Propane Tank Size Calculator — Gallons, BTU & Weight

BTU Sizing: How to Match Your Tank to Your Appliances

The starting point for any propane tank size calculator is your appliance BTU demand. British Thermal Units per hour, or BTU/hr, is the universal measurement of how much heat each propane appliance consumes. Your gas furnace, water heater, kitchen range, clothes dryer, fireplace, and any outdoor patio heaters all have a BTU/hr rating printed on a nameplate or sticker. To find your peak total demand, add up every appliance you might run simultaneously — a winter morning with the furnace cycling on while you run the hot water, cook breakfast, and dry clothes can easily hit 170,000 to 200,000 BTU per hour in a typical American home.

Once you have your total BTU/hr, two rules of thumb guide the propane tank size selection. The first is the 275 square feet per gallon rule: each gallon of working propane capacity heats approximately 275 square feet of moderately insulated home in a climate with typical winter temperatures. A 400-gallon working capacity tank (a 500-gallon tank at 80%) therefore supports about 110,000 square feet — well over what any single-family home needs, which is why this number is a safety margin rather than a hard target.

The second rule is the runtime rule: a well-sized tank should deliver at least 7–10 days of peak demand between propane deliveries. A 500-gallon tank at 80% holds 400 gallons, and each gallon of liquid propane contains roughly 91,500 BTU. That is 400 × 91,500 = 36.6 million BTU total. At a peak demand of 170,000 BTU per hour, the tank provides about 215 hours of runtime — or nearly 9 full days of continuous max load. This matches exactly why 500-gallon tanks are the default whole-house size. Our calculator on this page runs both checks automatically when you enter your BTU demand.

The 80% Fill Rule: Legal Requirement, Not a Suggestion

If you take away one thing from this page, make it this: every propane tank, portable or stationary, must stop at 80% of its water capacity when being filled. This rule comes from the National Fire Protection Association (NFPA 58), is enforced by the Department of Transportation for all portable cylinders, and is written into every state's fire code. It is not a voluntary guideline — it is a safety mandate that every certified propane delivery driver is trained to follow.

The reason is thermal expansion, which we cover in its own section below, but the practical effect is what matters for tank sizing. A "500 gallon" propane tank does not hold 500 gallons of liquid propane. It holds 500 gallons of water — that is its geometric internal volume. At the legal 80% fill, it holds 400 gallons of liquid propane. The remaining 100 gallons of headspace allows vapor pressure to build and absorbs expansion as the liquid warms. This is why you should always size your tank based on working capacity(water capacity × 0.80), not the nominal water capacity printed on the side.

Propane Weight: 0.51× the Weight of Water

Propane is significantly lighter than water. Water weighs 8.34 pounds per US gallon at 60°F, and liquid propane weighs approximately 4.25 pounds per gallon — which is about 51% of water's weight. The industry shorthand formula is therefore:

This weight matters for three reasons. First, it tells you how much your full tank adds to your patio, RV frame, or foundation — a full 1000-gallon stationary tank weighs over 4,000 pounds including the empty steel shell, so you need a proper concrete pad. Second, delivery companies price propane by the pound, and the weight scale on the delivery truck is how they know exactly how much they pumped into your tank. Third, you can use weight to estimate fill level on portable tanks: a 20 lb cylinder that weighs 32 pounds on your bathroom scale has about 16 pounds of propane (32 − 16 lb empty tare weight), or roughly 3.7 gallons — close to a full 80% fill.

Standard Propane Tank Sizes Chart

The table below lists every standard propane tank size you will encounter, from the 20 lb grill cylinder all the way to the 1000-gallon commercial tank. Note that the "working capacity" column is the legal 80% fill amount — the actual gallons of liquid propane you are allowed to have in the tank. The full weight is the combined weight of the empty steel shell plus the legal maximum propane fill.

