RV Propane Tank Sizes
Last updated: September 3, 2026
Quick answer
A "20 lb" tank holds 4.7 usable gallons (the 80% fill rule), a 30 lb holds 7.0, a 40 lb holds 9.4.
Multiply gallons × 91,502 for BTU. A 20 lb tank ≈ 430,000 BTU — about a week of mixed use,
or a day or two of hard furnace weather.
The conversion table
| Tank | Usable gallons* | Total BTU | Weight full | Typical use |
|---|---|---|---|---|
| 20 lb | 4.7 | 430,059 | ≈ 37–40 lb | Small trailers, portable BBQ-style bottle |
| 30 lb | 7 | 640,514 | ≈ 52–55 lb | The standard travel-trailer tank |
| 40 lb | 9.4 | 860,119 | ≈ 68–72 lb | Longer boondocking stretches |
| 100 lb | 23.6 | 2,159,447 | ≈ 170 lb | Site/equipment use, rarely mobile |
| ASME 30 gal (onboard) | 30 | 2,745,060 | ≈ 126 lb + tank | Motorhomes and 5th wheels, filled by weight |
| 2 × 30 lb | 14 | 1,281,028 | — | Changeover regulator = swap without stopping |
*Usable gallons already include the DOT 80% fill rule — a 20 lb cylinder's water capacity is ~25 lb, but propane is only put in to 80% to allow thermal expansion.
The constants behind every propane question
| Constant | Value | Why it matters |
|---|---|---|
| Energy per gallon | 91,502 BTU | Divide appliance BTU/h by this for gallons/hour |
| Energy per pound | ≈ 21,790 BTU | Tank weights and exchange refills are sold by the pound |
| Pounds per gallon | 4.2 lb | Converts between the two units above |
| 20 lb tank energy | ≈ 430,000 BTU | The "how long will it last" reference unit of RV camping |
Suppliers round the gallon figure to 91,500 BTU — the difference is noise. Our appliances run these exact constants; see the propane usage calculator.
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Refill vs exchange — the weight tells the truth
- Refill (by weight): you pay for what went in, and refill stations fill to the full 80%. A "20 lb" tank actually holds 20 lb of propane ≈ 4.7 gal.
- Exchange cages: typically 15 lb of propane in a 20 lb tank — the reason an exchange bottle "doesn't last as long". Check the stamped tare weight (T.W.) and compare against the fill weight printed on the cage.
- ASME onboard tanks (motorhomes) are filled by weight at refills and don't get swapped at all.
How long each tank lasts — quick sense check
A 30 lb tank (7 gal ≈ 640,000 BTU): gas-mode fridge alone ≈ 19 days; fridge + daily cooking + hot water ≈ 10 days; add a furnace burning 3 h/day of a 35k BTU rating and it drops to ≈ 4 days. Real runtimes depend on your burner hours — the calculator turns your habits into days-per-tank.
Sources:
- 91,502 BTU/gal, 4.2 lb/gal: propane industry standard energy content (propane supplier publications, e.g., Ferrellgas; Foster Fuels cites 91,452 BTU/gal — same constant, rounded).
- 80% fill rule for DOT cylinders: standard filling practice; tank gallon capacities (20 lb → 4.7 gal, 30 lb → 7 gal, 40 lb → 9.4 gal) follow from it.
- Tare/fill weight behaviour of exchange cylinders: printed on exchange cages; verify at purchase.
Related tools & tables
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