🔥 NFPA 58 Compliant LPG Engine
Propane Tank Calculator
Calculate liquid propane volume in gallons and liters, mass in kg and lbs, 80% OPD safe fill limit, thermal expansion, and appliance runtime.
Propane Density 0.493 kg/L • 4.114–4.24 lb/US Gal @ 60°F
Propane kg to Gallons Hub Calculation Mode:
International Metric Cylinders:
80% (Safe Full Fill)
0% (Empty) 20% (Low) 50% 80% (OPD Limit) 100% (Illegal)
🌡️ Ambient Temp:
Expansion: 0.0%
Visual Tank Profile 20 lb BBQ Tank
20% Vapor Space (Expansion) OPD Limit
80%
Remaining Propane Volume
4.00 US Gallons
15.14 Liters • 3.33 Imp Gal
Liquid Weight (lbs) 17.00 lbs
Liquid Mass (kg) 7.71 kg
Available Thermal Energy: 366,188 BTU
Equivalent Electricity: 107.3 kWh
⏱️ Estimated Appliance Runtime with Current Fuel
🥩 BBQ Grill ~30,000 BTU/hr (Med-High)
12.2 hrs
🔥 Patio Heater ~40,000 BTU/hr (Max)
9.2 hrs
🚐 RV Furnace ~20,000 BTU/hr (Continuous)
18.3 hrs
⚡ 5kW Generator ~75,000 BTU/hr (50% Load)
4.9 hrs
Interactive LPG Internal Cylinder Pressure Simulator
Propane exists inside closed cylinders in two-phase equilibrium (liquid boiling into vapor). The internal vapor pressure is dictated strictly by the tank's ambient temperature. Adjust the temperature slider below:
60°F
Cylinder Pressure State:
Standard Room Temperature (~92 psi)
Internal Vapor Pressure
92.4 psi
Gauge Pressure (PSIG) Liquid Thermal Expansion
0.0%
Relative to 60°F standard Relief Valve Safety Status
✅ Safe Normal
Relief triggers @ 250–312 psi
💰 Consumer Cost & Energy Economics
Propane Refill Cost vs Cylinder Swap Price & Value Analyzer
Most commercial exchange cages (Blue Rhino, Amerigas) fill 20-lb cylinders to only 15 lbs (75% full) for convenience, whereas bulk pump stations fill to the true 80% legal maximum (20 lbs / 4.7 gallons). Compare cost per pound, cost per gallon, and effective cost per 100k BTU.
$
$
Bulk Refill Total (20 lbs full)
$18.10
$0.91 / lb ($4.20/gal eq) Exchange Swap Rate
$1.53 / lb
$6.43 / gallon equivalent Savings by Refilling Tank
40.5% Cheaper
Save $12.33 per 20 lbs Cost per 100,000 BTU
$4.21 vs $7.08
Bulk Refill vs Swap Cage
🏡 Residential HVAC & Tank Engineering
Whole-Home Propane Tank Sizing & Annual Usage Wizard
Determine the optimal bulk ASME propane storage vessel (120, 250, 500, or 1,000 Gallons) based on your home square footage, climate heating zone, and installed gas appliances.
sq ft
Recommended Tank Size
500 Gallon Tank
400 Gal working capacity Estimated Annual Usage
810 Gallons / Year
1,514 kg propane Winter Refill Frequency
~2 Refills / Winter
Keep above 30% reserve Estimated Annual Fuel Cost
$3,118 / Year
@ $3.85/gal regional avg Standard Propane Tank Specifications & Fill Limits
Propane cylinders are sized by their nominal propane pound capacity (e.g. 20 lb, 100 lb) or water capacity (WC). Under NFPA 58 safety regulations, maximum safe liquid fill is capped at 80% of total volume.
