VW vs. VH vs. %: When Should You Use Viewport-Relative Units in CSS?

Understanding vh/vw vs. %: A Guide to Viewport-Relative Units in CSS

While % units measure the percentage of an element relative to its parent container, vw and vh units are specifically designed to measure the percentage of the viewport's height and width, respectively. This distinction becomes critical when the element's parent container is not the viewport itself.

In cases where the parent container's height or width is dynamic or differs from the viewport, vw and vh units ensure consistent sizing. For instance, if an element is set to 100% height within a parent div that's 1000px tall, its actual height could vary depending on the viewport's size.

Conversely, setting the same element to 100vh would ensure that it always occupies 100% of the viewport's height, regardless of the parent div's dimensions. This eliminates the potential for inconsistent sizing in responsive web design.

To illustrate this difference, consider the following code sample:

body,
html {
    height: 100%;
}

.parent {
    background: lightblue;
    float: left;
    height: 200px;
    padding: 10px;
    width: 50px;
}

.child {
    background: pink;
    height: 100%;
    width: 100%;
}

.viewport-height {
    background: gray;
    float: right;
    height: 100vh;
    width: 50px;
}

In this example, the child div within the parent div will have a height that's 100% of the parent div's height (200px). However, the viewport-height div will always have a height that's 100% of the viewport's height, even if the parent div's height were to change.

Therefore, vw and vh units provide more precise control over element sizing in relation to the viewport, making them particularly useful for creating responsive web layouts that adapt seamlessly to varying screen sizes and aspect ratios.

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