How Many Pounds of Propane in a 100 Gallon Tank?

Propane tanks are never filled to 100% — the 80% rule leaves room for the liquid to expand in warm weather. Here’s how the maths works and what it means for delivery, placement, and safety.

The Calculation

• Tank capacity: 100 gallons (this is the “water capacity” — total volume)
• Fill limit: 80% = 80 gallons of propane
• Propane weight: 4.24 lb per gallon
• Propane weight total: 80 × 4.24 = 339 lb

Wait — that gives 339 lb, not 420. The discrepancy: many sources quote the full 100 gallons × 4.24 = 424 lb as the theoretical maximum, but you should never see this in practice because of the 80% fill rule. The realistic figure is ~340 lb of propane plus ~170 lb for the steel tank = ~510 lb total for a full 100-gallon tank.

The National Propane Gas Association publishes safety standards including fill limits.

Why Only 80%? The Expansion Rule

Propane is stored as a liquid under pressure. Like all liquids, it expands when warmed — about 1.5% per 10°F rise. A tank filled to 100% on a cool morning could become dangerously over-pressurised by afternoon. The 20% vapour space (ullage) absorbs this expansion safely.

This is enforced by the OPD (Overfill Protection Device) — a float valve that stops filling at 80%. Every tank manufactured since 1998 has one. If a delivery driver ever bypasses it, that’s a serious safety violation.

What Does a 100-Gallon Tank Look Like?

Despite the name, a “100-gallon” propane tank is about 4 feet tall and 3 feet in diameter — roughly the size of a large water heater. Common residential sizes:

• 100 gallons: ~340 lb of propane — supplemental heating, fireplaces
• 250 gallons: ~850 lb — small homes
• 500 gallons: ~1,700 lb — standard whole-house (most common)
• 1,000 gallons: ~3,400 lb — large homes, farms

How Long Does 100 Gallons Last?

It depends entirely on usage. Propane contains ~91,500 BTU per gallon:

• Fireplace only (30,000 BTU/hr): ~240 hours of burn time
• Supplemental heating: 1–3 months in winter
• Cooking only: many months — a gas hob sips propane
• Whole-house heating: a 100-gallon tank is undersized; you’d refill constantly

Most homes with propane heating use 500–1,000 gallon tanks precisely because 100 gallons goes quickly in cold weather. For UK readers: propane (LPG) heating is common in rural homes off the gas grid. The Ofgem website covers off-grid heating options and consumer protections.

Placement and Safety Rules

• Distance from buildings: typically 10 feet minimum for 100-gallon tanks (check local codes)
• Surface: level concrete pad or compacted gravel — 500+ lb needs solid footing
• Ventilation: never in enclosed spaces; propane is heavier than air and pools in low spots
• Inspection: tanks need periodic recertification (usually every 10 years)

Propane has an excellent safety record when handled properly, but it demands respect. The distinctive “rotten egg” smell is added ethyl mercaptan — if you ever smell it strongly, leave the area and call emergency services. Our propane vs natural gas comparison covers the fuel choice in more depth.

Propane Tank Sizes: Full Comparison

• 20 lb (BBQ tank): ~4.7 gallons — the familiar barbecue bottle
• 100 lb: ~23.6 gallons — tall cylinder, popular for temporary heating
• 100 gallons: ~80 gallons usable — supplemental home use
• 250 gallons: ~200 gallons usable — small homes, hot water + cooking
• 500 gallons: ~400 gallons usable — the standard whole-house tank
• 1,000 gallons: ~800 gallons usable — large homes, farms, commercial

Don’t confuse “100 lb” and “100 gallon” tanks — the naming is a notorious trap. A 100 lb tank holds about 24 gallons; a 100-gallon tank holds about 80 usable gallons. Always clarify which unit a supplier means.

Buying vs Renting Your Tank

• Renting (common): the propane company owns the tank, handles maintenance and recertification. Monthly rental £30–£60/year equivalent. Downside: you’re usually locked to that supplier.
• Buying: a 100-gallon tank costs £400–£800 new. You own it, can shop suppliers for the best per-litre price, but handle maintenance yourself.

If you use significant propane, owning usually pays off within a few years through supplier competition. For minimal use (fireplace only), renting avoids the hassle.

Reading Your Propane Gauge

Tank gauges show percentage of capacity, not gallons. A “100-gallon” tank reading 50% holds ~40 gallons of propane (50% of the 80-gallon usable fill). Order a refill at 20–30% — never let it hit zero, as running dry requires a system pressure test and pilot relighting before reuse, often with a callout fee.

Many suppliers offer automatic delivery (they monitor and top up) versus will-call (you watch the gauge). Automatic costs slightly more per litre but eliminates run-out risk in midwinter — worth considering for heating-dependent homes.

Related: propane tank duration guide.

Frequently Asked Questions

How many pounds of propane in a 100 gallon tank?

~340 lb at the standard 80% fill (80 gallons × 4.24 lb/gallon).

Why can’t propane tanks be filled to 100%?

Liquid propane expands with heat; the 20% vapour space prevents dangerous over-pressurisation.

How much does an empty 100-gallon propane tank weigh?

Roughly 150–180 lb (the “tare weight” stamped on the tank).

How long will 100 gallons of propane last?

Weeks to months depending on use — fireplace-only lasts longest; whole-house heating depletes it fast.

What’s the difference between a 100 lb and 100 gallon tank?

A 100 lb tank holds ~23.6 gallons of propane; a 100-gallon tank holds ~80 gallons at 80% fill. Confusingly named!