How this calculator works
Film moves through the camera in discrete steps, and each frame occupies a fixed number of perforations. A foot of 35 mm four-perforation stock holds 16 frames, while a three-perforation pull-down fits 21.33 frames into the same foot because each frame is shorter. The frame rate then sets the run time: a thousand feet of four-perforation 35 mm is 16,000 frames, which lasts 666.67 seconds at 24 frames per second. Silent-era rates near 16 or 18 frames per second run the same print longer, and high frame rate formats run it shorter. The conversions here assume a constant frame rate and ignore leader, head and tail trims, which reduce the usable length.
Formula
frames = film length in feet x frames per foot. run time = frames / frame rate. film length in feet = frames / frames per foot. Rolls of 1000 feet = feet / 1000, and rolls of 400 feet = feet / 400.
Worked example
A 1000 foot roll of 35 mm four-perforation film holds 1000 x 16 = 16,000 frames. At 24 frames per second that is 16,000 / 24 = 666.67 seconds, or 11.11 minutes, and it is one 1000 foot roll or 2.5 rolls of 400 feet. Reversing the question, a one minute take at 24 frames per second needs 1440 frames, which is 1440 / 16 = 90 feet of 35 mm four-perforation stock. The same take on 16 mm, at 40 frames per foot, needs only 36 feet.
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Frequently asked questions
Why does 35 mm film fit different frame counts per foot?
Each frame spans a different number of perforations. The standard four-perforation frame fits 16 per foot, three-perforation fits 21.33 and two-perforation fits 32, which is why short pull-down formats stretch stock further.
What frame rate should I use for old footage?
Silent film was often shot between 16 and 20 frames per second, and early sound settled near 24. If you are timing a scan, use the rate the camera actually ran, not the 24 frame per second sound standard.
Do these figures include leader and waste?
No. A camera load usually has several feet of leader and a tail for handling, so the usable run time from a cartridge or roll is shorter than the arithmetic suggests.
How many frames are in a minute at 24 frames per second?
Exactly 1440. Multiply the frame rate by 60 for any rate, so 25 frames per second gives 1500 frames and 30 gives 1800 frames per minute.