Just did a small 'in-laws addition and likely went a bit heavy on the feeds.Is there a simple sq. ft./amp.Btw,per the builders layout.thanks.

My formula was, I guess still is even though I don’t do much residential anymore as follows:

Square footage of room times 8 for new construction or a tight house.

Square footage of room times 10 for an older / drafty house

It seemed to work fine I would fudge it depending on the room as well maybe a little more for the bathroom a little less for the bedroom.

Oh yeah forgot one important thing that’s the wattage!!

That's a good method, also you can go by the cubic foot, which makes for ceiling heights, some ceilings are pretty high. 2 watts per cu.ft.

In years past we installed 10 watts per sq ft. With new energy requirements (attic insullation, double pane windows, etc, allow us to install only 3 watts per sq ft. I've never known an inspector to check the wattage of installed heaters. When in doubt I install on the high side of 3W/sq.ft. I believe the inspectors allow a maximum of 3,ooo watts installed on a 20 amp 240 volt circuit.Here in Seattle it does not get below 30 degrees too often. bob