<easing-function>

The <easing-function> CSS data type denotes a mathematical function that describes the rate at which a numerical value changes.

This transition between two values may be applied in different situations. It may be used to describe how fast values change during animations. This lets you vary the animation's speed over the course of its duration. You can specify an easing function for CSS transition and animation properties.

Syntax

css

/* linear function and keyword */
/* linear(<point-list>) */
linear(1, -0.5, 0)
linear

/* cubic-bezier function and keywords */
/* cubic-bezier(<x1>, <y1>, <x2>, <y2>) */
cubic-bezier(0.42, 0.0, 1.0, 1.0)
ease
ease-in
ease-out
ease-in-out

/* steps function and keywords */
/* steps(<number-of-steps>, <direction>) */
steps(4, end)
step-start
step-end

Values

<point-list>

List of linear stops.

linear

Indicates that the interpolation goes linearly. This keyword represents the easing function linear(0, 1).

Graph of "Input progress" to "Output progress" showing a line extending from the origin to (1, 1).

<x1>, <y1>, <x2>, <y2>

<number> values representing the abscissas and ordinates of the P1 and P2 points defining the cubic Bézier curve. x1 and x2 must be in the range [0, 1], otherwise the value is invalid.

ease

Indicates that the interpolation starts slowly, accelerates sharply, and then slows gradually towards the end. This keyword represents the easing function cubic-bezier(0.25, 0.1, 0.25, 1.0). It is similar to ease-in-out, though it accelerates more sharply at the beginning.

ease-in

Indicates that the interpolation starts slowly, then progressively speeds up until the end, at which point it stops abruptly. This keyword represents the easing function cubic-bezier(0.42, 0.0, 1.0, 1.0).

ease-out

Indicates that the interpolation starts abruptly and then progressively slows down towards the end. This keyword represents the easing function cubic-bezier(0.0, 0.0, 0.58, 1.0).

ease-in-out

Indicates that the interpolation starts slowly, speeds up, and then slows down towards the end. This keyword represents the easing function cubic-bezier(0.42, 0.0, 0.58, 1.0). At the beginning, it behaves like the ease-in keyword; at the end, it is like the ease-out keyword.

Graphs of "Input progress" to "Output progress", of which "ease" shows a curved line quickly rising from the origin to (1, 1); "ease-in" shows a shallow curved line from the origin that straightens out as it approaches (1, 1); "ease-out" shows a straight diagonal line that slightly curves as it gets close to (1, 1); and "ease-in-out" shows a symmetrical, "S"-shaped line curving from the origin to (1, 1).

<number-of-steps>

A strictly positive <integer>, representing the amount of equidistant treads composing the stepping function.

<direction>

One of the following keywords that indicate when the jumps occur:

  • jump-start denotes that the first step or jump happens when the interpolation begins.
  • jump-end denotes that the last step or jump happens when the interpolation ends.
  • jump-both denotes that jumps occur at both the 0% and 100% marks, effectively adding a step during the interpolation iteration.
  • jump-none denotes no jump on either end. Instead, holding at both the 0% mark and the 100% mark, each for 1/n of the duration.
  • start is the equivalent of jump-start.
  • end is the equivalent of jump-end. This is the default.
step-start

Indicates that the interpolation jumps immediately to its final state, where it stays until the end. This keyword represents the easing function steps(1, jump-start) or steps(1, start).

step-end

Indicates that the interpolation stays in its initial state until the end, at which point it jumps directly to its final state. This keyword represents the easing function steps(1, jump-end) or steps(1, end).

Graphs of "Input progress" to "Output progress", of which "step-start" shows a hollow origin and a horizontal line extending from (0, 1) to (1, 1); and "step-end" shows a horizontal line extending from the origin to (1, 0) (hollow) and a point at (1, 1).

Description

There are three types of easing functions:

Linear easing function

The linear() function defines a piecewise linear function that interpolates linearly between its points, allowing you to approximate more complex animations like bounce and elastic effects. The interpolation is done at a constant rate from beginning to end. A typical use of the linear() function is to provide many points to approximate any curve.

When you define the linear() function, you specify the linear easing points as a list, as in, linear(0, 0.25, 1). This linear() function produces an easing function that moves linearly from 0, to 0.25, then to 1.

Graphs of "Input progress" to "Output progress", of which "linear(0, 0.25, 1)" shows a broken line connecting the origin, (0.5, 0.25), and (1, 1); and "linear(0, 0.25 75%, 1)" shows a broken line connecting the origin, (0.75, 0.25), and (1, 1).

