Box 15, the one box that does not add up
K-1 box 15 codes: M, N, AK and the form each credit feeds
Every other box on a Schedule K-1 can be checked against something. Box 15 cannot: the credits in it are not components of a total, they are separate claims that go to separate forms in separate years.
This is the box where a preparer who has been careful all the way through the K-1 can still be wrong, because the habit that works everywhere else — add the parts, compare with the whole — produces a number here that belongs to nothing.
Routing table
Box 15 by destination
The 2025 list runs from A to BC, skips a few letters, and ends at ZZ. What follows is arranged by where each credit ends up rather than by letter, because the destination is the part you need at the desk.
| Item | Codes | Lands on | Why it is easy to lose |
|---|---|---|---|
| Research | M | Form 6765, then Form 3800 | The K-1 gives your share of the credit, not the amount you can use. The usable amount is limited by your own tax, so the two figures are almost never the same and the K-1 figure is the ceiling. |
| Low-income housing | C, D | Form 8586, then Form 3800 | Two letters, one credit. Which line each letter takes has changed between years, so the letter and the year on the K-1 have to be read together. |
| Rehabilitation and rental property | E, F, G | Form 3468 | E names a rehabilitation expenditure and has no ambiguity. F and G say only "other rental", which is why they cannot be routed without the year's instructions. |
| The cooperative block | P, Q, R, S, T, U | Form 3468 as a carryforward | These are unused investment credits a cooperative allocated upward to the partnership and then to you. The year that generated them is not the year printed on the K-1, and both years belong in the record. |
| Employer credits | J, K, L, N, AE, AF, AG, AH, AV, BA, BB | Form 3800, mostly | Several are limited by wages you paid and by how many people you employ, which the K-1 cannot know. A partnership K-1 rarely carries these at all. |
| Energy and production credits | A, B, I, V, W, X, Y, Z, AA, AB, AC, AI, AJ, AK, AL, AM, AO, AW, AX, AY, AZ | The credit's own form | This is the part of the list that grew fastest after 2022. Codes near the end of the alphabet are new enough that software from two seasons ago may have no field for them at all. |
| Bond credits | AP, AQ, AR, AS, AT, AU | Form 8912 | They are claimed by the holder of the bond, not by the partnership that issued it. If you are not the holder, the line does not belong on your return. |
| Purchased credits | BC | Section 6418 election | A credit bought from another taxpayer rather than generated by the partnership. The transferor's identity and the election are part of the record, so this row is never just a number. |
| Backup withholding | O | Payments, not credits | It is tax already paid on your behalf. Totalling it with the credits above counts it twice; ignoring it understates what has already been paid in. |
| Left blank in the year's table | AN, BD to BG | The year's instructions | The 2025 table of contents gives these letters no description. Do not infer a meaning from the letter in front of them. |
| Everything else | ZZ | The attached statement | "Other" is a reference, not a value. It points at a statement the K-1 did not print. |
Form numbers above are the 2025 forms and they do move. Where the credit names its own form, that form is the authority for the year on your K-1, not this table. The box-by-box wording is in the box 15 entry in the box guide , and the general business credit that most of these reach is Form 3800.
There is no total to check here, and that matters
Boxes 4, 6 and 9 have something every reasonable reader would call a check. Box 4c is the sum of 4a and 4b, 6b is a part of 6a, 9b and 9c are parts of 9a, and the Item L capital account has to roll forward from its opening figure to its closing one. Each of those is an arithmetic fact about the paper, and each can be verified from the paper alone.
Box 15 has nothing like it. There is no total the credits should reach, because they are not components of one figure: they are independent claims, several with their own taxable-income limitation, several carried forward from a year that is not the year on this K-1. A page that promised to check box 15 by adding it up would be telling you something that is not true.
What can be checked is narrower and still worth having: that every letter printed in the box appears on the working sheet with its amount, that no letter was read as its first character alone, that STMT was kept as STMT instead of as zero, and that each row carries the page it came from. That is a completeness check rather than a correctness check, and the two should not be described as the same thing.
