Any good online housing appraisal calculators?

In a nutshell, NO. While they display sales in your area, they are unaware of the specific property details and thus can't reasonably reconcile (adjust) these sales between themselves, much less to your home. It will give you a vague idea, but the spreads I've seen have been as high as 70%. This could inadvertantly whip you into a frenzy before you find out the actual, supportable value. Simply be sure, if you have any influence, to engage a competent appraiser.

Look at zillow.com. Best way to get into an MLS calculator and look at recent comp sales. tba.

Destiny's Euphoria's answer is so true, I second that.

No because it depends on recent sales in your neighborhood. Appraisers look at homes that are like yours in size, # of BD, baths, sq. ft., condition and compare them to yours. They add or subtract the differences between your homes and recent sales and give you a number.

It greatly depends on your area. You could go online, if your city is online and search for recent similiar sales and guestimate that way.