WAV File Over 4 GB? RF64 and BW64 Explained, and How Long You Can Record Before You Hit the Limit

A standard WAV or Broadcast Wave file cannot be larger than about 4 GB, because its size fields are 32-bit numbers. RF64, published by the EBU, and BW64, its successor standardized by the ITU as Recommendation ITU-R BS.2088, get past the limit by storing the real sizes in a 64-bit ds64 chunk at the start of the file. If you deliver long or multichannel recordings, check that the client’s spec accepts RF64 or BW64 before you send; WavSpec reads RF64 and checks each file’s container, sample rate, bit depth and channel layout against the spec you pick.

This guide explains where the limit comes from, how long you can record before you reach it, and how the 64-bit formats stay compatible with ordinary WAV readers.

Check container, format and loudness for a whole batch before delivery.

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Why does a WAV file stop at 4 GB?

A WAV file is a RIFF file: a header with the ID RIFF, a size field, and then chunks such as fmt and data, each with its own size field. The EBU’s original RF64 specification, Tech 3306, states the cause plainly: the barrier comes from the 32-bit addressing in RIFF/WAVE and BWF, and with 32 bits a maximum of 4,294,967,296 bytes can be addressed. That is 4 GiB, which most software labels 4 GB.

Once the audio data passes that size, the 32-bit fields cannot describe it. A recorder or editor that keeps writing ordinary WAV past that point produces a file whose header is wrong, and other software may read only part of it, or refuse it.

How long can you record before you hit 4 GB?

Uncompressed PCM grows at a fixed rate: sample rate times bytes per sample times channels. The table below is our own arithmetic, dividing 4,294,967,296 bytes by that rate and ignoring the small header, so real files reach the limit a few seconds sooner.

Format	Bytes per second	Time to 4 GiB
44.1 kHz, 16-bit, stereo	176,400	about 6 h 45 min
48 kHz, 24-bit, mono	144,000	about 8 h 17 min
48 kHz, 24-bit, stereo	288,000	about 4 h 8 min
96 kHz, 24-bit, stereo	576,000	about 2 h 4 min
96 kHz, 32-bit float, stereo	768,000	about 1 h 33 min
48 kHz, 24-bit, 5.1 (6 channels)	864,000	about 1 h 22 min
48 kHz, 24-bit, 7.1 (8 channels)	1,152,000	about 1 h 2 min

Stereo voice work rarely gets close. A long multichannel stem, a location recording left running, or a 96 kHz float master of a feature-length programme can pass it within a single take.

What is RF64?

RF64 is the EBU’s 64-bit extension of RIFF/WAVE and BWF, first published in 2007 as Tech 3306. According to the specification, an RF64 file is identical to an ordinary WAV file except for two changes:

1. The first four bytes read RF64 instead of RIFF.
2. A mandatory ds64 chunk follows immediately after the header. It holds three 64-bit values: riffSize, dataSize and sampleCount, which replace the RIFF size field, the size of the data chunk and the sample count in the fact chunk.

The rule for reading is simple. If one of the old 32-bit fields holds -1 (hex FFFFFFFF), a reader uses the 64-bit value from ds64 instead. Every other chunk that is valid in a WAV file, including the Broadcast Wave bext chunk, stays valid in RF64.

The EBU’s current edition of Tech 3306 adds that the 2009 version introduced an r64m marker chunk to replace the cue chunk for files larger than 4 GB, because the EBU had seen ambiguous handling of cue in some products.

How does a recorder switch to RF64 mid-recording?

A recorder cannot know when it starts whether a take will pass 4 GB. Tech 3306 solves this with a placeholder. An RF64-aware application starts an ordinary WAV or BWF file with a JUNK chunk the same size as a ds64 chunk. Software that does not know RF64 ignores JUNK, as the original RIFF format allows.

If the recording passes 4 GB, the application:

1. Renames the JUNK chunk to ds64.
2. Writes the real RIFF size, data size and sample count into it.
3. Sets the three 32-bit fields to -1.
4. Changes RIFF to RF64 in the first four bytes, and keeps recording.

The practical result: short takes from such a recorder stay plain WAV files that any software can open, and only the takes that need it become RF64.

