Key takeaways
- Form 100S pays the greater of 1.5% of net income or $800. A newly incorporated corporation is exempt from the minimum in its first year.
- For a calendar-year 2026 return, the due date is March 15, 2027, and a weekend or holiday moves it to the next business day.
- The automatic six-month extension to file does not extend the time to pay, and it depends on the tax being fully paid.
- Check each shareholder's federal tax classification before preparing the return, because a partnership shareholder is not allowed.
What Form 100S covers
Form 100S is the California return for an S corporation. The corporation files it with the Franchise Tax Board (FTB) alongside its federal Form 1120-S, and the California tax is computed on Form 100S, not on the federal return.
Before the state file is opened, check three things in the client folder: the federal S election is in place, the date the corporation was incorporated or qualified in California is on file, and the legal classification of the entity matches what the client says. The LLC question is covered below, because the return depends on how the LLC is classified.
The $800 minimum and the 1.5% tax
The FTB says the S corporation tax is the greater of 1.5% of the corporation's net income or $800. The $800 is a floor, not an add-on: the corporation pays whichever amount is larger. A loss year produces no 1.5% amount, so the minimum is the tax due.
Worked example for tax year 2026. A corporation with California net income of $100,000 owes 1.5% times $100,000 for $1,500. A corporation with net income of $40,000 owes 1.5% times $40,000, or $600. The $800 minimum is therefore the tax. A corporation with a $25,000 loss owes the $800 minimum.
The break-even point is where 1.5% equals $800. That is about $53,333 of net income. Below that, the client pays the minimum. Above it, the rate controls. Put the break-even in the client summary, so a client with a small profit can see why the bill is $800.
Newly incorporated or qualified corporations are exempt from the minimum for their first year, according to the FTB page. Ask for the incorporation or qualification date before applying the $800. The exemption covers the first year only.
Due date for tax year 2026, filed in 2027
The FTB states that Form 100S is due on the 15th day of the third month after the close of the taxable year. For a calendar-year S corporation, the tax year 2026 return is due March 15, 2027. March 15, 2027 is a Monday, so no weekend shift applies to that return.
The weekend rule matters in years when the 15th falls on a weekend. The FTB says the filing date becomes the next business day when the due date falls on a Saturday, Sunday or legal holiday. The IRS instructions for Form 1120-S (2025) show the same pattern at the federal level: for calendar-year corporations, the due date is March 16, 2026, because March 15, 2026 falls on a Sunday. A tax year 2025 Form 100S moves the same way.
Set two calendar entries for each client: the original due date and the extended date. Do not copy a 2025 date into the 2026 file. The FTB guidance this page relies on does not carry a tax year label, so confirm the 2026 Form 100S version when the FTB posts it, and check the form's instructions for the year before filing.
Extensions: more time to file, not more time to pay
The FTB describes an automatic six-month extension to file for S corporations in good standing that have fully paid their tax liability. No request form is needed. The same guidance says an extension to file is not an extension to pay.
Decision rule: if the client has not paid the full estimate by the original due date, do not rely on the automatic extension. Estimate the tax from the year-end trial balance, collect the payment before March 15, 2027 for tax year 2026, and then file. If the estimate comes in short, pay the difference with the return and record the amount and the date.
Put the extended date on the calendar only after you confirm it on the FTB due-date page. The FTB extended due date for a calendar-year S corporation is generally October 15, which is longer than six months from March 15. Confirm the date on the FTB due-date page before calendaring it, and do not assume it matches the federal September 15 date.
LLCs: the classification and shareholder checks
The FTB S corporation page says an LLC classified as an association and taxable as a corporation for federal purposes may elect S corporation status, and must then file Form 100S. That is the rule to apply when the LLC has an S election. The return follows the federal classification and the S election, not the word LLC in the entity name.
The second check is shareholder eligibility, and it applies whether the owner is a person or an entity. The IRS says an S corporation may have individuals, certain trusts and estates as shareholders, and may not have partnerships, corporations or non-resident alien shareholders. If the shareholder list shows a multi-member LLC taxed as a partnership, stop. The corporation does not meet the shareholder rule as the client has it, and both the federal and state filings need review before anything is prepared.
The IRS Schedule B-1 section of Form 1120-S lists shareholders that were disregarded entities, trusts, estates or nominees. A disregarded entity on the cap table has to be checked against that schedule, so the federal and state shareholder records match.
Mistakes that cost the client money
Failure: the firm computes 1.5% on a $40,000 profit and bills $600. Fix: apply the greater-of rule, so the tax is the $800 minimum.
Failure: the client files the automatic extension and pays nothing. Fix: the extension depends on full payment of the tax, so the estimate goes out before the original due date.
Failure: the 2025 due date is carried into the 2026 file. Fix: set each year's due date from the rule and check the weekday on that year's calendar.
Failure: a partnership appears on the shareholder list and the return is prepared as if the S election were clean. Fix: check the federal classification of each shareholder before the return is prepared.
What to request from the client
Ask the client for four items in writing: the date the corporation was incorporated or qualified in California, the federal Form 1120-S draft or the trial balance it comes from, the list of shareholders with each shareholder's tax classification, and the estimated payment the client plans to make by the original due date.
A short request works best. Send this: “Please send the date the corporation was incorporated or qualified in California, and the list of shareholders with each shareholder's tax classification.” The answer decides whether the $800 applies, whether the extension is available and whether the return can be prepared at all.