• SOLVENT DIRECT INC
• Apr 23, 2025
• 2 min read

Understanding LPG Fill Ratios: What’s Legal, What’s Safe, and Why It Matters

When it comes to liquefied petroleum gas (LPG) used in extraction—like high purity butane, isobutane, and propane—there’s more to filling a cylinder than just “topping it off.” Behind every legally compliant fill is a standard governed by the Pipeline and Hazardous Material Safety Administration (PHMSA) under the Department of Transportation (DOT).

Let’s break it down for the inquiring minds looking for truth, justice, and extraction efficiency.

Fill ratios determine the maximum amount of liquefied gas that can legally be put into a DOT/UN cylinder. The ratios are based on the water capacity (WC) of the cylinder—essentially, how much water it holds—and are defined per gas type. These

standards are outlined in the U.S. DOT's

UN Recommendations on the Transport of Dangerous Goods.

Here’s the standard fill ratio per gas:

• Butane: 0.51 (Page 12)

• Isobutane: 0.49 (Page 13)

• Propane: 0.42 (Page 12)

🧮 Putting Math into Practice

Let’s use a common example: a LP 239 cylinder, typically referred to as a “100-pounder.” Here’s what the math says:

• Propane: 239 * 0.42 = 100.38 lbs ➡️ Filled to 100 lbs

• Butane: 239 * 0.51 = 121.89 lbs ➡️ Filled to 120 lbs

• Isobutane: 239 * 0.49 = 117.11 lbs ➡️ Filled to 116 lbs

Why the slight reduction? Industry standards. These numbers are slightly below the max to accommodate temperature fluctuations and ensure safety.

⚗️ The 70/30 Blend Breakdown

For common extraction blends like 70% butane / 30% propane, here's how it works:

• Propane: (239 0.42) 30% = 30.11 lbs

• Butane: (239 0.51) 70% = 85.32 lbs

• Total Max Fill = 115.43 lbs ➡️ Industry standard = 115 lbs

Some states (looking at you, Texas 👀) might try to simplify the blend math, but even their “simple” calculations—30 lbs propane + 84 lbs butane = 114 lbs—come within 1% of industry standards. In short: there’s no meaningful difference, but precision still matters.

Using the correct fill ratio:

• Keeps you compliant with federal safety regulations

• Protects against overfilling-related hazards

• Prevents unnecessary scrutiny from regulators and law enforcement

• Keeps your team and customers safe

So next time you see a cylinder marked "LP 239," you’ll know that science, math, and compliance all played a part in determining how much LPG it’s holding.

Learn more about the harsh economics of under-filled cylinders: https://www.solventdirect.com/post/fillweights

📄 Learn more about DOT LPG Regulations

For more compliance education, visit www.solventdirect.com/compliance

Engineered for Extraction™

Originally published at solventdirect.com.