Calculator
Example data table
| Scenario | Radius (ft) | Temp rise (°F) | Wind | Patio | Recommended BTU/hr |
|---|---|---|---|---|---|
| Compact seating | 5.0 | 15 | Calm | Semi-covered | 14,137 |
| Typical gathering | 6.0 | 18 | Light breeze | Open | 28,474 |
| Windy larger circle | 8.0 | 20 | Windy | Open | 72,382 |
Formula used
- Area = π × radius²
- Recommended = Area × TempRise × WindFactor × PatioFactor × AltFactor
- WindFactor: calm 12, breeze 14, windy 18
- PatioFactor: open 1.10, semi 1.00, covered 0.95
- AltFactor: 1.00 (low), 1.05 (≥5000 ft), 1.10 (≥8000 ft)
- Gas: BTU/hr = Flow × HeatValue × Efficiency
- Propane: flow in gal/hr or lb/hr (converted internally)
- Natural gas: flow in ft³/hr or therm/hr
- Wood: BTU/hr = lb/hr × BTU/lb × MoistureFactor × Efficiency
- MoistureFactor reduces usable heat as moisture rises.
How to use this calculator
- Select the fuel type used by your fire pit.
- Enter your seating radius to set the warmed area.
- Set your desired temperature rise above ambient air.
- Choose wind exposure, patio type, and your altitude.
- Optionally enter a fuel consumption rate for output estimates.
- Press Calculate to view results above the form.
- Use CSV or PDF downloads to save your plan.
Comfort output relates to seating distance
Heat needs rise with seating radius because warmed area grows with π×r². Doubling radius from 5 ft to 10 ft increases area fourfold, so comfort BTU climbs fast. For compact circles under 35,000 BTU/hr, people typically sit within 4–6 ft of the flame. For mixed groups at 6–8 ft, 35,000–59,999 BTU/hr often fits, assuming moderate shelter and efficient radiant output in practice.
Wind and shielding shift heat demand
Outdoor air movement is the biggest variable. This calculator applies wind factors of 12 (calm), 14 (breeze), and 18 (windy) to reflect convective losses. Patio shielding also matters: open patios use a 1.10 factor, semi-covered uses 1.00, and covered uses 0.95. If your space is windy and open, consider wind screens, a tighter seating ring, or stepping up one size band.
Efficiency and combustion drive usable warmth
Efficiency converts theoretical fuel heat into usable warmth. The model lets you choose 30–95%, with practical planning usually in the 60–80% range. Higher efficiency can come from well-designed burners, media that increases surface radiation, and controlled flame height. For wood, moisture reduces usable heat; the calculator scales output with a moisture factor that typically ranges from about 0.40 to 0.97.
Fuel planning ties BTU to consumption
Fuel inputs help estimate actual output and operating time. Propane commonly uses about 91,500 BTU per gallon and roughly 4.24 lb per gallon, so the same setup can be expressed as gal/hr or lb/hr. Natural gas often averages near 1,037 BTU per cubic foot, and 1 therm equals 100,000 BTU. Enter your current flow to compare it against the comfort recommendation.
Altitude and safety checks finalize selection
Use results as a sizing guide, then validate with equipment limits and clearance rules. Higher altitudes can affect combustion, so this calculator increases demand by 5% at 5,000 ft and 10% at 8,000 ft. If your fuel-based output is far below the recommendation, expect cooler comfort. If it is far above, reduce flow to improve safety, economy, and control.
FAQs
What BTU range is common for a backyard fire pit?
Many patio burners fall between 35,000 and 65,000 BTU/hr. Smaller setups can feel fine near 25,000 BTU/hr when sheltered. Windy, open seating circles often need 60,000+ BTU/hr to maintain comfort.
Why does seating radius change the recommendation so much?
The warmed area scales with radius squared. Increasing radius from 6 ft to 8 ft boosts area about 78%, so the comfort target rises similarly, especially when wind is present.
Should I size to the recommended BTU or my fuel-based BTU?
Use the recommendation as the comfort target, then compare your fuel-based output to see if your flow rate can meet it. If output is lower, tighten seating or increase flow within safe limits.
How accurate are propane and natural gas heat values?
They are typical planning values. Propane is often near 91,500 BTU per gallon. Natural gas varies by region, commonly around 1,000–1,100 BTU per cubic foot. Your utility or manufacturer specs are best.
How does wood moisture affect heat?
Wet wood spends energy boiling water, reducing usable warmth and increasing smoke. The calculator applies a moisture factor to reduce BTU/hr as moisture rises, so seasoned wood usually provides steadier comfort.
What safety steps should I follow after sizing BTU?
Confirm your unit’s rated output, use approved regulators and hoses, and keep required clearances from combustibles. Avoid enclosed spaces without ventilation. When in doubt, follow local codes and the manufacturer’s installation guide.