Boiler Horsepower Calculator

Boiler horsepower (BHP) is a steam-industry unit that rates a boiler's capacity to deliver steam — it isn't the same as mechanical horsepower. This calculator converts BHP into BTU per hour, kilowatts, and pounds of steam per hour.

Boiler HP Conversion Formulas

The American Society of Mechanical Engineers set the definition in 1876. One boiler horsepower is the power needed to evaporate 34.5 pounds of water per hour "from and at 212 °F" — water already at boiling point, turning entirely to steam. Multiplying 34.5 lb/hr by the latent heat of vaporization (about 970.3 BTU/lb) gives the familiar 33,475 BTU/hr figure.

How to Use This Calculator

1. Enter the boiler horsepower from the boiler's nameplate or spec sheet.
2. Read the heat output in BTU/hr and kW, and the steam rate in lb/hr.

Boiler HP vs. Mechanical HP — Not the Same Unit

Mechanical horsepower (746 W) measures work done by an engine or motor. Boiler horsepower (9,810 W) measures thermal energy delivered as steam — one BHP is about 13.15 times larger than one mechanical HP. They share a name for historical reasons only: early boilers were sized to feed steam engines of a given mechanical horsepower, with generous margin. Never substitute one for the other when sizing equipment.

Boiler Horsepower Conversion Table

Worked Example

Where 33,475 Actually Comes From

The number looks arbitrary until you see what it was built from. One boiler horsepower was defined as the ability to evaporate 34.5 pounds of water into steam per hour, at atmospheric pressure, starting from water already at 212 °F. Turning a pound of 212 °F water into a pound of 212 °F steam takes 970.3 BTU — the latent heat of vaporisation. Multiply the two:

The historical link explains the name. Early boilers were sold to run steam engines, so rating a boiler by how many engine horsepower it could feed made commercial sense. That relationship stopped being useful once steam moved into heating and process work rather than driving engines. The unit survived anyway. It's why a boiler horsepower is now roughly thirteen times a mechanical horsepower and shares nothing with it but a name. The what is boiler horsepower guide covers that history in more depth; the sections below deal with what the rating means operationally.

What "From and At 212 °F" Means for Your Boiler

Every boiler horsepower figure carries the qualifier "from and at 212 °F", and almost no real boiler operates that way. Feedwater usually arrives well below boiling, and steam is usually produced above atmospheric pressure. Both differences mean your boiler delivers less actual steam than its rating suggests.

The heat has to cover two jobs rather than one: raising the feedwater to saturation temperature first, then evaporating it. Feedwater at 180 °F needs about 32 BTU per pound of sensible heat before boiling even begins, and producing steam at 100 PSIG requires more total heat per pound than at atmospheric pressure. Combine the two and a nominal 100 BHP boiler delivers closer to 3,200 pounds of steam per hour rather than the 3,450 the rating implies.

Rated Output Is Not Fuel Input

The 33,475 BTU/hr figure describes energy delivered to the water. The fuel you've to buy is a larger number, because no boiler converts fuel to steam perfectly. Combustion efficiency for a modern firetube boiler typically runs 80–85%, and older units with poor burner control sit lower.

Sizing a gas supply, a burner or a flue from the output figure rather than the input figure is a common and expensive mistake. Use the input column — we size gas supply, burners and flues from it, never from output. The five-percentage-point difference between the last two columns is also worth noting on its own: on a 500 BHP boiler running continuously, it represents well over a million BTU per hour of fuel.

Rules of Thumb for Sizing and Support Systems

Boiler horsepower connects to several other quantities a plant engineer needs, and each has a working relationship worth keeping to hand:

The feedwater figure is the one people forget until commissioning. A 100 BHP boiler wants roughly 6.9 gallons per minute of treated feedwater at full fire. The pump, softener and chemical dosing all have to keep up with that continuously, plus blowdown losses on top.

The Horsepower Units That Get Confused With This One

Boiler horsepower is one of several units sharing the word, and mixing them up produces errors of an order of magnitude. These are the ones that actually appear on equipment:

The gap isn't subtle: a 100 boiler horsepower unit is nearly 981 kW, while a 100 mechanical horsepower engine is about 74.6 kW. Anyone who treats the two as interchangeable will be out by a factor of thirteen. For the engine-side units, the HP to PS converter handles the mechanical-to-metric conversion, and the HP to BTU calculator covers thermal output for mechanical horsepower.