Questions, answered with numbers

It depends entirely on which air tools. Burst tools - impacts, nailers, blow guns - run fine on 4 CFM with a 6 to 20 gallon tank. Continuous tools - ratchets held down, die grinders, sanders, spray guns - need the pump to keep up alone, so 5 CFM minimum and 12 CFM or more for sanding.

For nailing and inflation, 1 to 6 gallons. For burst automotive work, 6 to 20. For continuous tools, tank size is not the answer at all - you need pump output. Compare gallons multiplied by maximum PSI rather than gallons alone, because a 6-gallon tank at 200 PSI holds more usable air than an 8-gallon tank at 120.

No, and it is the most expensive misunderstanding in this category. The tank stores air; the pump supplies it. A big tank lets you draw more than the pump makes for longer, which helps burst tools and does nothing for continuous ones.

Manufacturers publish it as a duty-cycle average: the volume of free air a tool consumes per minute at a stated pressure over a stated load factor, usually 25%. That is why the same impact wrench can be listed at 4.2 CFM average and 22 CFM at load - neither is wrong, they answer different questions.

The pump-up test: drain the tank, disconnect everything, time the fill from 0 PSI to cut-out, then compute tank cubic feet times (final PSI divided by 14.7) divided by fill time in minutes. It gives average delivered CFM across the fill rather than CFM at a specific pressure.

4 to 5 CFM average at 90 PSI per the published figures, which is about 6.25 CFM after a sizing margin. Ingersoll Rand also publishes 22 CFM at load for its 231C, which is what the tank covers during the burst rather than what the pump must produce.

Ingersoll Rand publishes 4 CFM for a touch-up gun, 8 for a production gun and 19 for an undercoat gun. But spraying is the last step - the 6 to 10 CFM DA sander that prep requires is the real constraint, and no portable compressor supplies it.

Yes if you will own two or more air tools, or want a blow gun - there is no cordless equivalent to one. Probably not for a single tool with a good cordless version, and almost certainly not if the only job is tires, where a 12V pump in the car is more useful.

For nailing, genuinely yes. A 1-gallon unit at 0.7 CFM comfortably exceeds a brad nailer's published 0.5 CFM demand and weighs 20 pounds. It will not run any automotive air tool, so buy the 6-gallon unit if that is on your horizon.

If your garage adjoins a living space or you work in the evening, yes - 60 dBA against 82 dBA is a large real difference and a compressor you avoid using has no value. Just know that the quiet models in the small sizes deliver less flow than a cheaper pancake.

A portable tank compressor is worth it if you actually carry it. A 12V inflator is worth it if the job is tires, because it lives in the vehicle. Neither is worth buying as a substitute for the other - a 12V unit cannot run any air tool at all.

An oil-lubricated pump runs its pistons in an oil bath that reduces friction and carries away heat; an oil-free pump uses permanently lubricated seals. Oil-lubricated runs cooler and lasts longer but needs maintenance. Oil-free needs none and wears out sooner - and for burst work it is usually the right choice.

Single stage, almost certainly. Two stage makes sense for continuous 10-plus CFM tools, two tools at once, or hours of running. For nailing, tires, impacts and occasional grinding it is a 240V project you do not need.

3/8-inch internal diameter is the right default - it covers every tool up to a die grinder without starving an impact wrench. Ingersoll Rand specifies 3/8-inch minimum for its 1/2-inch impact, and most compressor kit hoses are 1/4-inch.

Two separate measurements: the pipe thread size, usually 1/4, 3/8 or 1/2 inch NPT, and the quick-connect pattern - M-STYLE, ARO, Lincoln or Tru-Flate. A fitting can have the right thread and the wrong pattern, which is why this is confusing.

Volts times amps: a 120V unit drawing 10 A uses about 1,200 W while the pump runs, and one drawing 11.5 A about 1,380 W. Energy use over an hour is small, because the pump only runs for a few minutes of it.

In order of likelihood: the tank drain valve is not fully closed, a leak in the fittings or hose, a failed check valve letting tank air bleed back, a pressure switch not tripping, or a tool drawing more air than the pump produces.

A 20-gallon unit at 4.0 CFM makes it practical zone by zone; a 30-gallon at 5.7 CFM makes it comfortable. Volume matters more than tank here, and pressure must be limited to whatever your irrigation system's own documentation specifies.

No, and we say so on every page. Recommendations come from manufacturer-published specifications, two published engineering references for tool demand, and a scoring formula published in full on our methodology page. Nothing here implies testing that has not happened.