An air gage (also spelled air gauge) is a measuring tool that uses a small flow of air, instead of a contact point, to check the size of a bore or shaft. You slide an air plug into a hole or an air ring over a shaft, and a readout shows the size to 0.00005″ or finer in about a second. Machine shops use air gages when they measure the same feature over and over to tolerances of a few tenths (.0001″–.0005″).
What an air gage looks like
A complete air gage has four parts. Most people picture only the first two.
| Part | What it is |
|---|---|
| Air tooling | A precision plug (for bores), ring (for shafts) or snap gage, made to one nominal size. Small jets in its surface do the measuring. |
| Readout | The display. It turns air pressure into a size and shows it against your tolerance, often with green, yellow and red zones. Older shops use dial or column readouts; newer ones use digital readouts like the Metro M5. |
| Setting masters | Certified rings or plugs of known size. You set the gage on them before measuring parts. |
| Air supply | Shop air, cleaned and held steady by a filter-regulator. See air supply requirements. |
A bench station such as the Air Gage Pro puts the readout, filter-regulator and quick-connects in one housing, so you plug in shop air and power and add your tooling.
How it measures, in one paragraph
Air flows out of the jets in the plug or ring. When the part surface is close to a jet, the air has a hard time escaping and pressure rises; when the surface is farther away, pressure drops. The readout converts that pressure change into a size. Because only air touches the part, nothing wears, scratches or deflects it, and the airflow blows coolant and chips off the measuring spot. For the full explanation, read how air gaging works.
Types of air gages
| Type | Measures | Typical use |
|---|---|---|
| Air plug (probe) | Inside diameters (bores) | Valve bodies, hydraulic cylinders, bearing seats, gun barrels. Thru-hole, blind and super-blind styles. |
| Air ring | Outside diameters (shafts, pins) | Spools, pistons, pins and ground shafts |
| Air snap gage | ODs, thickness or width from one side | Large shafts or measuring on the machine |
| Multi-jet and special tooling | Several features at once, or form | Taper, out-of-round, straightness, or many diameters on one part. See measuring form with air gaging. |
Not sure whether you need a plug or a ring? Read air ring vs. air probe.
When an air gage is the right tool
An air gage is a good fit when most of these are true:
- Tight tolerance. The feature is held to about .005″ total or tighter, often to tenths.
- Repeat measuring. The same size is checked many times a shift, in production or at the machine.
- Several operators. Results need to match between people and pass a gage R&R study.
- Delicate or dirty parts. Soft, polished or thin-walled parts, or parts covered in coolant.
- Data. You want readings in SPC software, Excel or a PLC. See getting air gage data into SPC, Excel or a PLC.
It is the wrong tool when you measure many different sizes in small lots, when tolerances are loose, or when you need a long measuring range. Each air plug or ring covers a short range around one nominal size, typically a few thousandths of an inch.
Air gage vs. other bore and shaft gages
| Air gage | Dial bore gauge | Micrometer / caliper | |
|---|---|---|---|
| Touches the part | No | Yes | Yes |
| Typical resolution | 0.00005″ or finer | 0.0001″–0.0005″ | 0.0001″–0.001″ |
| Speed per reading | About 1–2 seconds | 10 seconds or more (rocking to find size) | Several seconds |
| Operator skill needed | Low | High | Medium |
| Range of sizes per tool | One nominal size | Wide, with extensions | Wide |
More detail: air gage vs. dial bore gauge accuracy and why use air gaging.
Air gage terms you'll hear
- Jet (nozzle): the small opening in the tooling the air flows out of.
- Back-pressure: the pressure that builds up behind the jets; it changes with size.
- Single master / dual master: setting the gage on one master or on a min and max master. See single vs. dual master.
- Amplification: how much the readout magnifies the change in size, which sets range and resolution. See amplification explained.
- Restrictor: a fixed orifice between the regulator and the tooling that sets the gage's sensitivity.
- Mastering: setting the readout on the masters, usually at the start of a shift. See mastering and troubleshooting.
What an air gage costs
Digital readouts start at $1,595 for a Metro M5 1-input display. A complete Air Gage Pro bench station is $2,895. Dual-master air plugs start at $605, and masters are made to your size. The air gage price guide adds up complete systems.
Quick answers
Is it "air gage" or "air gauge"?
Both are correct and mean the same tool. "Gage" is the usual spelling in U.S. manufacturing; "gauge" is the general English spelling.
How accurate is an air gage?
Air gages resolve to 0.00005″ or finer and repeat very closely between readings and operators. Real-world accuracy depends on the masters, clean regulated air and stable part temperature. See 5 common reasons for incorrect readings.
Do I need a separate gage for every size?
You need separate tooling and masters for each nominal size, but one readout can run many gages. Swap the plug or ring and master again.
How small a hole can an air gage measure?
Standard air plugs start at .059″ (1.5 mm), and we have built special plugs for bores as small as about .038″. Price your size with the online quote. Blind holes need a blind or super-blind plug; see how close to the bottom of a blind hole an air gage can measure.
Do air gages need special air?
Clean, dry, oil-free shop air at about 60 psi, through a filter-regulator. The air gage itself uses only a small, steady flow.
Prices are in U.S. dollars as of October 2026 and may change; the price on each product page is current.