Why is a 25% to 100% linear ramp not simply 62.5% average speed?

It is 62.5% when speed changes linearly over output time. If speed changes linearly through source position, the slow portion occupies more output time, so the effective average is the logarithmic mean, about 54.1%.

Does changing playback speed change frame rate?

Not necessarily. Retiming changes how source frames map to the timeline. The project frame rate can stay fixed while the software repeats, drops, blends, or synthesizes displayed frames.

Why can the exact timeline frame count be fractional?

The mathematical duration can fall between timeline-frame boundaries. An edit must normally end on a whole frame, so the calculator shows the nearest frame and its timing difference.

Does the calculator upload my footage or inputs?

No. It does not open media. All calculations and report creation run locally in the browser, with no backend input tracking.

Video Speed Ramp Calculator — Playback Speed, Frames & Clip Duration

Calculate the playback duration, effective speed, and timeline frames for one video speed-ramp segment. Compare linear and eased curves, and specify whether the ramp is keyed across source positions or across output-timeline time.

Ramp inputs

Source clip length

Source duration

Use HH:MM:SS.sss or seconds. The duration is snapped to the nearest complete source frame.

Frame rate

Used to convert clip time to complete source and timeline frames.

Start speed (%)

Positive speeds only: 0.01% to 10,000%.

End speed (%)

Use the same value for a constant-speed segment.

Ramp curve

Controls how speed moves between the endpoints.

Ramp keyed across

Choose the horizontal axis represented by the editor’s speed curve.

Calculations, copying, and report creation stay in your browser. No footage or entered production values are uploaded.

Speed ramp results

Ramped playback duration —

Effective average speed—

Duration change—

Source duration used—

Source frames—

Timeline frames—

Nearest-frame timing error—

Ramp preview

Calculation model Enter valid values to calculate the ramp.

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How to use the video speed ramp calculator

Enter the source segment length. Use its duration or exact frame count. If you enter time, the calculator snaps it to the nearest complete source frame at the selected rate.
Enter start and end speed. Use percentages such as 25% for quarter speed, 100% for real time, or 400% for four-times speed.
Choose the curve. Linear changes at a uniform rate. Ease in delays most of the change, ease out makes most of the change early, and smooth ease in/out softens both endpoints.
Choose what the curve’s horizontal axis means. Select source clip position if the endpoints divide source frames; select output timeline time if the speed graph is explicitly plotted against the retimed timeline.
Round deliberately. The exact result may end between timeline frames. Use the nearest-frame count for planning and check how the NLE rounds the last frame.

Video speed ramp formulas

Let D be source duration, v(x) be playback speed as a decimal multiplier, and x run from 0 to 1 across the selected keyframe basis. For example, 25% speed is v = 0.25.

Source-position ramp: output duration = D × ∫₀¹ [1 ÷ v(x)] dx Timeline-position ramp: output duration = D ÷ ∫₀¹ v(x) dx Effective average speed = D ÷ output duration Exact timeline frames = output duration × timeline fps

When the endpoints are equal, both models reduce to the familiar constant-speed rule: output duration = source duration ÷ speed. For a linear source-position ramp with different positive endpoints a and b, the integral is ln(b ÷ a) ÷ (b − a). The calculator uses closed-form solutions where available and adaptive Simpson integration for the smooth ease-in/out source-position curve.

Why source-position and timeline-position ramps differ

“Linear speed ramp” is incomplete unless the horizontal axis is known. If speed rises linearly as the playhead moves through source positions, slow source sections consume more output time. A 25% → 100% ramp therefore spends a disproportionate amount of the finished clip near the slow end. Its effective speed is about 54.1%, not the arithmetic mean.

If speed rises linearly across output timeline time, equal timeline intervals receive equal weight. The same 25% → 100% ramp then averages 62.5%. Editing applications can expose retiming as frame-to-time remapping, speed segments, or application-specific transition handles, so reproduce the chosen curve in the NLE and treat this result as a planning calculation rather than interchange metadata.

Linear speed ramp examples

10-second source ramp	Source-position result	Timeline-position result	What changes
25% → 100%	18.484 s (54.10% effective)	16.000 s (62.5% effective)	The slow beginning has more weight in the source-position model.
100% → 400%	4.621 s (216.40% effective)	4.000 s (250% effective)	The real-time source region occupies more output time in the source-position model.
100% → 50%	13.863 s (72.13% effective)	13.333 s (75% effective)	Swapping endpoints preserves a linear curve’s duration within the same basis.
50% → 50%	20.000 s (50% effective)	20.000 s (50% effective)	A constant-speed segment is identical in both models.

Workflow assumptions and limits

This calculator models one monotonic, positive-speed segment at a constant project frame rate. It does not reproduce proprietary NLE bezier handles, segmented ramps, freeze frames, reverse playback, optical-flow quality, source timestamp irregularities, audio pitch processing, frame blending, motion blur, or variable-frame-rate media. A ramp reaching or crossing 0% needs a separate freeze/reverse model and is intentionally rejected. Confirm exact keyframe positions and final duration in the editing application before a sync-critical export.

Video speed ramp FAQ

How do I calculate the duration of a speed-ramped clip?

Divide source duration by the ramp’s effective average speed. Because variable speed weights portions of the clip differently, the effective speed depends on the curve shape and whether the ramp is measured over source position or output time.

Why is a 25% to 100% linear ramp not always 62.5% average speed?

62.5% is the arithmetic mean and applies when speed changes linearly over output-timeline time. When the change is linear through source position, the slow portion lasts longer on the output timeline; the effective average becomes about 54.1%.

What does 50% playback speed do to clip duration?

A constant 50% speed doubles the duration. A 10-second source segment becomes 20 seconds, before any rounding to timeline frames.

Does a speed ramp change the project frame rate?

Usually no. Retiming changes how source pictures map onto a fixed timeline frame grid. The editor may repeat, drop, blend, or synthesize pictures depending on its retiming-quality settings.

Can this calculator handle freeze frames or reverse playback?

No. Start and end speeds must both be greater than zero. A curve touching zero can imply an infinite-duration point under this continuous model, while a reverse ramp requires direction and crossing rules specific to the edit.

Why is the exact timeline frame count fractional?

The calculated duration can land between frame boundaries. The nearest complete frame is suitable for rough planning; the displayed timing error shows how far that rounded boundary is from the mathematical result.

Does the calculator upload my clip or production values?

No. The page does not inspect or upload media. Its calculations, copying, and text-report download run locally in your browser.

Production references

Adobe After Effects: time-stretching and time-remapping — explains time remapping, playback speed as source seconds per output second, frame redistribution, and speed graphs.
Apple Final Cut Pro: create variable speed effects — describes variable-speed segments and gradual speed transitions, also called ramps.
Apple Final Cut Pro: introduction to retiming clips — outlines constant and variable retiming and notes the application’s audio-pitch behaviour.