An 80 CF cylinder of welding gas lasts about 3-4 hours of actual welding time at standard flow rates. But “actual welding time” and “time in the shop” are very different numbers. A hobbyist might stretch that 80 CF tank across weeks or months because the arc is only running a fraction of the time you’re in the shop.
The Basic Formula
Gas consumption is simple math:
Arc Time (hours) = Cylinder Volume (CF) / Flow Rate (CFH)
An 80 CF cylinder at 25 CFH: 80 / 25 = 3.2 hours of arc time.
That’s trigger-pulled, arc-burning time. It doesn’t count setup, tacking, grinding, repositioning, grabbing a coffee, or staring at the joint wondering if it’s good enough. In a real shop session, your arc-on factor (percentage of time actually welding) is typically 20-40% for manual welding.
Arc Time by Cylinder Size and Flow Rate
| Cylinder Size | 15 CFH | 20 CFH | 25 CFH | 30 CFH | 35 CFH | 40 CFH |
|---|---|---|---|---|---|---|
| 20 CF | 1.3 hr | 1.0 hr | 0.8 hr | 0.7 hr | 0.6 hr | 0.5 hr |
| 40 CF | 2.7 hr | 2.0 hr | 1.6 hr | 1.3 hr | 1.1 hr | 1.0 hr |
| 80 CF | 5.3 hr | 4.0 hr | 3.2 hr | 2.7 hr | 2.3 hr | 2.0 hr |
| 125 CF | 8.3 hr | 6.3 hr | 5.0 hr | 4.2 hr | 3.6 hr | 3.1 hr |
| 250 CF | 16.7 hr | 12.5 hr | 10.0 hr | 8.3 hr | 7.1 hr | 6.3 hr |
| 330 CF | 22.0 hr | 16.5 hr | 13.2 hr | 11.0 hr | 9.4 hr | 8.3 hr |
Typical Flow Rates by Process
Different welding processes use different flow rates. TIG generally uses less gas than MIG because the gas cup is smaller and held closer to the work.
| Process | Typical Flow Rate (CFH) | Notes |
|---|---|---|
| MIG (GMAW) - short circuit | 20-30 | Standard range for most shop MIG |
| MIG (GMAW) - spray transfer | 30-45 | Higher flow for broader arc cone |
| TIG (GTAW) - steel/stainless | 15-25 | Depends on cup size |
| TIG (GTAW) - aluminum | 18-30 | AC welding runs slightly higher |
| Flux-core gas-shielded (FCAW-G) | 30-45 | Larger wire, wider arc |
| MIG with helium blend | 35-55 | Helium is lighter, needs more volume |
| TIG with gas lens | 10-18 | Gas lens allows lower flow rates |
TIG with a gas lens is the most gas-efficient setup. A gas lens replaces the standard collet body and produces a smooth, laminar gas flow that provides better coverage at lower flow rates. You can run 10-12 CFH with a gas lens where a standard setup needs 18-20 CFH. Over time, the gas savings are significant.
Converting Arc Time to Calendar Time
Hobbyists and small shops care about calendar time: how many weeks or months between cylinder changes. That depends on your arc-on factor.
Arc-on factor estimates:
- Hobby welder (weekend projects): 10-15% of shop time is arc-on. A 4-hour shop session produces about 30-40 minutes of actual welding.
- Small fab shop: 20-30% arc-on factor. Fitting, tacking, grinding, and repositioning take up the rest.
- Production welder (repetitive parts): 40-60% arc-on factor. Jigs and fixtures minimize non-welding time.
- Robotic welding: 60-85% arc-on factor.
| Usage Pattern | Shop Hours per Week | Arc-On Factor | 80 CF Cylinder Lasts |
|---|---|---|---|
| Light hobby | 2-4 | 15% | 3-6 months |
| Active hobby | 6-10 | 15% | 1-3 months |
| Side business | 15-25 | 25% | 2-4 weeks |
| Full-time small shop | 30-40 | 30% | 1-2 weeks |
| Production | 40+ | 50% | Less than 1 week |
A weekend hobbyist with an 80 CF cylinder can easily go 3-6 months between fills. That same cylinder in a busy one-person fab shop might last a week or two.
CO2 Cylinder Consumption
CO2 cylinders are measured by weight, not volume. Convert using the ratio: 1 lb of liquid CO2 = 8.7 CF of gas.
| Cylinder Size | Gas Volume (CF) | Arc Time at 25 CFH |
|---|---|---|
| 5 lb | ~44 CF | ~1.8 hours |
| 10 lb | ~87 CF | ~3.5 hours |
| 20 lb | ~174 CF | ~7.0 hours |
| 50 lb | ~435 CF | ~17.4 hours |
CO2 flow rates for MIG welding tend to run 5-10 CFH higher than argon mix flow rates because CO2’s arc behavior requires more volume for consistent shielding. Straight CO2 at 30-35 CFH is typical for short-circuit MIG on steel.
What Drains Your Cylinder Fast
If your gas runs out faster than the math suggests, one of these is the culprit.
Leaks. The number one cause of unexplained gas loss. Even a tiny leak at a fitting can drain a cylinder over a few days. Test every connection with soapy water. Pay special attention to the regulator-to-cylinder junction, hose fittings, and the gas solenoid inside the welder. A leaking solenoid lets gas bleed out any time the machine is powered on, even when you’re not welding.
Leaving the cylinder valve open overnight. If your machine’s gas solenoid has even a slight leak, an open cylinder valve means gas flows all night. Close the cylinder valve at the end of each session. Bleed the remaining pressure from the regulator by pulling the MIG trigger briefly, then close the regulator.
Flow rate set too high. Many welders crank the flow to 40-50 CFH thinking more gas equals better coverage. Above 30-35 CFH in a still shop, you’re wasting gas. The excess flow creates turbulence that actually degrades shielding. Set it to 20-25 CFH and only increase if you have drafts.
Pre-flow and post-flow settings. Machines with adjustable pre-flow and post-flow add gas usage before and after each weld. Pre-flow is usually 0.5-1 second. Post-flow ranges from 2-10 seconds. On short tack welds, the post-flow gas consumption can exceed the actual welding gas consumption. Reduce post-flow to 3-5 seconds for general work.
Wind. Welding in drafty conditions forces higher flow rates to maintain coverage. If you’re running 40+ CFH to fight a breeze, you’re burning through gas at nearly double the normal rate. Use wind screens or switch to flux-core for outdoor work.
Tracking Gas Usage
Keep a simple log to predict when you’ll need a refill:
- Note the regulator pressure when you start a session (e.g., 2,100 PSI on a full 80 CF tank)
- Note it again when you finish
- Track the pressure drop per session
For high-pressure inert gas cylinders, the relationship between pressure and remaining gas is roughly linear. A half-pressure reading (1,100 PSI on a 2,200 PSI full cylinder) means roughly half the gas remains.
CO2 is different. Because it’s stored as a liquid, the cylinder pressure stays nearly constant (about 830 PSI at 70F) until all the liquid has evaporated. Then the pressure drops rapidly. The only reliable way to track CO2 level is by weight. Weigh the cylinder and subtract the tare weight stamped on the shoulder.