Thermal Expansion: Why Propane Needs Headspace

Every liquid expands when warmed. Water expands about 0.02% per 10°F — almost unnoticeable. Propane is not water. Liquid propane expands approximately 1.5% in volume for every 10°F temperature rise. That is 75 times more expansion than water. This single physical property is the reason the 80% fill rule exists, and why every propane tank has a pressure relief valve (PRD) designed to vent vapor before rupture occurs.

Let's run the math on a 500-gallon tank. On a 40°F spring morning, a delivery driver fills the tank to 80% — 400 gallons of liquid propane. By mid-afternoon, the tank's temperature climbs to 90°F under direct sun. That is a 50°F rise, or five × 10°F increments. Each increment expands the liquid by 1.5%, so total expansion is 5 × 1.5% = 7.5%. Our 400 gallons of liquid has grown to 430 gallons — exactly 30 gallons short of the 500-gallon tank's total volume. The 100 gallons of ullage (headspace) we left at fill time has shrunk to 70 gallons, but the liquid still fits. If we had filled the tank to 100% that morning, the expanded liquid would be at 537.5 gallons — more than the tank can physically hold. The pressure relief valve would vent violently, spraying propane vapor that could ignite.

Underground tanks see a milder version of this story. Buried below the frost line, tank temperatures swing less dramatically — typically 10–20°F between seasons rather than 50–60°F above ground. But even a 20°F swing causes 3% expansion on a 500-gallon tank, or 15 gallons of extra liquid. The headspace handles that easily. Never be tempted to fill past 80% in winter because "the cold keeps it down." Spring is coming, and the ground will warm.

How to Check How Much Propane Is In Your Tank

Stationary tanks have a gauge — either a direct-read needle gauge or a wireless magnetic sensor that transmits to a mobile app. Portable cylinders require a different approach. The simplest and most accurate method is the warm water test: pour hot water down the side of the tank, then run your hand vertically up the metal. The tank wall is warm above the liquid level and cool where liquid propane is present — the propane is cold because it is evaporating inside the cylinder. Mark the transition point with a felt-tip pen, then measure the height and compare to the total height. A tank that is half covered with cool metal is roughly 50% full.

The scale method is even more precise if you know the tare weight (empty weight) of your cylinder, which is stamped into the tank collar. A standard 20 lb cylinder has a tare weight of 16 pounds. Weigh it on a bathroom scale, subtract 16, and divide by 4.25 — that gives you gallons remaining. A 20 lb cylinder weighing 29 pounds has 13 pounds of propane, or about 3.06 gallons remaining — roughly 77% of its 4-gallon working capacity.

Common Mistakes in Propane Tank Size Calculation

1. Sizing on Water Capacity Instead of Working Capacity

The single most common mistake. A homeowner shopping for a "500-gallon propane tank" thinks they are getting 500 gallons of usable propane. They are getting 400 gallons. If you calculated your BTU runtime on 500 gallons, you are overestimating by 20%. Always multiply water capacity by 0.80 first, then do your BTU and heating-area math. Every number in our calculator on this page uses working capacity — you can trust it.

2. Ignoring Peak vs. Average Demand

Your furnace cycles on and off all day — it only runs about 30–50% of the time in mild weather. But the tank needs to handle peak demand, not average. On a zero-degree morning when the furnace is running continuously while you also heat water, cook breakfast, and run a dryer, your peak BTU/hr is 100% of every appliance's rating. If you size the tank to average load, you will run out of propane on the coldest day of the year. Always sum the BTU/hr of all simultaneously-running appliances for sizing.

3. Forgetting That Propane Weighs Less Than Water

When you look at a delivery receipt, you will see "gallons delivered" and you might assume each gallon weighs 8.34 pounds. It does not — it weighs 4.25 pounds. A 100-gallon delivery adds about 425 pounds to your tank, not 834. This matters if you are monitoring fill by weight on a stationary tank pad scale, or if you are trying to estimate remaining gallons from a portable cylinder's total weight. Divide propane weight by 4.25 to get gallons, not by 8.34.

Frequently Asked Questions