Propane can power heaters, furnaces, generators, grills, camp stoves, water heaters, fireplaces, and other equipment during a blackout, but the supply is limited by the amount stored in your cylinder or tank. This free propane runtime calculator estimates how long that fuel may last based on container size, current fuel level, appliance input rating, and daily operating schedule.
Select a common propane cylinder or tank, or enter a custom amount. Then add the appliances you expect to operate and adjust their BTU-per-hour ratings and daily use. The results show estimated runtime, usable fuel, daily propane consumption, and the amount required for outages lasting up to 14 days.
Quick Answer
Divide the usable energy in your propane supply by the appliance’s input rating to estimate continuous runtime. One gallon of propane contains approximately 91,452 BTU, while a full 20-pound cylinder holds roughly 4.7 gallons. A 20,000-BTU heater would therefore run for approximately 21 hours on a full 20-pound cylinder under ideal continuous-use conditions before adding a planning reserve.
How Long Will My Propane Last?
Estimate propane runtime for heaters, furnaces, generators, cooking equipment, water heaters, and other propane appliances.
Your Estimated Propane Runtime
Propane Needed for an Extended Outage
| Duration | Propane Gallons | Propane Pounds | 20 lb Cylinders |
|---|
Appliance Use Breakdown
| Appliance | Qty. | Input Rating | Hours/Day | Propane/Day |
|---|
How this estimate was calculated
The calculator uses 91,452 BTU per gallon of propane and approximately 4.2 pounds per gallon. Appliance input BTU per hour is multiplied by quantity and daily operating time. Available propane is reduced by the selected reserve before runtime is calculated.
A thermostat-controlled furnace or heater may not fire continuously during every hour selected. Generator fuel use also changes with electrical load, so manufacturer consumption data at the expected load provides a better estimate than a general BTU preset.
How to Use the Propane Runtime Calculator
Begin by identifying the propane supply connected to the equipment. Portable cylinders are normally described by the pounds of propane they hold, while larger residential tanks are described by nominal gallon capacity. The calculator handles both types.
- Select the cylinder or tank size. Choose a common 20-, 30-, 40-, or 100-pound cylinder; a 100- through 1,000-gallon tank; or a custom amount.
- Enter the current fuel level. Use 100% for a full pound-rated cylinder. For a stationary tank, use the percentage shown on its gauge.
- Choose a fuel reserve. The default 10% reserve prevents the plan from depending on every last bit of fuel.
- Select an appliance. Choose a general preset or add a custom propane appliance.
- Verify the input rating. Replace the preset with the BTU-per-hour input shown on your actual appliance label, manual, or specification sheet.
- Enter quantity and daily hours. Include how many similar appliances will operate and how many hours they may run each day.
- Add additional equipment. Include other appliances drawing from the same propane supply.
- Calculate the result. Review runtime, daily fuel use, and the extended-outage table.
Planning Tip
Run more than one scenario. Compare continuous operation with a conservative schedule that uses heaters, generators, and cooking appliances only when needed. The difference shows how much propane can be saved through load management.
Understanding Your Propane Runtime Results
The calculator separates the available supply from the expected daily demand. This prevents a continuous-runtime estimate from being mistaken for the number of calendar days the fuel will last under an intermittent schedule.
| Calculator Result | What It Means |
|---|---|
| Runtime on your schedule | Estimated calendar days the usable supply may support the entered daily operating schedule. |
| Continuous runtime | Estimated operating hours if every selected appliance runs continuously at its complete input rating. |
| Usable propane | The calculated fuel remaining after the current fill level and selected reserve are applied. |
| Daily propane use | Estimated gallons and pounds consumed during one day of the entered schedule. |
| Extended-outage fuel | Propane required for 1, 3, 7, and 14 days, including an equivalent number of 20-pound cylinders. |
How Propane Runtime Is Calculated
The calculator converts the available propane supply into energy, measured in British thermal units. It then compares that energy with the combined daily input of the selected appliances.
The Basic Formulas
Available energy = usable propane gallons × 91,452 BTU
Daily appliance demand = BTU per hour × quantity × hours per day
Runtime in days = available energy ÷ total daily demand
Continuous hours = available energy ÷ combined hourly input
BTU input ratings describe how much fuel energy an appliance consumes. They do not necessarily equal the amount of useful heat delivered to the room or water. Appliance efficiency determines how much of that fuel energy becomes useful output.
Propane Pounds and Gallons Are Different
Portable cylinders are commonly identified by propane weight. A standard grill cylinder called a “20-pound tank” is designed to hold approximately 20 pounds of propane when properly filled. It does not hold 20 gallons.