Why the letters moved
The code list is not stable across years. Credits arrive, expire and are renamed, and the letters are reassigned rather than retired, so position in the alphabet tells you nothing about the credit. A return prepared against last season's list can route a perfectly correct amount to a form that is no longer the right one.
The practical consequence is that the year has to travel with the code. A row that says
M:1250 and nothing else is unreadable next season; a row that says
2025 · M:1250 · p.2 can still be explained three years later when the
carryforward comes back.
Four steps for the box
- 1 Read the letter before the amount. A credit amount means nothing until you know which credit it is. One hundred and twenty dollars under M and one hundred and twenty dollars under AK are two different forms, two different limitations and often two different years.
- 2 Match the code list to the year on the K-1. The letters were renumbered. A letter that carried one credit in the 2019 list carries a different one now, so a guide read without the year in front of it is a guess. The year is in the K-1 header, not beside the box.
- 3 Separate credits from payments. Code O is withholding. Everything else in this box is a credit. A worksheet that puts them in one column produces a total that is neither a credit nor a payment.
- 4 Keep the letter, the amount, the year and the page on one row. The row has to survive to next season, when the carryforward comes back and the only way to explain it is to find the paper it started on.
What goes wrong, and the shape of it
- A credit entered as a payment. Some K-1 software layouts still place box 15 beside the withholding boxes. A credits figure typed into a withholding field reduces the balance due by an amount nobody has verified.
- A two-letter code read as one. AE is not A, AD is not A, and AK is not A. The first letter alone usually belongs to a different credit that also exists on the list.
- The right credit in the wrong year. A carryforward has a year of origin that is not the K-1's year. Carried forward on the wrong line, it becomes a credit claimed twice or a credit lost.
- STMT entered as zero. A code that says STMT is telling you an amount exists elsewhere. Entering zero makes the row look complete and hides the attachment.
None of these is caught by looking at whether the box is filled in. All of them are caught by keeping the letter, the amount and the page together from the first moment the K-1 is read, which is what the reading step produces: one row per code, with the page it sat on.
Plain answers
Questions about box 15
What does box 15 code M mean on a K-1?
Credit for increasing research activities. It is your share of a research credit the partnership computed, and it goes through Form 6765 before it reaches the general business credit. What you can actually use is limited by your own tax liability, so the figure on the K-1 is a ceiling rather than the amount that ends up on the return.
What is box 15 code N?
Credit for employer social security and Medicare taxes, on tips and on certain employee wages. It is a credit the employer claims, and the amount depends on wages the employer paid, which is why a K-1 carrying it usually belongs to a partner who is also an employer in the same trade.
What is box 15 code AK?
Credit for oil and gas production from marginal wells. It is a production credit, claimed on the form that credit names for the year, and it is limited by the amount of tax attributable to the production. Like the other credits in this box, the K-1 amount is not by itself the amount that can be used.
Can I add up the box 15 amounts and enter the total?
No. Each code is a separate credit with its own form, its own limitation and, for carryforwards, its own year of origin. A total in this box is a number with no meaning, and entering it puts money in a place no form asked for. Keep one row per code.
My box 15 is blank. Is that an error?
Usually not. Most partnerships generate no credits, and a blank box is genuinely absent rather than unreadable. The distinction that matters is between a blank and a code reading STMT: a blank means nothing was allocated to you, while STMT means an amount exists and lives on an attachment you have not seen.
The letter on my K-1 is not in any list I can find.
Check the year in the K-1 header first, then read that year's Partner's Instructions for Schedule K-1 (Form 1065). The letters were renumbered, several additions since 2022 sit past AE, and the 2025 table leaves a few letters with no description at all. A letter without a description is a question for the issuer, not something to infer from its neighbour.
Keep the credit codes, and the page each one sat on
K1 FastTax reads every box 15 code and amount from a K-1 PDF into one row per code, keeps a letter with no printed amount as STMT rather than zero, and records the page so the attachment can be found again.