LB/HR ↔ SCFM Calculator

Convert lb/hr to SCFM or SCFH, or SCFM to lb/hr, for air or another gas at your chosen standard conditions, with kg/hr and metric flow options.

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LB/HR to SCFM Formula

Converting mass flow (lb/hr) to standard volumetric flow requires the molecular weight of the gas. The general formula, derived from the ideal gas law at 60 degrees F and 14.696 psia, is:

Where MW is molecular weight in lb/lb-mol and the ideal-gas molar volume is approximately 379.48276169 ft3/lb-mol at 60°F and 14.696 psia. The default gas is air. MW 66 is an illustrative gasoline-vapor assumption, not a universal fuel property.

Conversion Factors by Gas Type

The factor approximately 10.43526 applies to the illustrative MW 66 basis. Each gas or mixture needs its own molecular weight. These rounded ideal-gas factors use the listed approximate molecular weights at 60°F and 14.696 psia:

What is SCFM?

SCFM (Standard Cubic Feet per Minute) is a mass-equivalent volumetric flow rate normalized to a fixed reference condition. Because a gas expands at higher temperature and contracts at higher pressure, SCFM removes those variables by expressing flow as if the gas were at a defined standard state. The result is proportional to molar and mass flow regardless of actual operating conditions.

The word "standard" is not universal. These reference examples use different conventions; confirm the applicable equipment or contract specification:

A flow stated as 1,000 SCFM of air at the 60 deg F standard carries about 1.5% more mass than 1,000 SCFM at the 68 deg F standard. For combustion or custody transfer work, always confirm which reference standard applies.

SCFM vs ACFM

SCFM is flow corrected to reference conditions. ACFM (Actual Cubic Feet per Minute) is the real volumetric flow at the gas's operating temperature and pressure. The relationship is:

For a fixed gas, ideal behavior and matching wet/dry basis, raising absolute operating pressure reduces ACFM, while raising absolute temperature increases it. Blowers and compressors are sized in ACFM because that determines actual equipment size. Pipeline contracts and combustion air calculations use SCFM or lb/hr because those quantities are proportional to the mass of gas transferred.

How to Calculate SCFM from LB/HR

Example: A fuel system delivers 5,000 lb/hr of gasoline vapor (MW = 66) to a combustion unit. What is the equivalent SCFM at 60 degrees F and 14.696 psia?

Step 1: Identify the molecular weight. For this illustrative example only, assume gasoline-vapor MW = 66 lb/lb-mol; actual composition varies.

Step 2: Calculate the conversion factor:

Step 3: Divide mass flow by the factor:

For comparison, the same 5,000 lb/hr with propane MW 44.096 gives approximately 717.153 SCFM at this reference. Lighter molecules occupy more ideal-gas volume per unit mass at matching reference conditions.