Where to Find the Gauge
On a standard ASME propane tank (the large cylindrical tank typically buried or sitting next to your home, cabin, or farm building), the gauge is located on top of the tank under the protective dome or metal housing. You may need to lift a cover or cap to see it. The gauge is a round dial about 2–3 inches in diameter, typically with a colored scale ranging from 0 to 100.
On portable DOT cylinders (like the 20 lb tank you use for a gas grill), the gauge may be built into the valve/regulator assembly at the top of the cylinder, or it may not be present at all — some portable cylinders have no gauge, and you have to estimate the level by weight. If your cylinder has a float gauge, it works exactly the same way as a large tank gauge.
What the Percentage Scale Means
The gauge shows your propane level as a percentage of the tank's total volume:
- 100% — The highest the scale goes, but you will never see a tank here. Propane tanks are never filled to 100%.
- 80% — Full. This is the maximum fill level per propane safety regulations (NFPA 58). If your tank was just filled, it should read approximately 80%.
- 50% — Half full. This is a comfortable level with no immediate action needed.
- 30% — Reorder territory. Most propane suppliers recommend scheduling a delivery when you reach 20–30%.
- 20% — Call your supplier today if you haven't already scheduled a delivery.
- 10% or below — Critical. Call your supplier immediately. At this level, deliveries should be treated as urgent.
- 0% — Empty. If a fixed tank runs completely empty, most suppliers require a pressure test before refilling.
Why does a full tank read 80%, not 100%? Propane is a liquid under pressure that expands significantly when heated. NFPA 58 (the Liquefied Petroleum Gas Code) requires tanks to be filled to a maximum of 80% of total capacity. The remaining 20% is "vapor space" — room for the liquid to expand safely as temperature rises. A reading of 80% means your tank is completely full. This is not a defect or a filling error; it is the correct, safe fill level.
How a Float Gauge Works
Most residential propane tanks use a float gauge — the same basic mechanism as a car's fuel gauge. A float inside the tank rests on the liquid propane surface. As the level drops, the float drops with it. A rod connected to the float rotates a dial on the outside of the tank, and the dial shows the corresponding percentage.
Float gauges are generally accurate but not precise. They are designed as planning guides — enough to tell you whether your tank is full, half-full, or getting low. They are not calibrated instruments. A reading of 35% means "somewhere around a third full," not exactly 35.0%. For propane planning purposes, this level of accuracy is entirely adequate.
Converting Percentage to Gallons
To calculate how many gallons of propane you have remaining, multiply your tank's total capacity by the gauge percentage:
Gallons remaining = tank capacity × (gauge % ÷ 100)
Examples:
- 500-gallon tank at 60%: 500 × 0.60 = 300 gallons
- 250-gallon tank at 45%: 250 × 0.45 = 112.5 gallons
- 100-gallon tank at 25%: 100 × 0.25 = 25 gallons
- 20 lb cylinder (4.7 gallons) at 50%: 4.7 × 0.50 = 2.35 gallons
PropanePal does this calculation automatically when you log a gauge reading — just enter the percentage, and the app shows you the estimated gallons remaining.
What to Do With the Reading
Once you've read the gauge, you have a few options:
If you're above 30%
You're in good shape. Log the reading in PropanePal to update your usage rate and reorder date prediction. Make a note to check again in a week or two.
If you're between 20–30%
This is the standard reorder zone. Contact your propane supplier to schedule a delivery. Don't wait — delivery times vary by season and region. During winter heating season, delivery may take 3–7 days or more in some areas.
If you're below 20%
Call your supplier today. Mention that your level is below 20% and ask about the earliest available delivery. If you are also using propane for heat in cold weather, mention this to prioritize the urgency.
If the tank is empty
Contact your supplier immediately. Most suppliers require a pressure test and system inspection before refilling a tank that has run completely empty. This check ensures there are no leaks before propane is introduced back into the system. There is typically a fee for this service and it can add a day or more to your delivery timeline.
Logging Your Reading in PropanePal
Once you have a gauge reading, logging it in PropanePal takes about 10 seconds:
- Open PropanePal on your iPhone
- Select your tank from the dashboard
- Tap Add Reading
- Enter the percentage from the gauge
- Tap Save
PropanePal timestamps the reading, calculates gallons remaining, updates your usage rate, and projects your next reorder date. Do this weekly during active use periods for the most accurate predictions.
What About Portable Cylinders Without a Gauge?
Not all propane cylinders have a float gauge. Some 20 lb grill cylinders, in particular, have no gauge at all. For cylinders without a gauge, you can estimate the level by weight: weigh the cylinder on a postal or luggage scale, then subtract the tare weight (stamped on the collar as "TW" — typically 17–18 lbs for a 20 lb cylinder). The remaining weight is the net propane weight. Divide by 4.24 to get gallons (since propane weighs 4.24 lbs per gallon at 60°F).
PropanePal supports this method — enter the scale reading and tare weight, and the app calculates the level for you.
Safety reminder: If you ever smell propane near your tank or suspect a leak, do not check the gauge — leave the area immediately, do not operate any electrical switches, and call your propane supplier and emergency services from a safe distance. Never try to tighten a valve or check for leaks yourself.