Convert and add timecode at any frame rate

Timecode is a label, not a clock

Timecode gives every frame of a video an address written as hours, minutes, seconds and frames: HH:MM:SS:FF. Every field counts from zero, so the frame at 01:02:03:04 sits in hour 1, minute 2, second 3, and is frame 4 within that second: the fifth frame, because frame numbers start at 00. Editors, sound mixers, subtitlers and colorists all use these addresses so they can point at the same frame without confusion.

The important word is label. Timecode counts frames and groups them into seconds by a fixed number of frames per second. If the video really plays at that number of frames per second, the label and the clock agree. If the video plays slightly slower than the label assumes, the label falls behind the clock. Most of the confusion around timecode, and every reason this calculator exists, comes from that gap.

The format is defined by SMPTE, the Society of Motion Picture and Television Engineers, in its timecode standard, SMPTE ST 12-1. Professional editing and audio software follows it, which is why a timecode from one program means the same frame in another, as long as both use the same frame rate.

The frame rates and why they are odd numbers

Some frame rates are whole numbers. Others are a whole number multiplied by 1000/1001, which is why they come out as 23.976 or 29.97.

The 1000/1001 rates date from the introduction of color television in the United States. When the NTSC color system was adopted in 1953, the frame rate was lowered from 30 to 30000/1001, about 29.97, for technical reasons to do with fitting the color signal alongside the sound. Everything built on that system inherited the slower rate.

23.976 comes from the same place. Film runs at 24 frames per second. To transfer film to 29.97 video, the film is slowed by the same 1000/1001 factor, to 23.976, so that its frames map neatly onto the video frames. Many digital cameras still record at 23.976 because the footage is destined for systems built around 29.97.

25 and 50 come from the television systems used across Europe and much of the rest of the world, which never had the 1000/1001 adjustment. 24 is true cinema speed, used for theatrical DCPs.

Why the label drifts at 29.97

At 29.97 frames per second, the timecode still counts 30 frames to each labelled second, because a label of 29.97 frames per second is impossible to write. So each labelled second contains 30 frames, but 30 frames take slightly longer than one real second to play.

Here is the size of the problem over one hour. A label that reads one hour, counted at 30 frames per second, covers 60 times 60 times 30, which is 108,000 frames. At the real rate of 30000/1001 frames per second, 108,000 frames last 108,000 times 1001 divided by 30,000, which is 3,603.6 seconds. The label says one hour; the clock says one hour and 3.6 seconds.

For a feature film that is a curiosity. For television, where a program must fill an exact slot in a schedule, 3.6 seconds an hour is a real problem. Drop-frame timecode was created to solve it.

How drop-frame works

Drop-frame timecode keeps every frame of picture. It only skips some frame numbers, so the label catches up with the clock.

The rule for 29.97 drop-frame:

• At the start of every minute, frame numbers 00 and 01 are skipped. The label jumps from 00:00:59;29 straight to 00:01:00;02.
• Except every tenth minute. At minutes 00, 10, 20, 30, 40 and 50, nothing is skipped.

Why that pattern? The label needs to lose 108 frame numbers per hour, the 3.6 seconds times 30 frames. Skipping two numbers in every minute would lose 120, which is too many. Skipping two numbers in 54 of the 60 minutes loses exactly 108. Leaving out every tenth minute is how you get 54.

By convention, drop-frame timecode is written with a semicolon before the frames, 01:00:00;00, and non-drop with a colon, 01:00:00:00. Some programs use a full stop or comma instead of the semicolon. The punctuation is a signal, not a guarantee, so always check the project's settings.

59.94 drop-frame

59.94 is the same idea at double the rate. Editing software that labels 59.94 with 60 frame numbers per second skips frame numbers 00 to 03 at the start of each minute, except every tenth minute. The tool applies both rules.

Labels that do not exist

Because numbers are skipped, some drop-frame labels never occur. 00:01:00;00 and 00:01:00;01 are not frames. If you type one into the tool, it tells you the label is skipped and gives the next real frame. Software that receives a skipped label from a careless edit list may reject it or round it, so it is worth catching before you send a list to someone else.

