Form 1120-S: S-Corporation Tax Return, Complete Filing Guide

An S-corporation does not usually pay federal income tax itself. It reports what it earned and allocates that to its shareholders, who carry it on their own returns.

That structure creates two obligations people underestimate. The eligibility conditions have to keep being met, continuously, and an ownership change can end the status without anybody intending it. And each shareholder's basis has to be tracked year by year, because it decides what losses they can actually use.

This covers the eligibility rules, how income and deductions are allocated, what reasonable compensation means in practice, and the errors that come up most often.

S-corporations are pass-through entities, the corporation itself pays no federal income tax. Instead, income, losses, deductions, and credits flow through to shareholders and are reported on their individual returns. Form 1120-S is the annual information return that captures and allocates those items.

S-Corporation Eligibility Requirements

To file Form 1120-S, the corporation must have a valid S-election in place (filed on Form 2553). S-corp eligibility requires:

• Domestic corporation only
• No more than 100 shareholders
• Only allowable shareholders: US citizens/residents, certain trusts, and estates, no nonresident aliens or other corporations
• Only one class of stock (though differences in voting rights are permitted)
• Not an ineligible corporation type (certain financial institutions, insurance companies, etc.)

If any of these requirements are violated, the S-election terminates and the entity becomes a C-corporation going forward.

Filing Deadline

Pro Tip: If your S-corp has a fiscal year (not calendar year), the return is due the 15th day of the 3rd month after the end of the fiscal year.

Schedule K: Income, Deductions, Credits Allocated to Shareholders

Schedule K is the summary of all items that flow through to shareholders. Key line items:

Schedule K-1: Each Shareholder's Allocable Share

The corporation prepares a separate Schedule K-1 for each shareholder, showing their pro-rata share of each item from Schedule K. Allocation is based on stock ownership percentage and the number of days each share was held during the year (daily proration for mid-year ownership changes).

Pro Tip: Send Schedule K-1s to shareholders as soon as they are ready, shareholders need them to file their own returns, and delays cause friction.

Reasonable Compensation Requirement

Shareholder-employees of an S-corp must receive reasonable compensation (W-2 wages) for services rendered. This is one of the most-audited areas in S-corp returns.

• The IRS disallows the practice of taking all profits as distributions to avoid payroll taxes
• Reasonable compensation is based on what the position would pay in the open market
• Underpaying wages and taking excess distributions triggers SE tax reclassification
• Document your compensation rationale in writing each year

Shareholder Basis

Shareholders can only deduct S-corp losses to the extent of their basis in the S-corp. Basis consists of:

• Stock basis: original investment plus allocated income items, minus distributions and loss allocations
• Debt basis: loans the shareholder has personally made to the corporation (not guaranteed third-party loans)

Losses in excess of basis are suspended and carried forward until basis is restored. Tracking basis is the shareholder's responsibility, the corporation does not report basis on the K-1.

Built-In Gains Tax

If a C-corp converts to an S-corp, the built-in gains tax (BIG tax) may apply. Any pre-conversion appreciation that is realized within the recognition period (generally 5 years after conversion) is taxed at the highest corporate rate. This prevents C-corps from converting to S-corps solely to avoid corporate-level tax on appreciated assets.

LIFO Recapture

A C-corp that uses LIFO inventory accounting must include a LIFO recapture amount in its final C-corp return upon conversion to S-corp status.

Common Filing Mistakes

• Filing on April 15 instead of March 15, the S-corp deadline is a month earlier
• Not issuing K-1s to shareholders by the time the return is filed
• Paying unreasonably low shareholder salaries to avoid payroll taxes
• Failing to track shareholder basis, shareholders claiming losses they are not entitled to deduct
• Missing the S-election termination, violating any eligibility requirement silently terminates the election
• Not accounting for mid-year stock transfers when allocating K-1 items

Frequently Asked Questions

Written by

Ankit Sarawagi, CPA, CA, MBA

Ankit leads the finance team at Profitjets, where he has worked with 500+ businesses across the US on bookkeeping, tax and CFO-level strategy. He writes about the habits that keep small business books accurate all year. Connect on LinkedIn