Formula used
The calculator estimates usable fuel energy, then divides by the adjusted burn rate. Adjustments account for heat setting, wind, and cold conditions.
- Effective burn rate (BTU/hr) = Rated BTU/hr × Heat setting × Wind factor × Cold factor
- Usable fuel energy (BTU) = Fuel amount × Energy density × Efficiency
- Runtime (hours) = Usable fuel energy ÷ Effective burn rate
- Fuel use per hour = (Effective burn rate ÷ Efficiency) ÷ Energy density
How to use this calculator
- Select your fuel type and enter the fire pit’s rated BTU/hr.
- Set a heat percentage that matches your usual flame setting.
- Choose wind and cold factors that match your outdoor conditions.
- Fill in the matching fuel details (tank, wood, ethanol, or gas).
- Press Calculate runtime to view results above the form.
- Download a CSV or PDF report for planning and recordkeeping.
Fuel energy and capacity inputs
This calculator converts your available fuel into usable heat energy, then compares it against your chosen output. For propane, the tank size and fill level define stored energy. For wood, weight and moisture-driven energy density define the heat budget. For ethanol, volume and blend strength set the energy pool. For natural gas, supply is often continuous, but an optional therm limit can model a capped session.
Adjusted burn rate factors
Rated BTU per hour is rarely the whole story outdoors. Heat setting scales burner output to match real use. Wind and cold factors approximate extra demand created by gusts, heat loss, and comfort tuning. Efficiency represents losses from open-air combustion and imperfect heat transfer. Together these inputs produce an effective burn rate that reflects typical backyard conditions.
Interpreting runtime output
Runtime is reported as hours and minutes . If you enter a therm limit for natural gas, the tool estimates how long that allowance supports your selected settings. If no limit is entered, the result shows a continuous supply message while reporting consumption per hour. Use the effective burn rate and fuel use figures to compare scenarios consistently.
Cost and consumption planning
Optional pricing fields estimate cost per hour using the calculated fuel consumption rate. This is useful when you compare fuels or decide whether to lower the heat setting for longer gatherings. For propane and wood, consumption is expressed in pounds per hour. For ethanol, it is gallons per hour. For natural gas, it is therms per hour, with an additional cubic‑feet estimate for quick utility checks.
Practical garden operation tips
Plan a safety buffer by targeting 80 to 90 percent of the displayed runtime, especially for breezy evenings. Shield the pit from wind, keep the burner ports clean, and pre-stage your refill or extra logs before guests arrive. When using wood, smaller splits ignite faster but can raise consumption; larger pieces extend runtime but may reduce peak heat. Re-check inputs whenever conditions change.
FAQs
1) Why does wind reduce my expected runtime?
Wind pushes heat away and often leads to higher flame settings. The wind factor increases the effective burn rate, so the same fuel supply is consumed faster in exposed areas.
2) What efficiency value should I use?
Use 70–85% for most open fire pits. Higher values suit well-designed burners with shields, while lower values fit very open bowls, tall flames, or poor heat capture.
3) Can I estimate runtime for mixed wood loads?
Yes. Enter the total weight and choose the closest wood type, or use a custom BTU value. Mixed moisture levels can swing results, so treat the runtime as planning guidance.
4) Why does natural gas sometimes show “continuous supply”?
Natural gas is typically not limited by a tank. If you leave the therm limit blank, the tool avoids a session runtime estimate, but still reports fuel use and hourly cost.
5) How accurate are the energy density defaults?
They are reasonable averages for planning. Real values vary by fuel purity, wood moisture, and product labeling. If you know your fuel specification, enter custom values where available.
6) What should I do if my results feel too high or low?
Check the rated BTU/hr, reduce the heat setting to match real use, and adjust wind or cold factors. For wood, verify the weight and select the correct moisture category.
Example data table
| Scenario | Fuel | Fuel amount | Rated BTU/hr | Setting | Wind | Efficiency | Estimated runtime |
|---|---|---|---|---|---|---|---|
| Patio evening | Propane | 20 lb at 100% | 50,000 | 80% | Calm | 85% | ≈ 7h 21m |
| Fire bowl | Ethanol | 3 L | 15,000 | 100% | Breezy | 80% | ≈ 0h 59m |
| Garden gathering | Wood | 10 lb seasoned hardwood | 35,000 | 90% | Windy | 75% | ≈ 1h 54m |
Safety note
Always follow manufacturer instructions, keep fuel containers away from flames, and use outdoor fire pits in well-ventilated areas. If conditions are very windy, postpone use for safety.