Consider another example of the function: linear(0, 0.25 75%, 1). This produces a linear easing function that spends 75% of the time transitioning from 0 to 0.25 and the last 25% transitioning from 0.25 to 1.

The linear keyword is equivalent to the easing function linear(0, 1).

Cubic Bézier easing function

The cubic-bezier() functional notation defines a cubic Bézier curve. The easing functions in the cubic-bezier subset of easing functions are often called "smooth" easing functions because they can be used to smooth down the start and end of the interpolation. They correlate an input progress to an output progress, both expressed as <number>s. For these values, 0.0 represents the initial state, and 1.0 represents the final state.

Graph of "Input progress" to "Output progress" showing an "S"-shaped line curving from the origin to (1, 1) with the Bezier control points P1(0.1, 0.6) and P2(0.7, 0.2).

A cubic Bézier curve is defined by four points: P0, P1, P2, and P3. The points P0 and P3 represent the start and the end of the curve. In CSS, these points are fixed as the coordinates progress (the abscissa the input progress, the ordinate the output progress). P0 is (0, 0) and represents the initial progress and the initial state. P3 is (1, 1) and represents the final progress and the final state.

Not all cubic Bézier curves are suitable as easing functions because not all are mathematical functions; i.e., curves that for a given abscissa have zero or one value. With P0 and P3 fixed as defined by CSS, a cubic Bézier curve is a function, and is therefore valid, if and only if the abscissas of P1 and P2 are both in the [0, 1] range.

Cubic Bézier curves with the P1 or P2 ordinate outside the [0, 1] range can cause the value to go farther than the final state and then return. In animations, for some properties, such as left or right, this creates a kind of "bouncing" effect.

Graphs of the easing function "cubic-bezier(0.3, 0.2, 0.2, 1.4)", one of which shows the output progress going above 1 starting from a certain input progress, the other shows the output progress reaching and then staying at 1.

However, certain properties will restrict the output if it goes outside an allowable range. For example, a color component greater than 255 or smaller than 0 in rgb() will be clipped to the closest allowed value (255 and 0, respectively). Some cubic-bezier() values exhibit this property.

When you specify an invalid cubic Bézier curve, CSS ignores the whole property.

Each of the keywords ease, ease-in, ease-out, and ease-in-out is equivalent to a specific cubic-bezier() value.

Steps easing function

The steps() functional notation defines a step function that divides the domain of output values in equidistant steps. This subclass of step functions are sometimes also called staircase functions.

These are a few examples illustrating the steps() function:

css

steps(2, jump-start) /* Or steps(2, start) */
steps(4, jump-end) /* Or steps(4, end) */
steps(5, jump-none)
steps(3, jump-both)

Graphs of "Input progress" to "Output progress", of which "steps(2, jump-start)" shows horizontal lines extending 0.5 unit from (0, 0.5) and (0.5, 1), respectively, with hollow points at the origin and (0.5, 0.5); "steps(4, jump-end)" shows horizontal lines extending 0.25 unit from (0, 0), (0.25, 0.25), (0.5, 0.5), and (0.75, 0.75), respectively, with hollow points at (0.25, 0), (0.5, 0.25), and (0.75, 0.5), and a point at (1, 1); "steps(5, jump-none)" shows horizontal lines extending 0.2 unit from (0, 0), (0.2, 0.25), (0.4, 0.5), (0.6, 0.75), and (0.8, 1), respectively, with hollow points at (0.2, 0), (0.4, 0.25), (0.6, 0.5), and (0.8, 0.75); "steps(3, jump-both)" shows horizontal lines extending 1/3 unit from (0, 0.25), (1/3, 0.5), and (2/3, 0.75),respectively , with a point at (1, 1) and hollow points at the origin, (1/3, 0.25), (2/3, 0.5), and (1, 0.75).

Each of the keywords step-start and step-end is equivalent to a specific steps() value.

Formal syntax

<easing-function> = 
linear |
<linear-easing-function> |
<cubic-bezier-easing-function> |
<step-easing-function>

<linear-easing-function> =
linear( <linear-stop-list> )

<cubic-bezier-easing-function> =
ease |
ease-in |
ease-out |
ease-in-out |
cubic-bezier( <number [0,1]> , <number> , <number [0,1]> , <number> )

<step-easing-function> =
step-start |
step-end |
steps( <integer> [, <step-position> ]? )

<linear-stop-list> =
[ <linear-stop> ]#

<step-position> =
jump-start |
jump-end |
jump-none |
jump-both |
start |
end

<linear-stop> =
<number> &&
<linear-stop-length>?