Worked example: one hour of 29.97 drop-frame

The tool opens on 01:00:00;00 at 29.97 drop-frame. Here is the arithmetic it shows, step by step.

1. Minutes on the label. One hour and zero minutes is 60 minutes.
2. Tenth minutes. Of those 60 minutes, 6 are tenth minutes (0, 10, 20, 30, 40, 50), which skip nothing.
3. Minutes that skip numbers. 60 minus 6 is 54.
4. Numbers skipped. Two per minute, times 54, is 108.
5. Frames if nothing were skipped. Reading the label at 30 frames per second: 3,600 seconds times 30 is 108,000.
6. Actual frame count. 108,000 minus 108 is 107,892 frames.
7. Real time. 107,892 times 1001 divided by 30,000 is 3,599.9964 seconds, shown as 3,599.996.
8. Drift. The label reads 3,600 seconds. Real time minus label is about -0.004 seconds.

So one labelled hour of drop-frame is about 4 milliseconds short of a real hour. That is the remaining error after the correction, and it is small enough to ignore for any program length you are likely to deliver.

The tool also shows the same label at every frame rate in a table. 01:00:00:00 at 29.97 non-drop is 108,000 frames and 3,603.6 real seconds. At 30 it is the same 108,000 frames but exactly 3,600 seconds. At 23.976 it is 86,400 frames and 3,603.6 seconds. Comparing those rows is a quick way to spot a project set to the wrong rate.

Worked example: frame numbers to drop-frame labels

Going from a frame count back to a drop-frame label is where hand arithmetic most often goes wrong. Three cases, all of which the tool's tests check:

Frame 1,799. No minute boundary has been crossed yet, so nothing has been skipped. 1,799 frames at 30 per labelled second is 59 seconds and 29 frames: 00:00:59;29.

Frame 1,800. This is the very next frame. A plain count would label it 00:01:00;00, but in drop-frame that number is skipped, as is 00:01:00;01. So frame 1,800 is labelled 00:01:00;02.

Frame 17,982. A ten-minute block of drop-frame contains 9 minutes that skip two numbers and one that skips none. That is 10 times 1,800, minus 18, which is 17,982 frames. So frame 17,982 is the first frame of minute 10, and because minute 10 is a tenth minute, it keeps its 00: the label is 00:10:00;00.

Here is a less tidy case you can check by hand. The label 00:05:30;15 has 5 minutes on it, none of which is a tenth minute after zero, so 2 times 5, which is 10 numbers, have been skipped. Read at 30 per second, 5 minutes, 30 seconds and 15 frames is 330 times 30 plus 15, which is 9,915. Take away the 10 skipped numbers and the frame count is 9,905. In real time that is 9,905 times 1001 divided by 30,000, or 330.497 seconds.

Why 23.976 has no drop-frame form

Drop-frame works at 29.97 because the numbers fit. One label hour at 29.97 is 108 frames behind the clock, and 108 splits evenly into two skipped numbers in each of 54 minutes.

At 23.976 the same 3.6-second gap is 3.6 times 24, which is 86.4 frames per hour. That is not a whole number, so no simple pattern of skipped frame numbers can remove it. There is no standard drop-frame timecode for 23.976, and 23.976 timecode is always non-drop.

In practice that means a 23.976 timecode label is always slightly behind the clock:

• A label of 01:00:00:00 at 23.976 is 86,400 frames, which play for 3,603.6 seconds.
• A label of 01:30:00:00 is 129,600 frames, which play for 129,600 times 1001 divided by 24,000, or 5,405.4 seconds. That is 1 hour, 30 minutes and 5.4 seconds of real time.
• A label of 02:00:00:00 is 172,800 frames, which play for 7,207.2 seconds, or 2 hours and 7.2 seconds.

None of that is a fault. Every frame is accounted for; the label is just counting at 24 per second while the picture plays slightly slower. But it matters whenever someone asks for an exact running time.

Runtime drift and when it matters

There are three common situations in a book-to-film project where the gap between label and clock causes trouble.

