Forge Calculator – Chamber Sizing, Burner Output & Fuel Estimator | Free Tool

  • Adam
  • January 3, 2026

Professional forge calculator for blacksmiths and makers. Estimate chamber volume, burner BTU requirements, propane consumption, heat-up time, and heat-treat temperatures. Free, fast, mobile-friendly tool with export options.

Forge Calculator

Plan chamber size, airflow, burner output, and fuel needs for common forge setups

Educational Tool: This calculator provides estimates based on common rules-of-thumb. Forge performance depends heavily on design, insulation quality, airflow, fuel quality, and ambient conditions. All outputs are estimates for planning purposes only.

Forge Calculator

Configure your forge parameters below

Chamber Sizing

Account for actual workpiece volume

Real forges lose heat through openings and exhaust. Tune these sliders to match your setup.

Applied to required output (default 10%)

Estimated Forge Requirements

Build smarter, forge safer

Instant estimates for chamber + burner
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⚠ Safety: High heat + fire + carbon monoxide risk. Use ventilation, PPE, and follow your local safety guidance.
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Presets

Save and load your forge configurations (stored locally when "Remember my inputs" is enabled)

Detailed Breakdown

ParameterValueNotes
Click Calculate to see detailed breakdown

How It Works

This forge calculator uses established blacksmithing and thermal engineering rules-of-thumb to estimate forge requirements. The calculations are based on simplified models appropriate for hobby and small-scale forging operations.

Chamber Volume Calculation

For cylindrical forges, the internal volume is calculated using:

\( V = \pi r^2 L \)

Where r is the internal radius and L is the chamber length. For rectangular chambers, volume is simply length × width × height.

Burner Sizing (BTU Requirements)

The commonly cited rule-of-thumb for gas forge burner sizing is 300-450 BTU per cubic inch of chamber volume to reach welding heat (approximately 1,300°C / 2,370°F). This calculator uses an adjustable intensity factor:

\( Q_{req} = V \cdot k \cdot \Delta T \)

Where k is an intensity coefficient (adjusted for target temperature and forge type), and ΔT is the temperature differential from ambient to target.

Fuel Consumption

Fuel flow rate is estimated using:

\( FuelRate = \frac{Q}{EnergyDensity \times Efficiency} \)

Where Q is the heat input requirement, and efficiency accounts for combustion losses, heat losses through openings, and exhaust.

Heat-Up Time

The energy required to heat the forge lining from ambient to operating temperature is:

\( Energy = m \cdot c \cdot \Delta T \)

Where m is mass, c is specific heat capacity of the lining material, and ΔT is temperature rise. Heat-up time is then estimated by dividing total energy by available burner power, adjusted for heat losses.

Example: A cylindrical propane forge with 150mm diameter × 300mm length chamber has a volume of approximately 5.3 liters (324 cubic inches). To reach welding heat (~1,300°C), it would require approximately 145,000 BTU/hr (42.5 kW) with a 20% safety margin and typical heat loss factors.

Safety Notes

Critical Safety Information:

  • Carbon Monoxide (CO): Gas forges produce deadly CO. Always operate in well-ventilated areas with cross-ventilation. Install CO detectors.
  • Fire Risk: Forges reach temperatures exceeding 1,200°C (2,200°F). Keep flammables away, have fire extinguisher accessible.
  • Personal Protective Equipment (PPE): Wear safety glasses, heat-resistant gloves, natural fiber clothing (cotton/wool), closed-toe leather boots, and hearing protection.
  • Burn Prevention: Forge bodies, workpieces, and exhaust remain dangerously hot long after shutdown. Never touch without verification.
  • Pressure Equipment: Gas regulators and lines must be rated for forge use. Check for leaks regularly with soapy water test.
  • Structural Safety: Ensure forge stand is stable and non-combustible. Maintain clearances per local fire codes.

For comprehensive safety guidance, consult:

Frequently Asked Questions

How do I size a burner for my forge chamber?
Use the rule-of-thumb of 300-450 BTU per cubic inch for welding heat. For general forging (not welding), 200-300 BTU/in³ is often sufficient. Select the Chamber Sizing module, enter your dimensions, and the calculator will estimate required burner output.
How much propane does a gas forge use per hour?
A typical small hobby forge (5-10 liter chamber) running at full fire consumes approximately 2-4 liters (0.5-1 gallon) of propane per hour. Use the Fuel Estimate module with your burner output and efficiency settings for a personalized estimate.
How do I calculate forge chamber volume?
For cylindrical forges: V = π × radius² × length. For rectangular: V = length × width × height. Use internal dimensions (after insulation). The calculator handles conversions between metric and imperial units automatically.
What does BTU per cubic inch mean for forge sizing?
BTU (British Thermal Unit) per cubic inch is a measure of heat energy density. Higher values mean more intense heat. Welding heat requires ~400 BTU/in³, general forging ~250 BTU/in³, and heat treating ~150 BTU/in³.
How much airflow does my forced-air burner need?
Forced-air burners typically require 15:1 air-to-fuel ratio by mass (stoichiometric for propane). This translates to approximately 1 CFM of air per 1,000 BTU/hr output. Use the Burner/Airflow module to estimate requirements based on your chamber size.
How is forge heat loss calculated?
Heat loss occurs through radiation from openings, conduction through walls, and exhaust gases. This calculator applies multipliers (1.2-2.0×) to account for typical losses. Well-insulated forges with small openings have lower loss factors.
What insulation thickness do I need?
For ceramic wool blanket: 50-75mm (2-3 inches) minimum. For castable refractory: 40-60mm. For firebrick: 115mm (4.5 inches, standard brick). Thicker insulation improves efficiency but reduces internal volume.
Why include a safety margin in burner sizing?
Safety margins (typically 10-20%) account for: sub-optimal fuel quality, aging burners, partial blockages, high ambient temperatures, and worn insulation. It ensures your forge can still reach target temperature under less-than-ideal conditions.
How long does it take a forge to heat up?
Small gas forges typically heat to forging temperature in 10-20 minutes. Larger forges with thick refractory may take 30-60 minutes. Use the Heat-Up Time module to estimate based on your lining mass and burner output.
Can I export my forge calculations?
Yes! Click "Show detailed breakdown" below the results, then use "Export as CSV" to download all inputs, calculations, and outputs. You can also use "Print-Friendly Version" to create a PDF via your browser's print function.

About This Calculator

The Forge Calculator is developed and maintained by OmniCalculator.Space, a platform dedicated to providing high-quality, free engineering and technical calculators for makers, students, and professionals.

This tool synthesizes established blacksmithing practices, thermodynamic principles, and real-world forge building experience into an accessible planning resource. It's designed for:

  • Hobby blacksmiths planning their first gas forge build
  • Students learning about combustion and heat transfer
  • Makers estimating fuel costs and session planning
  • Knife makers sizing forges for heat treatment

Disclaimer

Educational Use Only: This calculator provides estimates based on simplified models and common rules-of-thumb. Actual forge performance varies significantly based on design details, build quality, insulation effectiveness, burner tuning, fuel quality, altitude, and ambient conditions. These results are NOT professional engineering advice.

Verification Required: Always cross-reference critical specifications with authoritative sources, manufacturer datasheets, and experienced builders. For commercial or production applications, consult a qualified engineer.

Safety Responsibility: Users are solely responsible for safe forge design, construction, operation, and compliance with local codes and regulations. Improper forge use can result in serious injury or death.

References & Further Reading

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