# useVelocity > Track the velocity of a motion value in units per second. **Source:** [https://motion.svelte.page/docs/use-velocity](https://motion.svelte.page/docs/use-velocity) --- `useVelocity` returns an augmented `MotionValue` tracking the velocity of a source's value in units per second. Pass a motion value (from `useMotionValue`, `useSpring`, `useScroll`, …) or any Svelte readable; the result composes with `useTransform`, `useSpring`, and the rest of the Tier 2 surface. ```svelte
Velocity: {xVelocity.current} px/s
``` ## Usage The returned value emits the rate of change in units per second and settles to `0` when the source stops moving. ```svelte
x.set(e.clientX)} > Move your pointer
``` ### With unit strings `useVelocity` parses numeric values from unit strings like `"120px"` or `"45deg"`: ```svelte ``` ## How it works 1. Creates an internal `motionValue` from `motion-dom` for timestamp-based velocity tracking 2. Subscribes to the source and forwards each emit to the tracker via `.set()` 3. Polls `motionValue.getVelocity()` on every `requestAnimationFrame` and writes the result to the returned motion value 4. Snaps the result to `0` and stops polling when velocity falls below `0.001` units/second; the next source emit restarts the loop 5. The source subscription and both internal motion values are torn down when the surrounding `$effect` scope unmounts ## Performance - **On-demand polling**: The RAF loop only runs while velocity is non-zero - **Auto-settle**: Stops polling when movement stops, avoiding idle CPU usage - **Cleanup**: Cancels the frame loop and unsubscribes the source at component unmount - **SSR-safe**: Returns a static `motionValue(0)` with no source subscription on the server ## Common patterns ### Momentum-based skew ```svelte
x.set(e.clientX)} > Skews with momentum
``` ### Velocity-driven blur ```svelte
Blurs when moving fast
``` ### Chaining velocities You can derive the velocity of a velocity to get acceleration: ```svelte ``` ## API Reference ### Parameters - **source** `MotionValue | Readable` — a motion value or Svelte readable whose value changes over time. ### Returns An `AugmentedMotionValue` — a real motion-dom `MotionValue` containing the current velocity in units per second, plus: - `.current` — Svelte 5 reactive getter (templates, `$derived`, `$effect`). - `.subscribe(run)` — Svelte readable store contract (powers `$velocity` template syntax). - All other `MotionValue` methods from motion-dom (`get`, `getVelocity`, `on`, etc.). ## When to use - **Momentum effects**: Skew, stretch, or blur elements based on how fast a value is changing - **Physics-based UI**: React to acceleration or deceleration of animations - **Gesture feedback**: Show speed-dependent visual feedback during pointer or scroll interactions - **Chained dynamics**: Derive acceleration by passing a velocity store back into `useVelocity` ## See also - [useTransform](/docs/use-transform) - Map velocity values to visual ranges - [useMotionTemplate](/docs/use-motion-template) - Compose CSS strings from reactive stores - [useTime](/docs/use-time) - Reactive time source for continuous animations --- Based on [Motion's useVelocity](https://motion.dev/docs/react-use-velocity) API.