How do I calculate audio file size?

Multiply the bitrate in bits a second by the length in seconds and divide by 8 to get bytes. 128 kbps for 60 minutes is 128,000 × 3,600 ÷ 8 = 57,600,000 bytes, or 57.6 MB. For uncompressed WAV or AIFF, the bitrate is sample rate × bit depth × channels.

How big is an hour of MP3 or WAV?

At a constant bitrate, an hour of MP3 is 28.8 MB at 64 kbps, 57.6 MB at 128 kbps and 144 MB at 320 kbps. An hour of WAV at 44.1 kHz, 16-bit stereo is 635.04 MB, and at 48 kHz, 24-bit stereo it is 1.04 GB. The table on this page lists every preset per minute and per hour.

How do I calculate the bitrate for a target file size?

Divide the size in bits by the length in seconds: bitrate = size in bytes × 8 ÷ seconds. 25 MB for 30 minutes is 25,000,000 × 8 ÷ 1,800 = 111.1 kbps, so pick a setting at or below that. For a video, take away the audio bitrate to get what is left for the picture.

Why does my file show a different size on my computer?

Because of decimal and binary units. In decimal, 1 MB is 1,000,000 bytes; in binary, 1 MiB is 1,048,576 bytes, and some software labels binary sizes as MB. A 100 MB file therefore shows as 95.37 in binary units. Switch the size units in the calculator to match.

How long can I record in 1 GB?

Divide the space in bits by the bitrate. 1 GB at 128 kbps is 1,000,000,000 × 8 ÷ 128,000 = 62,500 seconds, about 17 hours 21 minutes. At 44.1 kHz, 16-bit stereo WAV the same gigabyte holds about 1 hour 34 minutes. Use the Recording time mode for your own format and space.

Bitrate and Audio File Size Calculator

Work out how big a recording will be from its bitrate and length, the bitrate that makes a file fit a size limit, or how long you can record in the space you have. Presets for MP3, AAC and WAV, a video mode, decimal or binary units, and the working shown step by step.

How big a recording will be, from its format or bitrate and its length.

Your numbers

Format

Length

Hours

Minutes

Seconds

Size units

Storage makers use decimal units; some software counts in binary units, which makes the same file look smaller.

Result

File size

57.6 MB

57,600,000 bytes

Total bitrate

128 kbps · 960 KB a minute · 57.6 MB an hour

The working

128 kbps × 1,000 × 3,600 s ÷ 8 bits a byte = 57,600,000 bytes
57,600,000 bytes ÷ 1,000,000 = 57.6 MB

Assumes a constant bitrate. Variable-bitrate files and file headers make the real size a little different.

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How to calculate audio file size

File size = bitrate × length ÷ 8. The bitrate is in bits a second (kbps means thousands of bits a second) and a byte is 8 bits. An hour of 128 kbps MP3 is 128,000 × 3,600 ÷ 8 = 57,600,000 bytes, or 57.6 MB.

Uncompressed audio such as WAV or AIFF has no bitrate setting: its bitrate is the sample rate × the bit depth × the number of channels. At 44.1 kHz, 16-bit, stereo that is 44,100 × 16 × 2 = 1,411,200 bits a second, or 1,411.2 kbps, so an hour is 635.04 MB: 11.025 times the 128 kbps MP3. The file adds a small header on top.

The calculator assumes a constant bitrate. A variable-bitrate MP3 or AAC moves around its average, so its real size differs a little from the estimate.

Audio file size per minute and per hour

Worked out with the same formula as the calculator, in decimal units (1 MB = 1,000,000 bytes).

Format	Bitrate	1 minute	1 hour
MP3 64 kbps	64 kbps	480 KB	28.8 MB
MP3 128 kbps	128 kbps	960 KB	57.6 MB
MP3 192 kbps	192 kbps	1.44 MB	86.4 MB
MP3 256 kbps	256 kbps	1.92 MB	115.2 MB
MP3 320 kbps	320 kbps	2.4 MB	144 MB
AAC 96 kbps	96 kbps	720 KB	43.2 MB
AAC 128 kbps	128 kbps	960 KB	57.6 MB
AAC 256 kbps	256 kbps	1.92 MB	115.2 MB
WAV 44.1 kHz, 16-bit, stereo	1,411.2 kbps	10.58 MB	635.04 MB
WAV 48 kHz, 24-bit, stereo	2,304 kbps	17.28 MB	1.04 GB

Constant bitrate; WAV sizes leave out the small file header.

Video bitrate and file size

A video file holds a video stream and an audio stream, so add their bitrates before you multiply by the length. Ten minutes at 5 Mbps of video and 128 kbps of audio is (5,000 + 128) × 1,000 × 600 ÷ 8 = 384,600,000 bytes, or 384.6 MB.

Use the Video file size mode for that, and the Bitrate for a size mode with "Video with audio" to find the video bitrate that leaves room for the sound.

The bitrate to fit a size limit

Turn the formula around: bitrate = size × 8 ÷ length. To fit a 30-minute interview into 25 MB you need 25,000,000 × 8 ÷ 1,800 = 111,111 bits a second, so about 111 kbps; a 96 kbps file fits with room to spare.

Containers and headers take a little space beyond the audio itself. The overhead is 0% by default; if the file you export comes out slightly over the limit, set a few percent and use the lower bitrate. Check the limit on the service you are sending to: some count in decimal megabytes, some in binary.

MB or MiB: why sizes look different

A kilobyte can mean 1,000 bytes or 1,024. NIST notes that "megabyte" is used inconsistently, sometimes for 1,000,000 bytes and sometimes for 1,048,576, and that the IEC approved separate binary prefixes in 1998: kibi (KiB), mebi (MiB) and gibi (GiB) for the powers of 1,024.

Storage makers use decimal units. Apple says iOS 10 and earlier and OS X Leopard and earlier reported storage in binary units, and some software still counts that way. So a 100 MB file is 95.37 MiB: the same file, a different unit. Use the size-units switch to match the program you are checking against.

Sources

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