| Tank Type / Size | Water Cap (WC) | 80% Safe Gallons | Propane Weight | Tare Wt (TW) | Total Full Gross | BTU Content |
|---|---|---|---|---|---|---|
| 20 lb (BBQ / Patio) | 4.7 gal (39.2 L) | 4.0 gal (15.1 L) | 17.0–20.0 lbs (7.7–9.1 kg) | ~17.0 lbs | ~37.0 lbs | 366,188 BTU |
| 30 lb (RV / Trailer) | 7.1 gal (59.2 L) | 5.7 gal (21.6 L) | 30.0 lbs (13.6 kg) | ~24.0 lbs | ~54.0 lbs | 521,818 BTU |
| 40 lb (Commercial / RV) | 9.5 gal (79.2 L) | 7.6 gal (28.8 L) | 40.0 lbs (18.1 kg) | ~29.0 lbs | ~69.0 lbs | 695,757 BTU |
| 100 lb (Residential / Heater) | 23.6 gal (196.8 L) | 18.9 gal (71.5 L) | 100.0 lbs (45.4 kg) | ~68.0 lbs | ~168.0 lbs | 1,730,238 BTU |
| 120 Gallon ASME Tank | 120.0 gal (454.2 L) | 96.0 gal (363.4 L) | 403.2 lbs (182.9 kg) | ASME Plate | — | 8,788,512 BTU |
| 250 Gallon ASME Tank | 250.0 gal (946.4 L) | 200.0 gal (757.1 L) | 840.0 lbs (381.0 kg) | ASME Plate | — | 18,309,400 BTU |
| 500 Gallon Whole-Home Tank | 500.0 gal (1892.7 L) | 400.0 gal (1514.2 L) | 1,680 lbs (762.0 kg) | ASME Plate | — | 36,618,800 BTU |
Frequently Asked Questions
Everything you need to know about propane tank sizing, weights, and safety limits.
Why can a propane tank only be filled to 80% capacity?
Under the NFPA 58 Liquefied Petroleum Gas Code and OSHA standards, propane containers are legally limited to an 80% maximum fill level (enforced by the Overfill Prevention Device, or OPD valve). Propane expands significantly as temperature increases (approximately 1.5% for every 10°F temperature rise). The remaining 20% vapor space acts as an essential expansion cushion to prevent dangerous hydrostatic hydraulic pressure buildup or relief valve discharge.
How many gallons of propane are in a 20 lb BBQ tank?
A standard 20 lb BBQ tank holds approximately 4.7 gallons of total water capacity (WC), which translates to a maximum legal safe fill of 4.0 to 4.2 US Gallons (or ~17.8 to 20 lbs / 8.0 to 9.07 kg) of liquid propane at 60°F. Most commercial cylinder exchange stations fill tanks to 15 lbs (~3.5 gallons) for safety margin.
How much does a gallon of liquid propane weigh?
At standard conditions (60°F / 15.6°C), one US liquid gallon of pure propane weighs 4.20 pounds (1.905 kg), with a physical density of 0.493 kg/L to 0.504 kg/L (specific gravity ~0.504). Commercial LPG is typically 90–95% propane with minor ethane and butane fractions.
How do you calculate remaining propane by weighing a tank?
Look for the Tare Weight (TW) stamped on the tank collar (e.g., TW 17.0 lbs). Weigh the tank on a scale to find the Gross Weight (GW). Subtract the tare weight:
Divide remaining pounds by 4.20 to calculate remaining liquid gallons.
Remaining Propane (lbs) = Gross Weight - Tare WeightDivide remaining pounds by 4.20 to calculate remaining liquid gallons.
How many BTUs are in a gallon of liquid propane?
One US gallon of liquid propane contains approximately 91,547 BTUs (British Thermal Units) of thermal energy, equal to approximately 26.83 kWh or 96.58 MJ. A full 20 lb tank (4.0 gal) contains roughly 366,188 BTUs.
How does the warm water method work to check propane levels?
Pour a cup of warm tap water slowly down the outer side of the cylinder. Run your bare hand down the metal. The empty upper portion absorbing vapor will feel warm, but the liquid propane below will absorb the thermal heat and feel distinctly cold to the touch, revealing the exact liquid level line.