Liquid propane weighs approximately 4.2 pounds per gallon, although density changes somewhat with temperature and fuel composition. Using that conversion, a 20-pound cylinder contains roughly 4.7 gallons of propane.
| Cylinder Rating | Approximate Propane Gallons | Approximate Energy |
|---|---|---|
| 20 lb | 4.7 gallons | About 435,000 BTU |
| 30 lb | 7.1 gallons | About 653,000 BTU |
| 40 lb | 9.5 gallons | About 871,000 BTU |
| 100 lb | 23.8 gallons | About 2.18 million BTU |
These are approximate energy conversions, not instructions for filling a cylinder. Cylinders and tanks must be inspected and filled by qualified propane professionals according to their markings and applicable requirements.
Understanding Large Propane Tank Percentages
Stationary propane tanks are normally identified by nominal gallon capacity. A propane delivery does not fill the container completely with liquid because expansion space is required. A tank gauge commonly reads around 80% after a normal fill.
For calculator purposes, enter the percentage shown on the tank gauge. For example, a 500-gallon tank reading 60% contains approximately 300 gallons before applying the calculator’s optional planning reserve.
Gauge readings are approximate. Tank shape, temperature, gauge condition, and the position of the float can affect the displayed percentage. Do not plan critical heating or medical needs down to the exact calculated minimum.
Example: How Long Will a 20-Pound Propane Tank Last?
Suppose a 20-pound cylinder supplies a 9,000-BTU portable heater for eight hours per day. The cylinder contains approximately 4.7 gallons when full. After keeping a 10% planning reserve, the calculator treats about 4.3 gallons as usable.
The heater consumes approximately 72,000 BTU per day:
9,000 BTU per hour × 8 hours = 72,000 BTU per day
Estimated runtime with a 10% reserve: approximately 5.4 days
Actual heater runtime may differ because many heaters cycle rather than burn continuously at the selected setting. Cold weather can also reduce the rate at which a small cylinder vaporizes propane, especially when demand is high.
Why Actual Propane Runtime Can Be Different
Thermostat Cycling
A furnace or thermostatically controlled heater may not burn continuously during every scheduled hour. A well-insulated room may reach its temperature setting quickly, while a drafty home in severe cold may keep the burner operating for much longer.
Generator Electrical Load
Propane generators use different amounts of fuel at light, half, and full electrical load. A generator running only a refrigerator and a few lights may consume less than the same machine powering pumps, heating equipment, and kitchen appliances. Use the manufacturer’s propane-consumption chart at the expected load whenever available.
Temperature and Vaporization
Liquid propane must vaporize before most appliances can use it. Cold temperatures reduce vaporization, and a small cylinder may be unable to supply a high-demand appliance even though liquid fuel remains inside. Frost on a cylinder can indicate that propane is vaporizing rapidly and cooling the container.
Appliance Condition
Dirty burners, improper air adjustment, damaged regulators, blocked vents, incorrect pressure, and neglected maintenance can change performance or create a dangerous condition. A calculator cannot determine whether an appliance is operating safely.
Fuel Composition and Measurement
Propane energy content and density vary slightly. Tank gauges and appliance ratings also have limits. The calculator uses standard planning values, so its result should not be treated as an exact shutdown time.
Estimating Propane for Emergency Heating
Heating can consume propane much faster than cooking or occasional hot-water use. A 30,000-BTU heater operating continuously for 24 hours would consume approximately 7.9 gallons per day under the calculator’s standard conversion. Reducing operation to eight hours per day lowers the estimate to about 2.6 gallons.
Do not assume that operating an indoor heater around the clock is safe or appropriate. Use only equipment approved for the intended location, follow clearance and ventilation requirements, maintain working carbon-monoxide alarms, and never use outdoor cooking equipment as an indoor heater.
If propane is part of a wider cold-weather plan, compare it with the Firewood Calculator and review the guide to indoor-safe heating options during a blackout.
Estimating Propane for a Generator
A generator’s electrical wattage cannot be converted into propane use with perfect accuracy because engine efficiency changes with load, speed, design, temperature, altitude, and condition. The best source is the fuel-consumption table for the exact generator model.
If the manufacturer provides propane consumption in gallons or pounds per hour instead of BTU per hour, convert that rate before entering it:
- Gallons per hour × 91,452 = approximate BTU per hour
- Pounds per hour × 21,548 = approximate BTU per hour
Once you know the expected electrical load, use the Generator Runtime and Fuel Calculator for a generator-specific comparison. Use the Generator Size Calculator first if you have not determined the running and startup capacity required for your appliances.
