How to calculate heating load? (DIY Homeowner)

It isn't rocket science. It is a bit tedious. Go to Slant/Fin's website, under Homeowner, and wander around a bit and you will find their heat loss calculator. It asks you a number of questions about each room in your house, but provides decent and usable suggestions for the less obvious values. It doesn't cover all the bases -- if you have a superinsulated house, for instance, it has some problems -- but it works and does all the necessary math for you.

You need that information for either forced air or hot water heat (you do not need it for steam -- boiler size for steam is calculated in a different way). The reason you need it is that the boiler must be sized to match the expected maximum load, but no more. This is one of those situations when enough is enough, and more isn't going to help and will cost you money.

As for definition -- I presume of "heat loss" -- that is the number of BTUs per hour (BTUh) that the house will lose to the great outdoors. Nothing fancier than that. And the number you want is the number of BTUs per hour on the coldest nastiest day you expect to get.

I've never tried the mobile phone app. But when I last used the version for full-fledged PCs, I thought running the program on a tiny screen might be challenging.