The duration field is called “Approximate Length” because it is not always an exact length. For an Abstraction, this is the duration of the original version of the work, accurate to the minute. For television programs, the actual content runtime is always preferred, but the time slot’s duration can be used if the runtime is not known. Records lower in the registration tree (Edits, Clips, and Manifestations), should have progressively more accurate durations.
When the running time of an older work is not known it can be estimated by its physical length on 4-perf 35mm film, where there are 16 image frames per foot of film. This is only a rough approximation, but it is accurate enough to facilitate EIDR discovery and de-duplication. Film speed (in frames per second, or fps) was not broadly standardized until after the adoption of sound, so adjustments must be made by release year to help improve the approximation.
- Film Measured in Feet:
- 1915 or earlier: Divide 60 (average speed was 16fps)
- 1916-1920: Divide by 67.5 (average speed was 18fps)
- 1921-1925: Divide by 75 (average speed was 20fps)
- 1926-1929: Divide by 82.5 (average speed was 22fps)
- 1930 or newer: Divide by 90 (average speed is 24fps)
In all cases, round up to the nearest integer. This gives an approximate running time in minutes.
- Film Measured in Meters:
- Multiply by 3.28084, then apply conversion for Feet
- Film Measured in Reels:
- 1929 or earlier: 12 minutes
- 1930 or later: 10 minutes
- A Work only identified as a “Short” with no length or duration provided:
- 1910 or earlier: Use 5 minutes
- 1911 or later: Use 15 minutes
- A Work only identified as a “Feature” with no length or duration provided:
- 1929 or earlier: Use 60 minutes
- 1930 or later: 90 minutes
When converting different material, it may be helpful to know:
- 8mm film has 80 frames per foot
- Super 8mm film has 72 frames per foot
- 16mm film has 40 frames per foot
- 2-perf 35mm film has 32 frames per foot
- 3-perf 35mm film has 21.33 frames per foot
You can then apply the generic formula to convert feet to minutes: ceiling[(feet * frames-per-foot) / (frames-per-second * 60)].
 Do not include seconds in the Title record’s Approximate Length unless the work is < 10 minutes long.
 If the work is recorded on another film gauge, with a different image size, or the recorded speed in frames per second is known, then make the necessary adjustments before estimating the duration.
 Technically, there are a variety of physical reel sizes, but the standard single reel is assumed to hold 1,000 feet of film with average tension and loading, but could hold slightly more or less. The running time for the standard 35mm sound reel is assumed to be 10 minutes – 20 minutes for a double reel. For 4-perf 35mm film at 24fps, this works out to 11.44 minutes total running time, but allowing for head and tail leader and the fact that reels are rarely loaded to capacity (to help avoid unspooling), the average content duration tends to be about 10 minutes per single reel.