A hydronic baseboard heater is a water or steam-based radiator for keeping your home at a comfortable temperature, located unobtrusively at the foot of your wall. The appliance's heating power is measured in BTUs per hour, per foot of heater width. Calculating the BTUs for a hydronic baseboard heater is a simple matter of determining the room's heat needs and then finding how long a heater is necessary to provide it.
Measure the width and length of the room in question and multiply the two numbers to obtain the square footage.
Find how many BTUs per square foot are necessary for your home, as determined by where you live. In New England and the upper Midwest, 40 BTUs are needed for each square foot every hour; this requirement drops to 35 south of Philadelphia --- and only 25 per square foot along the upper West Coast.
Multiply your region's BTUs-per-hour-per-square foot requirement by the area of your floor to find the BTU requirement of the entire room.
Divide this number by the width of your intended baseboard heater in feet. The result is an estimate of the BTU-per-foot rating you should look for when making the purchase.