Video Bitrate to File Size: H.264 vs H.265 vs AV1 (and Why ProRes Is Huge)

“How big will this video be?” sounds like it should depend on resolution — 4K files are bigger than 1080p files, everyone knows that. But resolution only sets the size indirectly. The number that actually determines your file size is bitrate: how many megabits the encoder spends per second of footage. Two 4K files of the same length can differ by a factor of fifty depending on the codec and the bitrate it was given. Once you think in bitrate, file size stops being a mystery and becomes one line of arithmetic.

The only formula you need

file size = (video bitrate + audio bitrate) × duration ÷ 8

Bitrates are measured in megabits per second and file sizes in megabytes, so the divide-by-8 converts bits to bytes. A 10-minute video at 8 Mbps video plus 192 kbps audio is (8.192 × 600) ÷ 8 ≈ 614 MB. That’s the entire calculation — duration and bitrate in, size out. Resolution, frame rate and codec matter only because they change what bitrate you need for the footage to look good.

What bitrate do you actually need?

Delivery codecs are lossy: the encoder throws away detail to hit the bitrate you give it, and the art is choosing a rate where the losses are invisible. The widely used starting points (following YouTube and broadcast guidelines) for standard-frame-rate footage:

• H.264 (AVC): 2.5 Mbps at 480p, 5 at 720p, 8 at 1080p, 16 at 1440p, 45 at 4K.
• H.265 (HEVC): roughly 60% of H.264 — 5 Mbps at 1080p, 25 at 4K.
• AV1: the leanest of the three — about 4 Mbps at 1080p, 20 at 4K.

Frame rate scales the budget too: 60 fps footage carries about 1.5× the bitrate of 24/30 fps at the same resolution, and 120 fps about 2×. More frames per second means more picture to describe each second, though not proportionally more — consecutive frames at high frame rates are very similar, and modern codecs exploit that redundancy. The Video Bitrate & File Size Calculator has this whole matrix built in: pick a resolution, frame rate and codec and it shows the recommended rate and the resulting file size for your duration, alongside every other codec for comparison.

H.264 vs H.265 vs AV1: the compression dividend

Why can H.265 use 5 Mbps where H.264 needs 8 for the same 1080p quality? Each codec generation spends dramatically more encoding computation to find better ways of describing the picture — larger and more flexible block structures, smarter motion prediction, better entropy coding. The dividend compounds at higher resolutions: at 4K, H.265 needs roughly 25 Mbps against H.264’s 45, and AV1 does it in about 20. Over an hour of 4K footage that’s the difference between roughly 20 GB (H.264) and 9 GB (AV1) — same length, same perceived quality.

So why does anyone still use H.264? Compatibility and encode cost. H.264 plays on effectively every device made in the last fifteen years; AV1 decoding only became widespread with recent hardware. And newer codecs encode slower — AV1 in software can be an order of magnitude slower than H.264. The practical rule: H.264 for maximum compatibility, H.265 for camera recording and Apple-centric delivery, AV1 when your platform supports it and storage or bandwidth is the constraint.

ProRes: enormous, and correctly so

Then there’s the row in the table that looks like a typo: ProRes 422 at 1080p wants about 147 Mbps — eighteen times H.264 — and ProRes 4444 around 220. At 4K, ProRes 422 runs near 492 Mbps, which turns an hour of footage into roughly 220 GB. That’s not waste; it’s a different job. ProRes is an intra-frame codec: every frame is compressed independently, with no motion prediction between frames. That makes each frame instantly seekable and cheap to decode — exactly what an editing timeline needs to scrub smoothly — and it preserves far more color detail for grading. Delivery codecs achieve their tiny sizes precisely by making frames depend on their neighbours, which is fine for playback and miserable for editing. The rule of thumb: edit in ProRes, deliver in H.264/H.265/AV1, and never archive your only copy at a delivery bitrate you might regret.

Audio: the rounding error that isn’t always

A 192 kbps stereo AAC track adds about 0.19 Mbps to the total — invisible next to 45 Mbps of 4K video, but real money at the low end. On a 1.5 Mbps 480p H.265 encode, that same audio is over 11% of the file. If you’re squeezing a long video onto a size budget, the audio track is worth a look before you starve the video any further.

Will it fit?

The point of the arithmetic is usually a practical question: does this recording fit on the card, the disc, the drive? An hour of 1080p30 H.264 at 8 Mbps is about 3.7 GB — fits on a DVD with room to spare. The same hour at 4K60 in ProRes 422 is around 330 GB — you’re shopping for another SSD. The bitrate calculator answers this directly with a “fits on” column mapping each codec’s file size to real media, from DVDs through Blu-rays and SD cards up to terabyte drives.

Next time you’re sizing a shoot, an export or an upload, skip the guesswork: put your duration, resolution, frame rate and codec into the Video Bitrate & File Size Calculator and read the sizes for every codec at once. And if the video is destined for a live stream rather than a file, the Streaming Bitrate Calculator works the same logic against your upload bandwidth instead of your storage.