What exactly is one boiler horsepower?

One boiler horsepower is the energy needed to evaporate 34.5 pounds of water per hour at 212 °F into dry saturated steam at atmospheric pressure. That equals 33,475 BTU per hour, or about 9.81 kW of gross thermal output.

Why does boiler horsepower differ from mechanical horsepower?

Mechanical horsepower (746 W) measures shaft work, while boiler horsepower measures heat output and is roughly 13 times larger at 9.81 kW. The two share a name for historical reasons but are not interchangeable units.

What does the efficiency input do?

Gross output is the heat delivered to the steam. Fuel input is always larger because combustion is not perfect. The tool divides output by your efficiency to estimate the fuel input rate, for example 82 percent efficiency raises a 100 BHP output to roughly 4.08 million BTU/hr of fuel.

What does from-and-at 212 °F mean?

It is a standard reference condition: feedwater already at 212 °F being turned to steam at 212 °F. Real boilers with cooler feedwater and higher pressures produce somewhat less steam per BHP, so treat the steam figure as a nominal rating.

Can I size a boiler with this tool?

Use it to translate between rating units and to estimate fuel demand, but final sizing must account for actual feedwater temperature, operating pressure, blowdown, and load diversity. Treat the result as a first-pass nameplate check.

Boiler Horsepower & Steam Output Calculator

Boiler ratings appear in several different units depending on the manufacturer and region. This calculator converts between boiler horsepower, BTU per hour, kilowatts, and pounds of steam per hour, and applies an efficiency factor so you can also see the fuel input the boiler must consume.

How it works

The fixed definition of a boiler horsepower drives every conversion:

1 BHP = 34.5 lb/hr steam × 970.3 BTU/lb = 33,475 BTU/hr
      = 9.81 kW gross thermal output

From a known output the tool scales linearly to each unit. To find the fuel the burner must supply, it divides the gross output by your efficiency:

fuel input = gross output / efficiency

So an 82 percent efficient boiler delivering 100 BHP of steam must burn roughly 33,475 × 100 / 0.82 ≈ 4.08 million BTU/hr of fuel.

Example and notes

A 100 BHP boiler outputs about 3.35 million BTU/hr, 981 kW, and 3,450 lb/hr of steam from-and-at 212 °F. Remember the steam figure is a nominal rating: with cooler feedwater or higher operating pressure the real evaporation rate falls, so always confirm against the manufacturer’s steam tables for your actual feedwater temperature and pressure before committing to a final selection.