Mac manufacture date by serial number: decoding the year-and-half-year letter#

To pull a Mac's manufacture date out of a 12-character serial (every Mac from 2010 through early 2021), the work happens at position 4. That single letter encodes both the year and which half of the year the device was assembled. X is 2018 H2. H is 2012 H1. The letter cycle repeats every decade, so F could mean 2011 H1 or 2021 H1, and the product itself resolves which.

This is the format every "decode my Apple serial" guide is built around, and it stopped being issued on new Macs in early 2021. Understanding what position 4 does, and what it doesn't, is the difference between a clean date read and a decade-off misidentification. For the full position-by-position legacy decoder (factory, year, half-year, week, configuration), see the pillar guide to Apple serial number lookup.

The format around position 4#

The legacy 12-character Mac serial decomposes as PPPYWSSSCCCC:

Position 4 is doing two jobs at once: it tells you both which year a Mac was assembled and whether it was assembled in the first or second half of that year. Pairing it with position 5 yields the exact production week.

This packed encoding is one of the reasons offline decoders (EveryMac, chipmunk.nl, Beetstech) could return the model and manufacture week from a serial alone, with no Apple-side lookup. Position 4 is half the date math.

The year letter table#

The table is community-derived from extensive reverse engineering through the MacRumors WikiPost on decoding Apple serials and acidanthera's macserial. Apple has never officially published it.

The 20-letter cycle starts at C (2010 H1) and runs alphabetically, skipping certain letters, until it wraps around at C again ten years later (2020 H1). The cycle is what produces the decade ambiguity.

The decade ambiguity, in practice#

A C in position 4 could mean 2010 H1 or 2020 H1. The format provides no in-band signal for which. In practice, the product itself resolves it unambiguously:

• The chassis design and ports tell you a generation. A MacBook Air with two USB-C ports and a notched display is not a 2010 Mac.
• The configuration code at positions 9-12 maps to a specific SKU. Apple's legacy XML endpoint returns the marketing model (including its release year) purely from the configuration code, ignoring the year letter.
• About This Mac (or system_profiler SPHardwareDataType if the Mac boots) shows the marketing model directly. The serial is corroborating, not load-bearing.

The decade collision has never been a real problem on Macs because hardware doesn't lie about its generation. The collision matters only when you are decoding a serial from a photograph, no other identifiers visible, and can't tell from the image whether the device is a clamshell iBook or a thin Apple silicon laptop. In that one edge case, the year letter alone won't disambiguate.

For practical purposes, Apple stopped issuing new structured serials in early 2021. The M1 Pro / M1 Max MacBook Pro (October 2021) was among the first Macs to ship with the new randomized format. Existing legacy-format SKUs continued for a window (the 24-inch iMac with M1 still used a 12-character serial despite launching in mid-2021), but the last common letters in real-world circulation are D (2020 H2) and F (2021 H1).

Which letters are omitted, and why#

The table has visible gaps. A, B, E, I, O, U never appear in position 4. The reasons:

1. Vowel exclusion across the serial alphabet. Apple omits the vowels A, E, I, O, U from serials globally, to prevent the random middle characters from accidentally spelling words (especially profane or trademarked ones). The year slot inherits this constraint.
2. Digit-lookalike exclusion. Apple Support 102858 explicitly states that Apple registration and hardware-product numbers "use the numbers 0 (zero) and 1 (one) instead of the letters 'O' or 'I'." The same OCR-friendly rule applies to position 4.

B is the only omission without an obvious public-record rationale. Most reverse-engineering references simply note it as omitted by convention.

The omissions are part of why the cycle is exactly 20 letters long, and therefore why the decade-wrap occurs at all. Twenty half-years is ten years.

How position 4 pairs with position 5#

Position 4 tells you the half-year. Position 5 tells you the week within that half-year. The full mechanics of that week character, including the alphabet substitution and the +26 adjustment, are in how to read the production-week character in a Mac serial.

For first-half letters (C, F, H, K, M, P, R, T, W, Y), position 5's value translates directly to a calendar week 1-26. For second-half letters (D, G, J, L, N, Q, S, V, X, Z), you add 26 to derive the absolute calendar week.

Worked example: C02XG...

• C02: Quanta Computer (Tech Com subsidiary), China. See the factory-code reference for the full plant table.
• X: second half of 2018.
• G: week 13 of that half-year, so calendar week 13 + 26 = week 39 of 2018, late September.

You now know the device was assembled at a Quanta plant in late September 2018, without consulting Apple's servers or any third-party database. The start-to-finish decode walkthrough runs through the full example position by position.

What position 4 does not encode#

The year letter tells you when. It does not tell you what.

• The model lives entirely in the 4-character configuration code at positions 9-12.
• The factory lives in positions 1-3.
• Whether the device is new, refurbished, or a service replacement lives in the order number, not the serial. The F in MNEH3LL/A (an order number) means refurbished; an F in position 4 of a serial means H1 of 2011 or 2021.

Conflating these is the main source of "F prefix" confusion in the secondary market. The same letter does different jobs in different identifiers.

Position 4 on post-2021 randomized serials#

Once Apple switched to randomized 8-to-14 character serials in early 2021, the year encoding goes away entirely. On any Mac newer than late 2021 (every M2, M3, M4 model), position 4 is just a random alphanumeric character. There's no decade ambiguity to resolve because there's no year to decode in the first place. The serial as a whole is a "dumb token" that only Apple's servers can resolve to a model and a date. The post-2021 randomized-serial workflow covers what still works on those units.

For Macs from late 2021 onward, manufacture date is not recoverable from the serial. The closest substitute is the Boot ROM Version in System Information, which roughly bounds the firmware build window but is not the assembly date. For purchase-date-grade certainty, the Apple Account device list, the original receipt, or an Apple Support inquiry are the only reliable paths.

What this means for the reader#

If you are decoding a Mac serial from the 2010-to-early-2021 window, position 4 is your year-and-half-year handle. Look it up in the table, pair it with position 5 to derive the production week, and use the configuration code at positions 9-12 to confirm the marketing model. The decade ambiguity is a non-issue in practice because the hardware itself tells you which decade you're in.

If the serial is 10 characters and looks random, you are outside the legacy format. Position 4 carries no information. Cross-check the configuration via the order number, Model Identifier, or A-number plus EMC. None of those changed in 2021, and all of them remain decodable offline. For the model and year themselves, the Macfax serial lookup resolves serials from either era, including the randomized ones no letter table can read. For warranty and ownership history, Apple's Check Coverage is the canonical lookup regardless of which serial era a Mac falls into.