What K-1 box codes are, in plain English

K-1 box codes are the letters (A, B, C, and so on, extending into AA, AB, AM and beyond) that a pass-through entity uses to tag amounts inside a single numbered box on a Schedule K-1. A code tells you what a figure is and where it belongs on your return. When a box can hold several different kinds of items, the entity lists each one with its code and dollar amount, usually pointing to an attached supporting statement for the detail.

The numbered boxes are broadly consistent in meaning: income items, then deductions, then credits, then other information. But the box numbers themselves differ by form. Box 13 is credits on a 1041 K-1, credits on a 1120-S K-1, and other deductions on a 1065 K-1. That is exactly why a cross-entity reference matters, and why reading schedule k-1 box codes off the wrong form’s map is such a common error.

Three ideas hold across every K-1. The entity itself generally pays no income tax; it passes items through to you to report. The numbered box tells you the category; the code letter tells you the specific item and its destination. And a coded amount almost always references a statement that carries the number you actually enter.

Form 1065: the partnership K-1, boxes 1 to 23

The partnership K-1 is the deepest of the three. Part III runs boxes 1 through 23, and the heavily coded boxes are 11 (other income), 13 (other deductions), 15 (credits), and 20 (other information), where the private-market items live. Here is the full Part III map.

Box 20 is where private-market complexity concentrates. The codes that come up constantly:

• Code Z: section 199A information, the qualified business income figures that drive the deduction on Form 8995 or 8995-A.
• Code N: business interest expense, the input to the section 163(j) limitation on Form 8990.
• Code AM: section 1061 information, the carried-interest reporting where a holding period of more than three years is generally required for long-term capital gain treatment.

A note on distributions: a cash distribution is tax-free only to the extent of your outside basis under section 731; anything above basis is capital gain. And a loss in Box 1 is not automatically deductible. It has to clear basis (section 704(d)), at-risk (section 465), and passive activity (section 469) limits, which is why Boxes 22 and 23 flag multiple activities. For the reader-friendly walkthrough of these, see the companion piece K-1 Box Codes, Decoded, and for the partnership form end to end, Schedule K-1 (Form 1065), Explained.

Form 1120-S: the S-corporation K-1, and how it differs

The S-corporation K-1 looks similar but is shorter and shifted. Its boxes run 1 through 17, and there is no self-employment box at all, because S-corporation income generally is not subject to self-employment tax (reasonable compensation is paid as W-2 wages instead). Because the box count is smaller, the numbers do not line up with the 1065: credits sit in Box 13, not Box 15, and other information is Box 17, not Box 20.

Two differences trip up preparers most. First, there is no Box 14 self-employment line to look for; do not go hunting for it. Second, section 199A information is Box 17, Code V on the S-corporation K-1, but Box 20, Code Z on the partnership K-1. Same deduction, different address. Basis mechanics differ too: an S-corporation shareholder tracks stock and debt basis (Box 16 feeds it), rather than the partner’s single outside-basis figure.

Form 1041: the estate and trust K-1

The Form 1041 K-1 reports a beneficiary’s share of an estate’s or trust’s income, deductions, and credits. Its boxes run 1 through 14 and are organized around the kinds of income a fiduciary distributes. The coded boxes here are 11 (final-year deductions and carryovers), 12 (AMT items), 13 (credits), and 14 (other information).

Note the placement carefully, because it differs from both other forms. On the 1041 K-1, AMT items are Box 12 and section 199A information is Box 14, Code I. There is no separate international checkbox box in the same way; a fiduciary reports foreign items within the numbered boxes and any required statements. Directly apportioned deductions in Box 9 are a fiduciary-specific concept: depreciation and similar deductions the trust allocates directly to beneficiaries rather than absorbing at the entity level.

Schedules K-2 and K-3: the international layer

Schedules K-2 and K-3 are the international-reporting schedules for pass-through entities. Schedule K-2 is the entity-level schedule attached to the return (Form 1065 or 1120-S); Schedule K-3 is the partner or shareholder version that reports each owner’s share of items of international tax relevance. Together they replaced the old patchwork of foreign lines that used to sit inside the K and K-1 (the former Box 16 foreign transactions on the 1065 K-1 and Box 14 on the 1120-S K-1).

They were first required for the 2021 tax year (returns filed in 2022). The point was standardization: instead of scattered, inconsistently formatted foreign entries, owners now receive a structured K-3 they can use for the foreign tax credit (Form 1116), GILTI, foreign-derived income, PFIC reporting, and related items.

The checkbox mechanics are the part preparers most need to get right:

• On the partnership K-1 (Form 1065), Box 16 is a single checkbox. When it is checked, a Schedule K-3 with the international detail is attached, and you should not expect line-item foreign amounts in that box itself.
• On the S-corporation K-1 (Form 1120-S), the equivalent checkbox is Box 14. Checked means a Schedule K-3 is attached.

A domestic filing exception can relieve certain entities with limited or no foreign activity from furnishing K-2/K-3, but a partner or shareholder who needs the information (for a foreign tax credit, for example) can still request it, which triggers the entity’s obligation to provide it.

For a private-market fund with foreign holdings, the K-3 is where the real international work lives. It is long, it is highly coded by part and column, and it is the schedule most likely to arrive late in the season.

How coded amounts flow to the Form 1040

A K-1 is only useful once each coded amount reaches the right line of the individual return. The routing is broadly consistent across the three forms, keyed to what the item is rather than which box number carried it.

The reason this matters operationally: the same economic item is coded differently depending on the entity that issued the K-1, but it has to land in the same place on the return. A preparer holding partnership, S-corporation, and trust K-1s for one client is effectively translating three code maps into one 1040. Do that by hand across hundreds of documents and the translation, not the tax, is what breaks.

Reading codes by hand versus K1x

Understanding the code maps is one thing. Applying them, correctly, across every K-1 a fund or a family office receives is another. Each entity formats its supporting statements a little differently, and someone has to read every Box 13, Box 17, or Box 20 code and route it into the return. Here is the honest comparison.

The pattern is simple. K-1 box codes are not intellectually hard once you know the maps; they are operationally punishing at scale, especially when a single client’s returns pull from all three entity types. K1 Aggregator ingests inbound K-1s, 1099s, and W-2s and extracts the coded data straight into returns, which is why practitioners describe the shift as 1 week’s work, 80 K-1s, 8 minutes, 1 platform.

Stop translating code maps by hand.
See K1 Aggregator read Box 13, Box 17, and Box 20 codes across a full stack of 1065, 1120-S, and 1041 K-1s and map them straight into your returns. Book a Demo

Conclusion

A K-1 is a routing document. The numbered box tells you the category, the code letter tells you the specific item and where it goes, and the supporting statement carries the figure you actually enter. Learn the three code maps, mind the differences between them, and confirm every letter against the current-year instructions, because the code lists change annually.