What is Form 1120-S, and what does an online seller need for it?

Form 1120-S is the annual federal income tax return for an S corporation. The IRS says it is used to report the income, gains, losses, deductions, credits and other information of a domestic corporation or other entity for any tax year covered by an election to be an S corporation.^[1][2] The figures on it come from the business's books: sales, cost of goods sold, operating expenses and the balance sheet.

The example shows how a year of the demo store's books lines up with the main areas of the return. Each figure is a total the books already hold at year-end; the return organises them into the IRS's lines and schedules.

Under the current Instructions for Form 1120-S, a corporation or other entity must file Form 1120-S if it elected to be an S corporation by filing Form 2553, the IRS accepted the election, and the election remains in effect.^[2] The election stays in effect until it is terminated, and the return is not filed for any tax year before the year the election takes effect.^[2]

An LLC can make the election too. The Instructions for Form 2553 say a corporation or other entity eligible to elect to be treated as a corporation uses Form 2553 to elect S status, and an eligible entity that meets the tests described there is treated as a corporation from the effective date of the S election without filing Form 8832 separately.^[3] Form 2553 is generally filed no more than 2 months and 15 days after the beginning of the tax year the election is to take effect, and the instructions describe relief for late elections.^[3]

For an online store, this usually means a single-member or multi-member LLC that chose S corporation tax treatment. Whether the election suits your store is a question for your tax preparer; the bookkeeping is the same either way, with payroll added once there is an S election.

An S corporation generally files Form 1120-S by the 15th day of the 3rd month after the end of its tax year.^[2] For calendar-year corporations filing for 2025, the 2025 instructions give the due date as March 16, 2026, because March 15 fell on a Sunday.^[2]

More time to file is requested on Form 7004, generally by the regular due date of the return.^[2] The IRS describes Form 7004 as an automatic 6-month extension of time to file certain business returns, and the Form 7004 instructions say it does not extend the time to pay any tax due.^[4][5]

For a calendar-year store, that puts the year-end close in January and February: the books for December need to be reconciled, inventory counted and the balance sheet tied out early enough for your tax preparer to work from them before mid-March.

Schedule K-1 shows each shareholder's separate share of the corporation's income, deductions and credits. A copy of each K-1 is attached to the Form 1120-S filed with the IRS, and each shareholder gets a copy.^[2] The corporation generally prepares a K-1 for every person who was a shareholder at any time during the tax year, and gives it to them on or before the day the Form 1120-S is required to be filed.^[2]

Each shareholder then reports their share on their own return. That is why the S corporation's books matter to every owner: an error in the store's profit flows straight through to each owner's K-1.

Form 1120-S draws on a full set of year-end books, not just a sales total. The parts that come straight from bookkeeping are:

• The income statement: gross receipts, returns and allowances, cost of goods sold and each category of deduction.
• Cost of goods sold on Form 1125-A. Form 1120-S filers attach Form 1125-A if they report a deduction for cost of goods sold, and the amount carries to the return.^[6][2] For a store, that means beginning inventory, purchases and ending inventory that tie to the count. See how to work out cost of goods sold.
• Schedule L, balance sheets per books, for the beginning and end of the year. The instructions say the balance sheets should agree with the corporation's books and records, and Schedule L is not required if the corporation answers “Yes” to question 11 on Schedule B, a small-corporation test on receipts and assets.^[2]
• Schedule M-1, which reconciles income per books with income per return; it is likewise not required with a “Yes” to question 11. Corporations with total assets of $10 million or more file Schedule M-3 instead.^[2]
• Schedule M-2, the analysis of the accumulated adjustments account (AAA) and related accounts. The instructions recommend that every S corporation maintain the AAA, which tracks accumulated undistributed net income and is used to work out the tax effect of distributions.^[2]

The instructions also note that a corporation must use an accrual method for sales and purchases of inventory unless it is a small business taxpayer.^[2] The difference between the two methods is covered in cash vs accrual for online sellers.

An S corporation owner who works in the business is usually also an employee. The IRS says S corporations must pay reasonable compensation to a shareholder-employee for services provided to the corporation before non-wage distributions may be made to that shareholder-employee.^[7] The Form 1120-S instructions state that distributions and other payments to a corporate officer must be treated as wages to the extent they are reasonable compensation for services rendered.^[2][7]

The IRS lists factors used in determining reasonable compensation, including training and experience, duties and responsibilities, time and effort devoted to the business, payments to non-shareholder employees, and what comparable businesses pay for similar services.^[7] Its page on S corporation employees adds that officers who provide more than minor services and receive, or are entitled to receive, payments are subject to federal employment taxes, and that being a shareholder does not change this.^[8]

For the books, this means owner pay runs through payroll as wages, distributions are recorded separately in equity, and the two are never mixed. The amount itself is a judgement for you and your tax preparer.

The 2025 Instructions for Form 1120-S set the late-filing penalty, for returns with no tax due, at $255 for each month or part of a month (up to 12 months) the return is late or missing required information, multiplied by the total number of persons who were shareholders at any time during the tax year.^[2] If tax is due, 5% of the unpaid tax per month or part of a month is added, up to 25%.^[2] A separate penalty of $340 per Schedule K-1 can apply for each failure to furnish a K-1 to a shareholder on time or with correct information.^[2]

At those amounts, a two-shareholder store filing three months late with no tax due would face 255 × 3 × 2 = $1,530, before any K-1 penalties. The instructions say the penalty is not imposed when a failure is due to reasonable cause.^[2] The amounts are adjusted over time, so check the instructions for the year you are filing.

Most of the work on Form 1120-S is already done if the books are closed every month: marketplace payouts reconciled to sales, fees and refunds; inventory tracked as an asset with cost of goods sold moving as units sell; owner wages through payroll and distributions recorded in equity; and a balance sheet that ties to the bank, the card and the loan statements.

When that is in place, year-end is a review of twelve closed months, not a reconstruction. Your tax preparer receives an income statement, a balance sheet with opening and closing figures, a cost of goods sold schedule and an equity roll-forward that agree with each other, with time to spare before the March due date.

If your store has an S election and the year's books are not yet closed, our year-end close for e-commerce sellers reconciles every marketplace and bank account, ties out inventory and cost of goods sold, and hands your tax preparer year-end books they can work from.