once, understand it for good. The safe-harbor rule and the mistakes that trigger the underpayment penalty, explained in plain English.
Runs entirely in your browser. Your numbers never leave your machine — no account, no server, nothing transmitted.
Brandable. Bake in your business name and colors on the setup page if you want to hand this to clients as your own reference tool.
Two tools, verified against the source
A self-contained toolkit — no login, no server, everything opens in your browser and prints clean.
- CALCULATOR
Safe-Harbor Estimator
Four inputs — prior-year tax, prior-year AGI, this year's estimate, what's already paid — and a live per-quarter number.
- AUTO
90/100/110% handled for you
The calculator applies the smaller-of-two-thresholds rule automatically, including the AGI-triggered 110% switch.
- REMINDERS
Download a calendar file
One click generates a .ics file with all four 2026 deadlines — works with Google Calendar, Apple Calendar, Outlook, anything.
- REFERENCE
The rule, explained plainly
What the underpayment penalty actually is, why it's calculated per-quarter, and the four mistakes that trigger it most.
- HONEST
Tells you what it can't do
This doesn't calculate your actual tax liability — it does arithmetic on numbers you provide. The disclaimer says exactly where a CPA takes over.
- VERIFIED
Checked against IRS.gov
The percentages, threshold, and 2026 due dates were verified against the IRS's own published estimated-tax guidance.
For the self-employed who owe quarterly and would rather know now
- Freelancers and solo operators without an employer withholding tax for them
- Small-business owners paying themselves outside a W-2
- Anyone who's been surprised by an underpayment penalty before
- Owners comfortable pulling two numbers off last year's tax return
Not a substitute for a CPA — if your income is irregular or your situation is anything but simple, this kit tells you exactly when to hand it to a professional instead.
Straight answers
- Does this calculate my actual taxes?
- No. It applies the safe-harbor formula to numbers you provide — your prior-year tax, prior-year AGI, and your own estimate for this year. It doesn't know your deductions, credits, or the details of your return.
- Is this a replacement for a CPA?
- No, and the disclaimer says so plainly. It's a fast, honest arithmetic tool for the common case. If your income is irregular, you have multiple income sources, or anything about your situation is unusual, talk to a CPA or enrolled agent.
- Does it cover state estimated taxes?
- No — federal only. Most states have their own separate estimated-tax rules and deadlines, which this kit doesn't touch.
- How do the deadlines get on my calendar?
- The calculator generates a standard .ics file you download and import into whatever calendar app you use — Google, Apple, Outlook, all support it.
- What happens after I buy?
- Your success page unlocks an instant ZIP download. You can bake in your business name and brand colors on the setup page first, or grab the default kit right away.
Download once, yours permanently. No subscription, no upsell, no account to keep paying for.
14-day money-back guarantee · instant download · Refund policy
An arithmetic tool, not tax, legal, or accounting advice. No outcome or penalty waiver is guaranteed — see the full disclaimer inside the kit.