What drop-frame skips
The calculator opens at 29.97 drop-frame with 00:00:59;29 plus one frame. The answer is 00:01:00;02: the labels ;00 and ;01 do not exist at the start of that minute. Change the first line to 00:09:59;29 and the answer is 00:10:00;00, because every tenth minute keeps all its labels.
The rule in full:
- At 29.97 fps, frame numbers 00 and 01 are skipped at the start of every minute except minutes 00, 10, 20, 30, 40 and 50.
- At 59.94 fps, frame numbers 00 to 03 are skipped on the same schedule.
- Only labels are skipped. Every frame of picture and sound is still there.
Why it exists
29.97 video plays 1000/1001 slower than 30 frames a second, but timecode still counts 30 labels a second. Counting every frame (non-drop) makes the labels fall behind the clock by 3.6 seconds an hour, which is 86.4 seconds a day. Skipping 108 labels an hour removes that drift: one labelled hour of 29.97 drop-frame is 107,892 frames and plays in 3,599.996 seconds.
Frame counts in drop-frame
| Span | 29.97 DF frames | 29.97 NDF frames | 59.94 DF frames |
|---|---|---|---|
| A minute that skips labels | 1,798 | 1,800 | 3,596 |
| Ten minutes | 17,982 | 18,000 | 35,964 |
| One hour | 107,892 | 108,000 | 215,784 |
| 24 hours | 2,589,408 | 2,592,000 | 5,178,816 |
Doing drop-frame math by hand
Subtracting drop-frame labels directly goes wrong whenever a minute boundary sits between them. From 00:00:59;20 to 00:01:00;10 the labels suggest 20 frames, but two labels were skipped, so the real distance is 18 frames. The safe method is to turn both labels into frame counts, do the math on the counts, and turn the result back into a label. That is what this calculator does on every line.
To check a label you typed, the calculator also refuses labels that drop-frame never uses, such as 00:01:00;00, and tells you the labels on either side.
Drop-frame or non-drop-frame?
The same frames can carry either kind of label, so switching between them changes the numbers on screen, not the picture. Frame 108,000 is 01:00:00:00 in non-drop but 01:00:03;18 in drop-frame. To move a timecode between the two, use the frame rate converter: pick 29.97 DF and 29.97 NDF, and the "same frames" result gives the other label.
Broadcast deliveries in 29.97 regions usually ask for drop-frame so programme length matches the clock. 23.976, 24, 25, 50 and 60 never use drop-frame. The main timecode calculator covers every rate.
Questions
Which frame numbers does drop-frame skip?
At 29.97 fps, frame numbers 00 and 01 at the start of every minute, except minutes 00, 10, 20, 30, 40 and 50. At 59.94 fps, frame numbers 00 to 03 on the same schedule.
How many frames are in an hour of 29.97 drop-frame?
107,892. Non-drop counts 108,000 for the same label, which plays 3.6 seconds longer than a real hour.
How do I write drop-frame timecode?
With a semicolon before the frames, such as 01:00:00;00. Non-drop uses colons only. The calculator accepts either when you type and writes drop-frame with a semicolon.
Is there drop-frame at 23.976 or 25 fps?
No. SMPTE defines drop-frame only for 29.97 and 59.94 fps. Every other rate is non-drop.