Timecode maths without counting on your fingers

What the tool answers

Edit decision lists, act lengths for a broadcast slot, the duration between two marks, how long a reel is in feet: all of it is timecode arithmetic, and all of it goes wrong at the same two places — the frame digits that don’t carry over at 100, and the frame numbers that drop-frame skips. The Calculate tab reads a whole column of values at once: one per line to add up, or + - * / inside a line. Every line shows its running total, so a typo is easy to find.

The Convert tab answers the other daily question: footage shot at 25 fps goes into a 23.976 timeline — does the clip get longer, by how much, and what happens to the sound? Below the numbers the tool shows which source frame ends up in each target frame.

How we calculate

Internally everything is a frame count. A non-drop timecode at 25 fps is simply hours × 90,000 + minutes × 1,500 + seconds × 25 + frames. NTSC rates run at 30000/1001 frames per second, so a timecode that counts 30 labels per second drifts 3.6 seconds per hour behind the wall clock. Drop-frame fixes the labels, not the pictures: it skips frame numbers ;00 and ;01 at the start of every minute except every tenth. An hour of 29.97 DF therefore has 107,892 frames and ends almost exactly on clock time.

Conforming keeps every frame and changes only the playback speed: the factor is target rate ÷ source rate, the pitch shift 12 × log₂(factor) semitones if the sound is sped up with the picture. Converting keeps the duration and maps each target frame to the source frame that is on screen at that moment — which means repeated or skipped frames.

frames (DF 29.97) = (h×3600 + m×60 + s) × 30 + f − 2 × (total minutes − ⌊total minutes ÷ 10⌋) real seconds = frames ÷ (30000/1001) pitch (semitones) = 12 × log₂(target fps ÷ source fps)

Where the tool is wrong

• Interlaced material. The tool counts frames. For 50i or 59.94i, field order and 2:3 pulldown of interlaced fields are a job for the edit system or a standards converter; the cadence preview shows progressive frames.
• Timecode is a label. A camera with free-run timecode or a recorder that jammed against a drifting clock has gaps and jumps that no arithmetic can see. Sum durations from the edit, not from camera roll timecodes.
• Pitch is the raw figure. Most edit and audio systems can conform with pitch correction; the semitone value is what you hear without it.
• Plain numbers. A bare number after + or − counts as frames, after × or ÷ as a factor. If you mean seconds, write 2s.

From the edit

A 52-minute documentary is cut at 25 fps for a German broadcaster: 78,000 frames. A US distributor asks for a 23.976 version. Conformed, every frame stays and the film runs 54:13 — 4.1 % slower, the voices 72 cents lower, roughly three quarters of a semitone, which an audience notices in music and narration. Converted, it stays at 52:00, but one source frame in 25 is dropped and pans start to stutter.

The usual way out is a conform with pitch correction on the audio stems, and a check of the new runtime against the slot before anyone books the mix. The tool gives both numbers in one view, along with the frame count the mix stage will ask for.