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Worked example
Real numbers from this tool, line by line.
$10,274.61 is what a US Shopify seller was carrying on 2026-09-21 after crossing one state's economic nexus threshold on 2025-11-14 and collecting no sales tax in the ten months since.
That number is not a penalty notice. Nobody has written to them. It is the arithmetic of ten quiet months — the number their bookkeeper could not produce on the call, because the threshold lookups everyone reaches for stop at the word "yes".
Here is the run, exactly as the defaults in Nexus Exposure Clock 2026 produce it. The state uses the $100,000-or-200-transactions test. The seller did $142,000 and 260 orders into that state in the rolling twelve months, and passed the line on 2025-11-14. Since then they have averaged $12,500 a month into that state. The combined state and local rate they should have been charging is 7.25%. The state's failure-to-file and pay penalty is 10%, interest runs at 9% a year, and a voluntary disclosure agreement there limits the lookback to 36 months.
Run on 2026-09-21: ten full months collecting nothing. $9,062.50 of tax they should have charged and did not. $906.25 of penalty. $305.86 of interest. $10,274.61 carried today.
The line that matters to an owner is the next one: six more quiet months add $6,491.02 — not because the rate changes, but because the meter never stopped. And the third line: $906.25 is still saveable through a voluntary disclosure — the penalty, waived, because the seller came forward first. That saving exists only until the state's first contact letter arrives. After that the same money is simply owed.
WHY THE HAND CALCULATION IS ALWAYS WRONG IN THE SAME DIRECTION
Most people, asked to price this, multiply: total sales since crossing, times the rate, times the interest rate, once. That is wrong twice over. Tax accrues at the end of each month, so the month that closed last week has had no time to accrue interest at all, while the month of last December has had nine. Interest has to be run per month, not against a total. In the run above, the flat method produces $815.63 of interest where the monthly accrual produces $305.86 — off by a factor of nearly three, in the direction that makes an owner distrust the whole schedule when the state's notice disagrees.
The second error is upstream. There is no single $100,000 threshold. States run at least four shapes: sales only at $100,000, sales or 200 separate transactions, sales only at $500,000, and sales plus a transaction count. Several states have dropped their transaction counts, so a seller with 260 small orders and $60,000 of sales can be over the line in one state and under it in the next. A general-purpose assistant will confidently apply the wrong shape, because that shape was true somewhere, once.
WHAT THE FREE TOOL DOES
Everything above. One state, priced end to end: the threshold test, the months uncollected, tax, penalty, interest, the total carried today, the cost of six more quiet months, and the part a voluntary disclosure still saves. In the popup or on the free web page, with no licence key, no run limit and no watermark. Your figures never leave the tab.
The $60 key sits on a different axis entirely: it exports every state you have run to .csv so the schedule can live in the working papers, and it sets dated browser reminders before a lookback window or a quarter closes. Neither of those changes an answer. They just stop you recomputing it.
Done by hand, a bookkeeper at $75 an hour needs roughly four hours per state to rebuild the year by destination and test the threshold — about $300 per state, repeated every time the owner asks again.
Questions people ask
What does this actually work out?
You give it the threshold test your state uses, the date you passed it, your average monthly sales into that state and the rate you should have charged. It returns the tax you never collected, the penalty on it, interest accrued month by month, the total you are carrying today, what six more silent months add, and the part a voluntary disclosure can still save.
Who is it for?
US Shopify, Etsy, Amazon and SaaS sellers who ship or sell into states where they have no office, and the bookkeepers and fractional controllers who close their books. It is built for the moment you already know you crossed a threshold and need to tell an owner what the silence has cost so far, in dollars.
Why not just ask a chatbot?
A chatbot will hand you a $100,000 threshold and stop. It does not know the month you crossed, it cannot accrue interest per month on your own figures, and it will happily apply one state's transaction count to a state that dropped transaction counts. The number an owner needs is arithmetic on your numbers, not a remembered threshold.
What is free and what needs the key?
Every calculation is free and unlimited: one state, priced end to end, in the popup or on the web page, with no key and no watermark. The $60 licence key adds two things on a different axis - a .csv export of every state you have run so it can live in the working papers, and dated browser reminders before a lookback window or quarter closes.
What would this cost done by hand?
A bookkeeper at $75 an hour needs roughly four hours per state to rebuild twelve months of sales by destination, test the threshold and build the accrual schedule - about $300 per state, repeated every time an owner asks again. A multi-state nexus study from a state and local tax specialist is quoted well above that.
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