I have a 1800 sqft ranch considering replaceing boiler there is no tag with the BTU rating on it. I don't know what size to get

Do a manual J for the amount of heat loss then calculate your boiler size from there. It is the ONLY way to be sure of your sizing requirements.

You could also solicit 3 bids from contractors and see what they spec. You would also then have an idea if you still want to try this on your own or to take one of those bids.

Around 85,000, but you must also go by the length of baseboard you have in you home. Or do you have cast iron radiators?

"around 85,000," 85,000 what?

BTU, 1800 sf home will go somewhere between 78,000 and 84,000 but depending on the amount of windows and the what area you are in.

I do not trust guesstaments. They cost too much in wasted fuel from improper sizing.

Might as well just use this chart. Then convert the BTU requirements from the A/C chart.

Nice chart. Bet you could actually get someone to use this. Lol

Trust me some people in the HVAC biz use something like it every day on sales calls. If the public only new how much extra utility money they spend over the lifetime of the equipment due to improper sizing there would be civil insurection. LOL

Now more than ever before due to the cost of utilitys and the Government involvement with possible additional taxes on all forms of energy accuracy in calculations will take on a new meaning.