How we calculate
Formulas, data sources and caveats for the BLKRIP calculators.
The calculators use open formulas, and every number comes from a specific source. The interface of each tool states what is used and when it was last checked.
Formulas
- File size
- size = (video bitrate + audio bitrate) × time ÷ 8
- Reverse duration calculation
- time = size × 8 ÷ (video bitrate + audio bitrate)
- Frame number for a timecode
- frame number = hours × 3600 × nominal fps + minutes × 60 × nominal fps + seconds × nominal fps + frame within the second
- NDF timecode drift
- drift = (actual frames ÷ nominal fps) − real time
- Shoot size
- bitrate = codec coefficient × pixels per second; size = bitrate × 1,000,000 × (hours × 3600) ÷ 8
- Frame safe area
- safe.x = W × left%, safe.y = H × top%, safe.width = W − left% − right%, safe.height = H − top% − bottom%; frame share = safe.width × safe.height ÷ (W × H)
Data tiers
Every number is marked with the tier of its source. The tier is visible in the interface — each tool has its own.
A — primary source
The value comes from a vendor document: the target bitrate of the codec from the Apple white paper. It is a published value and can be checked against the source.
B — our measurement
The value comes from our own ffmpeg measurement on a synthetic 1920×1080@25 source. Such a measurement is a lower bound: a test pattern contains almost no detail or texture movement, so real footage produces a higher bitrate.
Sources and verification dates
- Apple ProRes white paper — target bitrates for ProRes 422 Proxy, LT, 422, 422 HQ, 4444 and 4444 XQ at 1920×1080 @29.97 fps. Verified 03.11.2026.
- Our ffmpeg 9.0.2 measurement (libx264 preset slow CRF 18, libx265 preset slow CRF 20, prores_ks) on a synthetic 1920×1080@25 source. Verified 03.11.2026.
- SMPTE 12M timecode standard — frame numbering and the drop-frame rule (two frame numbers dropped at the start of every minute except every tenth). Verified 03.11.2026.
Caveats
- Bitrate units are decimal: 1 kbit/s = 1000 bit/s. Size in GB and TB is binary (1024³ and 1024⁴).
- Our ffmpeg measurement is a lower bound on a test source. For disk planning with ProRes, use the Apple target bitrates.
- Drop-frame is exact for 29.97 and 59.94 fps. For 23.976 fps only NDF applies, so the 3.6 s per hour drift remains.
- The real bitrate depends on how detailed the picture is: it is higher on detailed scenes. Calculator results are planning figures, not a guaranteed file size.