What Does This Boiler Horsepower Calculator Do?
Boiler horsepower is an old but still common way to rate boiler size in US, Canada and many Asian markets. One boiler horsepower does not mean shaft power. It means the boiler can evaporate 34.5 lb of water per hour from and at 212°F. That is 15.65 kg per hour in metric.
This calculator helps you convert between BHP and real system values. If you know actual steam flow, pressure and feedwater temperature, it will estimate BHP for your system. If you know BHP from nameplate, it will estimate how much steam you will get at your feedwater condition and what kW or BTU/h that equals. You can also configure the calculation to use the classic from and at standard for quick comparison.
For full feedwater system sizing after you know BHP, check our boiler feed pump calculation system to configure pump head, NPSH and flow for that BHP rating.
Boiler Horsepower Formula
All BHP formulas come from one definition:
1 BHP = 34.5 lb/h from and at 212°F = 33,475 BTU/h = 9.81 kW = 15.65 kg/h
BHP = Steam Flow (lb/h) ÷ 34.5 = Steam Flow (kg/h) ÷ 15.65
BHP = Steam Flow × (h_g − h_feed) ÷ 33,475 (BTU/h basis)
BHP = Steam Flow (kg/h) × (h_g − h_feed) (kJ/kg) ÷ 35,316
Steam Flow = BHP × 33,475 ÷ (h_g − h_feed)
kW = BHP × 9.81 | BTU/h = BHP × 33,475
h_g is saturated steam enthalpy at boiler pressure, h_feed is feedwater enthalpy. If you check from and at box, we use h_fg = 970.3 BTU/lb = 2257 kJ/kg and ignore actual pressure.
How to Use the Calculator
- Select mode. Steam Flow to BHP if you measured flow and want rating. BHP to Steam Flow if you have nameplate and want expected flow at your site.
- Enter flow or BHP. Use kg/h or lb/h based on unit switch.
- Enter boiler pressure and feedwater temperature. Needed for actual enthalpy. If you want quick from and at estimate, check the box.
- Click Calculate BHP. No result shows until you click. Reset clears and hides results.
Understanding the Results
Boiler Horsepower BHP
Main rating number. A 100 BHP boiler is about 981 kW or 3.34 MMBTU/h heat output. In US many rental boilers are sold by BHP.
From and At Evaporation
This is the classic rating: how much steam the boiler would make if feed was 212°F and pressure atmospheric. Use it to compare boilers from different makers on same basis.
Actual Steam Flow
At your feedwater temp and pressure, you get less steam than from and at because you need more heat per kg. This calculator shows both so you can estimate real capacity.
Heat Output kW and BTU/h
Use kW for electric or metric projects, BTU/h for US. 1 BHP = 9.81 kW. If you need burner size, divide by boiler efficiency.
Worked Example
You measure 1500 kg/h steam at 10 bar g, feedwater 85°C.
| Step | Value | Note |
|---|---|---|
| Saturated hg at 10 bar g | 2778 kJ/kg | From steam tables |
| Feed enthalpy | 85 × 4.186 = 356 kJ/kg | |
| Delta h | 2422 kJ/kg | |
| BHP actual | 1500 × 2422 ÷ 35316 | 102.9 BHP |
| From and at flow | 102.9 × 15.65 | 1610 kg/h from and at |
| Heat | 102.9 × 9.81 | 1009 kW = 3.44 MMBTU/h |
If you used from and at only, BHP would be 1500 ÷ 15.65 = 95.8 BHP. Actual is higher because feed is colder than 212°F and pressure raises enthalpy.
What Values Should You Enter?
Steam Flow
Average flow, not peak. If you have superheater, use flow after superheater. Higher flow means higher BHP rate.
Boiler Pressure
Operating pressure at drum. Sets hg. At 10 bar hg is about 2778 kJ/kg, at 20 bar it is 2799 kJ/kg then slowly drops.
Feedwater Temperature
Colder feed needs more heat per kg, so BHP for same steam flow goes up. Hot feed from economizer lowers BHP needed.
BHP Input
When you know BHP, enter nameplate BHP. Calculator will estimate steam flow at your site conditions, not just from and at.
From and At Checkbox
Check for quick comparison between boilers. Uncheck for accurate site estimate that includes feed temp and pressure.
Unit System
Metric uses kg/h and kW, US uses lb/h and BTU/h. BHP value itself is same in both.
Boiler Horsepower vs Real Capacity
Rating vs Actual
BHP is fixed rating, actual steam changes with feedwater and pressure. A 100 BHP boiler makes 3450 lb/h from and at, but about 3000 lb/h at 85°C feed and 10 bar.
Why BHP Still Used
US rental and small firetube market uses BHP for quick sizing. 1 BHP roughly needs 1 sq ft of heating surface historically, now just a capacity label.
kW and BTU/h
For burner and fuel sizing, convert BHP to kW or BTU/h. Divide by boiler efficiency to get fuel input rate.
Evaporation Rate
From and at evaporation rate is constant 34.5 lb/h per BHP. Actual evaporation rate is lower when feed is cold.
Feed System Impact
Higher BHP needs higher feedwater flow. Configure pump flow as steam flow plus blowdown. Check NPSH with hotter feed.
Efficiency Link
BHP to fuel: Fuel kW = BHP × 9.81 ÷ efficiency. At 80% efficiency, 100 BHP needs about 1226 kW fuel.
Common Mistakes
- Mixing from and at with actual: If you divide actual flow by 34.5 without correcting for feed temp, BHP will be low.
- Using lb/h vs kg/h factor wrong: 34.5 is lb/h, 15.65 is kg/h. Mixing gives 2.2x error.
- Forgetting feed enthalpy: Some use hg only, not hg minus hf feed. That overstates BHP.
- Superheated steam: BHP is defined for saturated. For superheated, use hg superheated, not saturated, or rate will be low.
- BHP is not motor HP: 1 BHP boiler is about 13.1 motor HP equivalent heat, not shaft power.
Frequently Asked Questions
What is 1 boiler horsepower in kW?
1 BHP = 9.81 kW heat output, which is 33,475 BTU/h. It equals 34.5 lb/h evaporation from and at 212°F.
How to convert BHP to kg/h steam?
From and at: kg/h = BHP × 15.65. At actual conditions: kg/h = BHP × 35,316 ÷ (h_g - h_feed).
How to estimate BHP from steam flow?
BHP = steam flow (kg/h) × (h_g - h_feed) ÷ 35,316. For quick from and at, BHP = flow kg/h ÷ 15.65 or lb/h ÷ 34.5.
Is BHP same as HP?
No. Boiler HP is heat rating. Motor HP is shaft power. 1 BHP is about 13 times larger in heat than 1 motor HP.
Why does actual steam per BHP vary?
Because from and at assumes 212°F feed and atmospheric steam. Colder feed or higher pressure needs more heat per kg, so you get less kg/h per BHP.
Can I use this for superheated boilers?
Yes, enter superheated hg if you know it. This calculator uses saturated hg at your pressure for simplicity. For accurate superheated BHP, use superheated enthalpy.
Related Boiler Calculators
After you rate BHP, you may need these for your system:
Basis: 1 BHP = 33,475 BTU/h = 9.8105 kW = 34.5 lb/h from and at 212°F. Steam tables 1-100 bar interpolated for hg and hf. Feedwater enthalpy approximated as 4.186 kJ/kgK. For acceptance use certified steam tables.