Propane Safety Rules
Carbon Monoxide and Fire Warning
Never operate a propane generator, grill, camp stove, patio heater, or other outdoor-only appliance inside a home, garage, basement, crawlspace, shed, tent, porch, or carport. Carbon monoxide can accumulate quickly even when doors or windows are open.
- Follow the appliance and cylinder manufacturers’ instructions.
- Keep cylinders upright and protected from excessive heat and physical damage.
- Do not store or use cylinders where prohibited.
- Never modify valves, regulators, connectors, or safety devices.
- Check connections using an approved leak-detection method.
- Do not use a flame to search for leaks.
- If you smell propane, avoid flames, sparks, switches, and ignition sources; leave the area and contact the propane supplier or emergency services from a safe location.
- Maintain working smoke and carbon-monoxide alarms.
- Have damaged, rusty, leaking, or questionable equipment inspected before use.
Common Propane-Planning Mistakes
- Confusing pounds with gallons: A 20-pound cylinder contains roughly 4.7 gallons, not 20 gallons.
- Using appliance output instead of input: Fuel consumption should be calculated from the input rating.
- Assuming continuous operation: Thermostatically controlled equipment may cycle, while a generator may follow a limited schedule.
- Ignoring generator load: Propane use changes as electrical demand changes.
- Planning to empty: Keep a reserve for weather, gauge error, unexpected use, and delayed delivery.
- Ignoring cold-weather vaporization: A small cylinder may not support a large appliance in very cold conditions.
- Combining incompatible equipment: Regulators, hoses, connectors, pressure, and appliance requirements must match.
- Using outdoor appliances indoors: This creates a serious carbon-monoxide and fire hazard.
Before the Next Outage
- Record the size and current level of every propane supply.
- Find the input rating or consumption chart for each appliance.
- Calculate a normal schedule and a cold-weather high-use scenario.
- Keep a reasonable fuel reserve.
- Inspect cylinders, tanks, regulators, hoses, and connections.
- Test smoke and carbon-monoxide alarms.
- Confirm which equipment is approved for indoor or outdoor use.
- Arrange deliveries before the tank reaches a critical level.
Propane Runtime Calculator FAQs
How long does a 20-pound propane tank last?
Divide the cylinder’s available energy by the appliance input rating. A full 20-pound cylinder contains roughly 430,000 to 435,000 BTU. A 20,000-BTU appliance running continuously may therefore operate for approximately 21 hours under ideal conditions before adding a reserve. Thermostat cycling and real-world conditions can change the result.
How long will a 20-pound tank run a 30,000-BTU heater?
A full cylinder may provide roughly 14 hours of continuous theoretical runtime before adding a reserve. Actual runtime depends on fuel level, burner setting, thermostat cycling, temperature, regulator performance, and whether the cylinder can vaporize propane quickly enough.
How many gallons are in a 100-pound propane tank?
One hundred pounds of propane equals approximately 23.8 gallons using 4.2 pounds per gallon. The cylinder itself weighs additional pounds, so total cylinder weight is not the same as propane weight.
Why does a 500-gallon propane tank stop near 80% when filled?
Propane tanks require vapor space so the liquid can expand as temperature changes. A normal delivery therefore does not fill the tank’s complete water-volume capacity with liquid propane. Enter the percentage shown on the tank gauge rather than automatically entering 100%.
Can I calculate propane generator runtime from generator wattage?
Not accurately from wattage alone. Generator design and electrical load strongly affect fuel consumption. Use the manufacturer’s gallons-per-hour or pounds-per-hour rating at a comparable load whenever possible.
Should I keep a propane reserve?
Yes. A reserve protects against gauge error, colder weather, higher appliance use, delivery delays, and fuel that may not be practically available under current conditions. The calculator defaults to a 10% planning reserve.
Does cold weather reduce the amount of propane in the tank?
Cold weather does not make the stored fuel disappear, but it reduces propane pressure and vaporization. A small or nearly empty cylinder may be unable to supply a high-demand appliance even though liquid propane remains.
Build a Complete Emergency Fuel Plan
Propane runtime is only one part of outage planning. If propane will power a generator, determine the required electrical capacity with the Generator Size Calculator, then compare operating schedules with the Generator Runtime and Fuel Calculator.
Visit the Preparedness Calculators hub for water, food, battery, solar, firewood, generator, and garden-planning tools. The Blackout Survival Hub can help you connect fuel planning with refrigeration, water, heating, cooking, communication, lighting, and sanitation.
Key Takeaway
Estimate propane runtime from the usable fuel supply, the actual input rating of each appliance, and a realistic daily schedule. Keep a reserve, account for cold-weather and generator-load differences, follow every safety requirement, and verify critical plans with manufacturer data before an outage.