What is BW64, and is it different from RF64?

In 2015 the ITU published Recommendation ITU-R BS.2088, the BW64 format. The current EBU Tech 3306 explains that BW64 contains the ds64 chunk and the axml chunk and adds a chna chunk, so a file can carry large multichannel audio together with Audio Definition Model metadata from ITU-R BS.2076. Because the Audio Definition Model describes loudspeaker positions itself, BW64 drops the 18-position channel mask found in RF64.

To avoid two competing 64-bit specifications, the EBU says that ITU-R BS.2088 “should exclusively be used”. The ITU page lists BS.2088-2, dated November 2025, as the version in force, with the 2015 and 2019 editions superseded.

For a delivery engineer the differences that matter are:

Question	RF64 (EBU Tech 3306)	BW64 (ITU-R BS.2088)
Breaks the 4 GB limit	Yes, with ds64	Yes, with ds64
Carries Broadcast Wave metadata	Yes	Yes
Object and scene-based audio metadata	No	Yes, with chna and axml
Current status	Superseded by BS.2088, per the EBU	In force, BS.2088-2

Why does a client reject a file over 4 GB?

A rejected large file usually points to one of three causes:

• The file was written as plain WAV past the limit. Its header cannot describe its length, so the receiving system sees a broken or truncated file.
• The receiving tool does not read RF64 or BW64. A file that is perfectly valid still fails because the first four bytes are not RIFF.
• The spec calls for a specific container. Some delivery specs name WAV or BWF explicitly. A 64-bit file can be a spec failure even when it plays.

The fixes follow from the cause. Split the programme into reels or parts below 4 GB if the spec allows; ask whether RF64 or BW64 is acceptable; or deliver at the bit depth and channel count the spec actually requires, since a 32-bit float master is twice the size of a 16-bit file.

Pick a client spec, drop in the folder, and get a pass or fail per file.

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How do you check a batch before you deliver?

Checking file size alone is not enough, because the container, sample rate, bit depth and channel layout all have to match the spec too. WavSpec loads a batch of audio files and checks each one against a named delivery specification. It reads WAV and Broadcast Wave, with bext and iXML preserved, RF64, AIFF, FLAC, MP3, AAC and M4A, in mono, stereo, 5.1 and 7.1.

For each file it checks loudness, true peak and sample peak per ITU-R BS.1770-5, along with sample rate, bit depth, channel layout and container. It marks every criterion pass, fail or advisory. The built-in specs include EBU R 128, UK DPP AS-11, ATSC A/85 and a generic 48 kHz, 24-bit post-delivery template, and you can create your own named spec with your client’s rules.

The report comes in PDF, HTML, CSV and JSON, with the SHA-256 digest of every delivered file. Corrected files go to a separate delivery folder and your originals are never overwritten. Processing runs locally, with no account and no upload.

If you are also normalizing long Broadcast Wave files, our guide to normalizing audio without losing bext metadata covers the metadata side.

Frequently asked questions

Is RF64 the same as BWF?

No. RF64 is an extension of WAV and BWF that adds 64-bit sizes. An RF64 file can carry the same bext metadata as a Broadcast Wave file, but the header starts with RF64, so a reader must support it.

Can a WAV file be 2 GB and still fail?

Yes, if the receiving software has its own lower limit or the file breaks the spec in another way, such as the wrong sample rate. Size is only one of the checks.

Will splitting a file at 4 GB affect loudness compliance?

It can. Integrated loudness is measured over the whole programme, so each part measured on its own may read differently. Check the spec to see whether it measures parts or the whole.

Does WavSpec convert WAV to RF64?

WavSpec’s Store listing says it reads RF64. It does not list conversion to RF64 among its corrections, so plan the container in your recorder or editor.

Try WavSpec

WavSpec is a one-time purchase with a free trial in the Microsoft Store; the listing shows the current price and system requirements. Run it on the files you are about to send and read the container line in the report first. More detail is on the WavSpec product page.

Prove the batch meets the spec, with a report you can send with the files.

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Last updated 23 October 2026. Published by Automata Labs, a software company in Canada.