Declaring a runtime. Festivals, competitions and distributors ask for the running time, often with strict category limits for short films. If your timeline reads 00:39:58:00 at 23.976, the real running time is 39 minutes and 58 seconds plus 0.1 percent, which is about 2.4 seconds more. Near a category limit, a few seconds can matter. Use the tool's real seconds, not the label, and check the exact rule of the festival you are entering.

Filling a broadcast slot. Television schedules run on the clock. A program delivered for a broadcast slot at 29.97 is usually timed in drop-frame so its label matches real time. If you deliver non-drop by mistake, a 50-minute program runs 3 seconds longer than its label.

Changing frame rate. Converting a 24 fps film to 25 fps by playing every frame faster, a common practice for European television, shortens the running time by 4 percent. A 96-minute film becomes 96 times 24 divided by 25, or 92.16 minutes, and its sound rises in pitch by roughly 0.7 of a semitone unless it is corrected. Converting between 23.976 and 24 changes the length by only 0.1 percent, but sound and subtitles must still be adjusted to match.

Adding and subtracting timecode

Timecode is not ordinary decimal arithmetic. Seconds roll over at the frame rate, not at 100, and drop-frame labels skip numbers. The reliable way to add or subtract is to convert both values to frame counts, do the arithmetic on whole frames, and convert the result back. That is exactly what the tool does.

Worked example: adding at 25 fps

Take an illustrative shot that starts at 00:59:50:20 and lasts 00:00:10:10, at 25 fps.

1. First value in frames. 59 minutes and 50 seconds is 3,590 seconds. 3,590 times 25 is 89,750, plus 20 frames is 89,770.
2. Second value in frames. 10 seconds times 25 is 250, plus 10 frames is 260.
3. Add. 89,770 plus 260 is 90,030 frames.
4. Back to timecode. 90,030 divided by 25 is 3,601 seconds with 5 frames left over. 3,601 seconds is 1 hour, 0 minutes and 1 second. The result is 01:00:01:05.

Adding the fields one by one would give 20 plus 10, which is 30 frames, and 30 is not a valid frame number at 25 fps. Working in frames removes that whole class of mistakes.

Worked example: subtracting across a drop

At 29.97 drop-frame, subtract one frame from 00:01:00;02. In frames, 00:01:00;02 is 1,800. Take away 1 and you have 1,799, which is 00:00:59;29. Doing the same subtraction on the labels, by taking one from the frames field, would give 00:01:00;01, a label that does not exist.

Durations: in, out and off-by-one

Edit software usually treats the out point as the first frame after the shot, so the duration is simply out minus in. A shot from 01:00:10:00 to 01:00:12:00 at 24 fps is 48 frames long, not 49. If you write down the last frame of the shot instead of the frame after it, add one frame to get the duration. Many timing errors in edit lists come from mixing up these two conventions.

Timecode, subtitles and seconds

Subtitle files such as SRT do not use frames. They time each subtitle in hours, minutes, seconds and milliseconds. To move between the two, the frame rate must be exact:

• At 24 fps, one frame lasts 41.667 milliseconds.
• At 23.976, one frame lasts 1001 divided by 24,000 seconds, which is 41.708 milliseconds.
• At 25 fps, one frame lasts 40 milliseconds.
• At 29.97, one frame lasts 1001 divided by 30,000 seconds, which is 33.367 milliseconds.

A subtitle file timed against a 23.976 picture and played against a 24 fps version of the same film will drift by 0.1 percent, about 3.6 seconds by the end of an hour. If subtitles slowly slide out of sync over a long film, a frame-rate mismatch is the usual cause. The tool's seconds mode converts seconds to the nearest whole frame at the rate you pick, and the subtitle reading speed checker works directly on SRT timings.

Reading the calculator's result

Each result has the same five parts, whichever operation you choose.

