Update 8/4/2010: Note that in at least WordPress 3.0 there is a new filter specifically designed for changing the length of the excerpt: excerpt_length. Instead of re-implementing the entire excerpt-creation function you can now simply return the appropriate value.

How can I change the length of the_excerpt() in Wordpress without editing core files?

Well, first we need to figure out exactly where the excerpt text is truncated. Checking wp-includes/default-filters.php, around line 128 we see:

add_filter('get_the_excerpt', 'wp_trim_excerpt');

Ok, so that’s how it’s cut off. The first step in our fix is to prevent the default function from running and truncating the text at the default length. The easiest way to do this? Simply remove the filter by adding a line to your theme’s functions.php file:

remove_filter('get_the_excerpt', 'wp_trim_excerpt');

Great. So now we’ve got the_excerpt() looking exactly like the_content(). Now we need to create our own pretty little function to handle truncating the_excerpt to the length we need. Since those crazy Wordpress devs have already thought of everything, we’ll use their original function as a template for our new one. The original function can be found in wp-includes/formatting.php, around line 779:

function wp_trim_excerpt($text) { // Fakes an excerpt if needed
	global $post;
	if ( '' == $text ) {
		$text = get_the_content('');
		$text = apply_filters('the_content', $text);
		$text = str_replace(']]>', ']]>', $text);
		$text = strip_tags($text);
		$excerpt_length = 55;
		$words = explode(' ', $text, $excerpt_length + 1);
		if (count($words) > $excerpt_length) {
			array_pop($words);
			array_push($words, '[...]');
			$text = implode(' ', $words);
		}
	}
	return $text;
}

See the line $excerpt_length = 55;? That’s the one we want to change!

Take your new wp_trim_excerpt function and stick it in your theme’s functions.php file. Be sure to rename it to something unique. You should end up with something similar to this:

function custom_trim_excerpt($text) { // Fakes an excerpt if needed
	global $post;
	if ( '' == $text ) {
		$text = get_the_content('');
		$text = apply_filters('the_content', $text);
		$text = str_replace(']]>', ']]>', $text);
		$text = strip_tags($text);
		$excerpt_length = 75;
		$words = explode(' ', $text, $excerpt_length + 1);
		if (count($words) > $excerpt_length) {
			array_pop($words);
			array_push($words, '[...]');
			$text = implode(' ', $words);
		}
	}
	return $text;
}

Note that I only made two changes: 1) renamed the function to “custom_trim_excerpt”, and 2) changed the excerpt_length to 75.

Great, we’re almost there! One step left. Now we have to tell Wordpress to use our new custom function to truncate the_excerpt. We’ll do this by adding a new filter, similar to the one we removed at the very beginning. Stick this line in your theme’s functions.php file:

add_filter('get_the_excerpt', 'custom_trim_excerpt');

Be sure to substitute your new function’s name, assuming you didn’t use the same one I did, and you should be all set!

Note: This guide was written using Wordpress 2.2. The exact procedure may vary with versions, but should be fairly similar.