/*
 *
 * Textarea Line Count - version 1.2
 *
 * http://mosttw.wordpress.com/
 *
 * Licensed under MIT License: http://en.wikipedia.org/wiki/MIT_License
 *
 * Copyright (c) 2010 MostThingsWeb

 Permission is hereby granted, free of charge, to any person obtaining a copy
 of this software and associated documentation files (the "Software"), to deal
 in the Software without restriction, including without limitation the rights
 to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 copies of the Software, and to permit persons to whom the Software is
 furnished to do so, subject to the following conditions:

 The above copyright notice and this permission notice shall be included in
 all copies or substantial portions of the Software.

 THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 THE SOFTWARE.
 */


(function($){
    $.countLines = function(textarea, o){
	// The textarea
	var ta;

	if (typeof textarea == "string")
	    ta = $(textarea);
	else if (typeof textarea == "object")
	    ta = textarea;

	if (ta.size() != 1)
	    return;

	// Get the textarea value
	var value = ta.val();

	var options = $.extend({
	    recalculateCharWidth : false,
	    chars : "",
	    charsMode : "random",
	    fontAttrs : ["font-family", "font-size", "text-decoration", "font-style", "font-weight"]
	}, o);
	var masterCharacters = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890";
	var counter;
	switch (options.charsMode){
	    case "random":
		// Build a random collection of characters
		options.chars = "";
		masterCharacters += ".,?!-+;:'\"";
		for (counter = 1; counter <= 12; counter++)
		    options.chars += masterCharacters[(Math.floor(Math.random() * masterCharacters.length))];
		break;
	    case "alpha":
		options.chars = masterCharacters;
		break;
	    case "alpha_extended":
		options.chars = masterCharacters + ".,?!-+;:'\"";
		break;
	    case "from_ta":
		// Build a random collection of characters from the textarea
		if (value.length < 15)
		    options.chars = masterCharacters;
		else
		    for (counter = 1; counter <= 15; counter++)
			options.chars += value[(Math.floor(Math.random() * value.length))];
		break;
           case "custom":
                // Already defined in options.chars
                break;
	}

	// Decode chars
	if (!$.isArray(options.chars))
	    options.chars = options.chars.split("");

	// Generate a span after the textarea with a random ID
	var id = "";
	for (counter = 1; counter <= 10; counter++)
	    id += (Math.floor(Math.random() * 10) + 1);

	ta.after("<span id='s" + id + "'></span>");
	var span = $("#s" + id);

	// Hide the span
	span.hide();

	// Apply the font properties of the textarea to the span class
	$.each(options.fontAttrs, function(i, v){
	    span.css(v, ta.css(v));
	});

	// Get the number of lines
	var lines = value.split("\n");
	var linesLen = lines.length;

	var averageWidth;

	// Check if the textarea has a cached version of the average character width
	if (options.recalculateCharWidth || ta.data("average_char") == null) {
	    // Get a pretty good estimation of the width of a character in the textarea. To get a better average, add more characters and symbols to this list
	    var chars = options.chars;

	    var charLen = chars.length;
	    var totalWidth = 0;

	    $.each(chars, function(i, v){
		span.text(v);
		totalWidth += span.width();
	    });

	    // Store average width on textarea
	    ta.data("average_char", Math.ceil(totalWidth / charLen));
	}

	averageWidth = ta.data("average_char");

	// We are done with the span, so kill it
	span.remove();

	// Determine missing width (from padding, margins, borders, etc); this is what we will add to each line width
	var missingWidth = (ta.outerWidth() - ta.width()) * 2;

	// Calculate the number of lines that occupy more than one line
	var lineWidth;

	var wrappingLines = 0;
	var wrappingCount = 0;
	var blankLines = 0;

	$.each(lines, function(i, v){
	    // Calculate width of line
	    lineWidth = ((v.length + 1) * averageWidth) + missingWidth;
	    // Check if the line is wrapped
	    if (lineWidth >= ta.outerWidth()){
		// Calculate number of times the line wraps
		var wrapCount = Math.floor(lineWidth / ta.outerWidth());
		wrappingCount += wrapCount;
		wrappingLines++;
	    }

	    if ($.trim(value) == "")
		blankLines++;
	});

	var ret = {};
	ret["actual"] = linesLen;
	ret["wrapped"] = wrappingLines;
	ret["wraps"] = wrappingCount;
	ret["visual"] = linesLen + wrappingCount;
	ret["blank"] = blankLines;

	return ret;
    };
})(jQuery);

http://mosttw.wordpress.com/2010/09/14/textarea-line-count-jquery-plugin/

Textarea Line Count - Version 1.2

JavaScript