Baseboard Heater Length Calculator

What this calculation does not cover

• This sizes the heater, not the circuit that feeds it. It does not check breaker or conductor size, the continuous-load allowance electrical codes apply to fixed electric space heating, or whether an existing circuit has capacity left for the load. That is a separate branch-circuit calculation, and in most jurisdictions permitted and inspected work.
• The watts-per-foot figure is tied to the heater's nameplate voltage. Output on a fixed resistance element follows the square of the applied voltage, so a 240 V-rated unit on a 208 V supply gives roughly three-quarters of its rating and on 120 V about a quarter — read the nameplate voltage before the nameplate wattage. The model also assumes an electric resistance element; hydronic baseboard is rated in BTU/hr per foot at a stated supply water temperature and is not what this calculates.
• The target wattage is taken as given and never checked. Insulation, glazing area, ceiling height, air leakage, exposure and climate do not enter the arithmetic, so a wattage that came from a floor-area rule of thumb produces a length that is only as good as that rule of thumb.
• It returns one unit from the common short stock lengths and does not split a load across two or more heaters, balance them between walls, or look for the longer cabinets some manufacturer lines carry. Once the requirement passes what a single stock unit supplies, the layout becomes a design decision this does not make.
• It says nothing about whether the length fits. Door swings, stair carriages, window returns, mechanical chases and furniture all shorten the usable run, and the manufacturer's clearances to finished floor level, carpet, drapes and receptacles govern where the cabinet may go — a convector that cannot draw air across its fins does not deliver its rating regardless of length.