Can I calculate the number of effective (output) rows for html text?

You could get an approximate answer by dividing the number of characters in the text by the average number of characters per line in a block of the given width. It won't be perfectly accurate, but unless the block is very narrow, you will be right a good deal of the time.

Well, it depends on the font-size and font-face and any font-styling, e.g. bold, italic etc. If you want something standard, you'll need to use a monospace font (e.g. Courier family) - they look ugly as hell usually, but they do the job. You also have to take account of wrapping. If the word too long to fit in a line, it will appear on the next, effectively wasting a few (or more!) characters space on the end of the first line.

However, if you do use a monospace font, you should be able to calculate this quite easily. You'll have a $max_no_chars_per_line variable, so should be straightforward. You also need to know which characters allow a wrap, e.g. '-', ' ' (most punctuation except things like quotemarks) etc.

Thinking about it, there's quite a bit involved with this. Remember that css properties may affect your output (e.g. padding) and that different browsers may display your text differently.

I'm sure that there must be a script out there for this. Somebody, somewhere must have done this before. Sounds like a job for javascript more than php, I'd say - if you're looking for an on-the-fly update.