<linear-stop-length> =
<percentage>{1,2}

Examples

Comparing the easing functions

This example provides an easy comparison between the different easing functions using an animation. From the drop-down menu, you can select an easing function – there are a couple of keywords and some cubic-bezier() and steps() options. After selecting an option, you can start and stop the animation using the provided button.

HTML

html

<div>
  <div></div>
</div>
<ul>
  <li>
    <button class="animation-button">Start animation</button>
  </li>
  <li>
    <label for="easing-select">Choose an easing function:</label>
    <select id="easing-select">
      <option selected>linear</option>
      <option>linear(0, 0.5 50%, 1)</option>
      <option>ease</option>
      <option>ease-in</option>
      <option>ease-in-out</option>
      <option>ease-out</option>
      <option>cubic-bezier(0.1, -0.6, 0.2, 0)</option>
      <option>cubic-bezier(0, 1.1, 0.8, 4)</option>
      <option>steps(5, end)</option>
      <option>steps(3, start)</option>
      <option>steps(4)</option>
    </select>
  </li>
</ul>

CSS

css

body > div {
  position: relative;
  height: 100px;
}

div > div {
  position: absolute;
  width: 50px;
  height: 50px;
  background-color: blue;
  background-image: radial-gradient(
    circle at 10px 10px,
    rgba(25, 255, 255, 0.8),
    rgba(25, 255, 255, 0.4)
  );
  border-radius: 50%;
  top: 25px;
  animation: 1.5s infinite alternate;
}

@keyframes move-right {
  from {
    left: 10%;
  }

  to {
    left: 90%;
  }
}

li {
  display: flex;
  align-items: center;
  justify-content: center;
  margin-bottom: 20px;
}

JavaScript

js

const selectElem = document.querySelector("select");
const startBtn = document.querySelector("button");
const divElem = document.querySelector("div > div");

startBtn.addEventListener("click", () => {
  if (startBtn.textContent === "Start animation") {
    divElem.style.animationName = "move-right";
    startBtn.textContent = "Stop animation";
    divElem.style.animationTimingFunction = selectElem.value;
  } else {
    divElem.style.animationName = "unset";
    startBtn.textContent = "Start animation";
  }
});

selectElem.addEventListener("change", () => {
  divElem.style.animationTimingFunction = selectElem.value;
});

Result

Using the cubic-bezier() function

These cubic Bézier curves are valid for use in CSS:

css

/* The canonical Bézier curve with four <number> in the [0,1] range */
cubic-bezier(0.1, 0.7, 1.0, 0.1)

/* Using <integer> is valid because any <integer> is also a <number>. */
cubic-bezier(0, 0, 1, 1)

/* Negative values for ordinates are valid, leading to bouncing effects. */
cubic-bezier(0.1, -0.6, 0.2, 0)

/* Values greater than 1.0 for ordinates are also valid. */
cubic-bezier(0, 1.1, 0.8, 4)

These cubic Bézier curves definitions are invalid:

css

/* Though the animated output type may be a color,
   Bézier curves work with numerical ratios. */
cubic-bezier(0.1, red, 1.0, green)

/* Abscissas must be in the [0, 1] range or
   the curve is not a function of time. */
cubic-bezier(2.45, 0.6, 4, 0.1)

/* The two points must be defined, there is no default value. */
cubic-bezier(0.3, 2.1)

/* Abscissas must be in the [0, 1] range or
   the curve is not a function of time. */
cubic-bezier(-1.9, 0.3, -0.2, 2.1)

Using the steps() function

These easing functions are valid:

css

/* There are 5 treads, the last one happens
   right before the end of the animation. */
steps(5, end)

/* A two-step staircase, the first one happening
   at the start of the animation. */
steps(2, start)

/* The second parameter is optional. */
steps(2)

Note: If the animation contains multiple stops, then the steps specified in the steps() function will apply to each section. Therefore, an animation with three segments and steps(2) will contain 6 steps in total, 2 per segment.

These easing functions are invalid:

css

/* The first parameter must be an <integer> and
   cannot be a real value, even if it is equal to one. */
steps(2.0, jump-end)

/* The amount of steps must be non-negative. */
steps(-3, start)

/* There must be at least one step. */
steps(0, jump-none)

Specifications

Specification
CSS Easing Functions Level 1
# easing-functions

Browser compatibility

BCD tables only load in the browser

See also