1. The verdict restates the question and the answer in one line, such as "01:00:00;00 at 29.97 fps drop-frame = 107,892 frames". It turns into a warning if the result is negative or runs past 24 hours, and a failure if a value could not be read or names a frame that does not exist.
2. The metrics give the timecode, the frame count, the real seconds, the same real time written as a clock, and the difference between real time and the time the label reads.
3. The breakdown shows the arithmetic. For drop-frame rates it counts the minutes on the label, the tenth minutes, the skipped numbers and the final frame count. For other rates it shows the frame count divided by the exact frame rate.
4. The table repeats your first value at every frame rate. A timecode keeps its label from row to row, a frame count keeps its count, and a value in seconds keeps its real time.
5. The notes flag anything specific to your rate, such as the 3.6-second drift of 23.976, or a semicolon typed for a rate that has no drop-frame form.

Where timecode turns up in a book-to-film project

Even a short adaptation passes through many hands, and timecode is how they talk about the same moment.

• On set. Cameras and sound recorders each stamp their files with timecode so picture and sound can be lined up later. Two common modes are record run, where the counter only advances while recording, and free run, where it runs continuously, often set to the time of day. Devices are synchronized, or jammed, to the same source at the start of a shooting day.
• In the edit. Shot logs, notes from the director and lists of changes all refer to timecode. An edit decision list, the plain-text file that records which source frames go where, is written entirely in timecode.
• In sound. Lines that must be re-recorded, sound effects to add and music cues are all listed by in and out timecode. A music cue sheet, which reports each piece of music used and how long it plays, needs accurate durations, which come from subtracting in from out.
• In visual effects and color. Shots sent out for effects work or grading are identified by their timecode ranges, often with a few frames of extra handle at each end.
• In delivery. Subtitle files, closed captions and quality control reports are timed against the program timecode of the final master.

Every one of those steps assumes everyone uses the same frame rate and the same drop or non-drop choice. A mismatch anywhere in the chain shows up as notes landing on the wrong frame, music cues reported a few seconds long, or subtitles drifting out of sync.

Conventions you will meet in delivery documents

Start timecode. Many delivery specifications ask the program to start at 01:00:00:00 rather than at zero, with any countdown or leader placed just before it. Longer films split into reels sometimes start each reel at a different hour. The exact convention is set by the distributor or broadcaster, so follow the delivery schedule you receive.

Drop or non-drop for 29.97 deliveries. Broadcasters working at 29.97 often ask for drop-frame so the label matches the schedule. Some post-production houses prefer non-drop because every label exists. Neither is wrong; what matters is that every file in a delivery uses the same choice, and that the choice is written on the paperwork.

23.976 is non-drop. If a document asks for 23.976 drop-frame, query it. It almost certainly means 29.97 drop-frame or 23.976 non-drop.

Separator characters. A semicolon before the frames means drop-frame by convention. Some systems use a full stop or comma for drop-frame instead. The tool accepts all of them and reads the rate from the frame rate you choose, not from the punctuation.

Common mistakes the calculator prevents

Assuming 29.97 and 30 are interchangeable. The same label is 3.6 seconds an hour longer at 29.97 non-drop than at 30. The table in the result shows both side by side.

Typing a skipped drop-frame label. 00:01:00;00 and 00:01:00;01 do not exist. The tool rejects them and names the next real frame.

Frames higher than the rate allows. At 25 fps the frames field runs from 00 to 24. A value of 25 is an error, not the next second. The tool reports it.

Quoting a runtime from the label. At 23.976 and at 29.97 non-drop, the label runs slow. Quote the real seconds for anything with a strict time limit.

Adding timecode field by field. Frames and seconds overflow at different points, and drop-frame skips numbers. Convert to frames first, as the tool does.

Sources and rules this page relies on

• SMPTE ST 12-1, Time and Control Code, published by the Society of Motion Picture and Television Engineers, which defines timecode, the HH:MM:SS:FF address and the drop-frame counting rule for 29.97. The four-number form at 59.94 is the same rule at double the rate, as editing software applies it.
• The NTSC color television standard adopted in the United States in 1953, which set the 30000/1001 frame rate that 29.97, 59.94 and 23.976 are based on.
• Digital Cinema System Specification, published by Digital Cinema Initiatives, for 24 frames per second as the standard rate of theatrical DCPs.

The arithmetic on this page follows directly from those definitions and is checked by the tool's tests. Delivery requirements, such as which timecode a broadcaster wants and where the program should start, vary by distributor. Confirm them in the delivery schedule you are given.