What size air compressor do I need for a farm workshop? — UK buyer's guide
The compressor size listed on the box is rarely the number that matters. Tank size tells you the reserve. CFM (or FAD in litres per minute) tells you whether the compressor can keep up with your tools. Get that wrong and a 200-litre tank still won't run a die grinder for more than 30 seconds at a time. This guide covers everything you need to choose right first time.
Free Air Compressor Size Calculator
Tick your tools, get your CFM and tank size recommendation
CFM reference — what each tool needs
CFM figures are at 90 PSI, which is the standard operating pressure for most air tools. Continuous-use tools (grinders, spray guns) must be matched to the compressor's sustained output — a large tank cannot compensate if the CFM is too low.
| Tool | CFM | L/min |
|---|---|---|
| Blow Gun / Air Duster | 2 | 57 |
| Car / Van Tyre Inflation | 2.5 | 71 |
| Tractor Tyre Inflation | 5 | 142 |
| ½″ Impact Wrench | 4.5 | 127 |
| ¾″ Impact Wrench | 8.5 | 241 |
| Air Chisel / Hammer | 6 | 170 |
| Die Grinder | 5 | 142 |
The 25% rule: always size the compressor at 25% above your peak CFM demand. A compressor working at 100% capacity runs hot, wears faster and will struggle to keep up in cold weather. The rule also gives headroom if you add a tool later.
FAD vs swept volume — the number that matters
Most UK compressor specs show two air output figures. One is the number to use. One is marketing.
FAD — Free Air Delivery
The actual measured output at the outlet, at a stated pressure (usually 7 bar / 100 PSI). This is what your tools will actually receive. Always compare compressors on FAD.
Swept Volume / Displacement
The theoretical maximum of the pump cylinder — always higher than what the compressor delivers. Budget brands use this to appear more capable than they are. Ignore it when comparing models.
A compressor advertising "350 L/min" in large text may show FAD of only 180 L/min in the small print. The 350 figure is swept volume. This is the most common confusion when buying — always check the FAD line on the specification sheet.
The converter: 1 CFM = 28.3 L/min. If a tool's manual quotes CFM, multiply by 28.3 to get the L/min FAD you need from your compressor.
Oil-free vs oil-lubricated — the farm verdict
Oil-free
Light / occasional use onlyNo oil changes
Lighter and portable
Lower upfront cost
Less maintenance
Runs significantly hotter
Louder under load
Shorter pump lifespan
Struggles with sustained use
Fine for the odd tyre top-up or a nail gun. Not suitable as a daily farm workshop compressor.
Oil-lubricated
Recommended for farm workshopsRuns cooler — handles long sessions
Quieter under load
Much longer pump life
Better for continuous-use tools
Regular oil checks required
Heavier and less portable
Higher upfront cost
The right choice for any farm workshop that uses an impact wrench, grinder or spray gun regularly.
Direct drive vs belt drive
Both types are oil-lubricated. The difference is how the motor connects to the pump — and it matters significantly for farm workshop longevity.
Direct Drive
Motor shaft connects directly to the pump. Fast build times, compact and lower upfront cost. Runs the pump at full motor speed — more heat, more wear, louder. Better suited to light workshop use.
Belt Drive — Recommended
A pulley and belt runs the pump at a lower RPM than the motor. Less heat, less noise, longer pump life. Higher upfront cost — paid back quickly in a farm workshop that runs the compressor daily.
For any farm workshop compressor above 50L that will see regular use, specify belt-drive. The pump runs at roughly half the RPM of the motor — this alone doubles the service life of the pump valves and piston rings in real-world use.
Single phase vs three phase — what your supply can run
The compressor's motor size determines the power supply needed. Buying the wrong spec for your supply is the most expensive mistake — you either can't start it or you need a new workshop consumer unit.
| Supply | Max motor | Typical compressor size |
|---|---|---|
| 13A single phase (standard socket) | ~2.2 kW / 3 HP | Up to 50L, ≤6 CFM |
| 16A single phase (blue plug) | ~2.2–3.0 kW / 3–4 HP | 50–100L, up to 9 CFM |
| 32A single phase (workshop feed) | ~4.0–5.5 kW / 5.5–7.5 HP | 100–200L, up to 16 CFM |
| 3-phase (400V) | 7.5 kW+ / 10 HP+ | 200L+, 20+ CFM industrial |
Large compressor motors have a high inrush current on startup — often 6–8× the running current. A 32A breaker that comfortably runs the motor at load may trip on start-up. If this happens, a soft-starter or star-delta starter is the fix — not a bigger breaker.
Four common farm workshop setups
These are worked examples — use the calculator below for your specific tools.
Light farm use
Tyre inflation (cars and light trailers) plus an occasional impact wrench. One tool at a time, infrequent use.
6–7
CFM
Recommended tank
50L
Type
Mid-range oil-lubricated
Power supply
16A single phase
Regular farm workshop
Impact wrench, tractor tyre inflation, occasional die grinder. The most common UK farm workshop setup.
8–9
CFM
Recommended tank
100L
Type
Workshop oil-lubricated, belt-drive
Power supply
16–32A single phase
Belt-drive recommended if die grinder or angle grinder in regular use
Heavy workshop / two users
Impact wrench and grinder running simultaneously, large AG tyre inflation, air chisel work, two people.
16–18
CFM
Recommended tank
150L
Type
Large belt-drive oil-lubricated
Power supply
32A single phase — check supply
At this demand level, verify your workshop has a 32A feed — standard 13A sockets cannot supply this motor size
Spray painting or sandblasting
HVLP spray gun or siphon sandblaster — both run continuously and demand sustained high flow.
15–18
CFM
Recommended tank
200L
Type
Large belt-drive, twin-cylinder
Power supply
32A single phase or 3-phase
For spray work the compressor must keep up in real time — a large tank alone is not enough. Undersizing causes runs and orange peel in the finish.
Tank size and maintenance — what they don't tell you
A bigger tank extends the time before the compressor needs to cut back in — useful for intermittent tools like impact wrenches. It does nothing for sustained-use tools like grinders. The compressor's CFM output is the ceiling; no tank overcomes a flow deficit.
Steel tanks rust internally. Every compressor cycle draws in atmospheric air, which contains moisture. That moisture condenses inside the tank and accumulates at the bottom. On a farm workshop compressor in regular use, drain the condensate at least once a week — more often in summer or humid conditions. Fit an auto drain valve and forget about it.
An undrained tank corrodes from the inside. The first sign is usually rust in the air line — by which point the tank wall may already be thinning. A tank failure under pressure is extremely dangerous. Check the tank exterior annually; any sign of external rust, weeping or deformation — take the compressor out of service immediately.
Compressors & accessories on Amazon
Always check the FAD figure on the spec sheet before buying — not just the tank size or motor HP.
50L Oil-Lubricated Workshop Compressor
Covers the light–medium farm workshop: tyre inflation, impact wrench, blow gun. Look for FAD ≥140 L/min on the spec sheet.
100L Belt-Drive Oil Compressor
The recommended buy for most UK farm workshops — covers ½″ impact, tractor tyres, die grinder and two tools at once.
150–200L Trade Belt-Drive Compressor
For heavy workshop demand, two simultaneous users or spray painting. Check for 3-phase supply at this power level.
Tractor Tyre Inflator Chuck & Hose
Heavy-duty clip-on chuck rated for large AG tyre pressures — standard inflators won't seal on tractor valves.
Auto Tank Drain Valve
Prevents internal rust — the most common cause of compressor tank failure. Fits most standard workshop tanks.
Retractable Air Hose Reel (10–15m)
Keeps the workshop tidy and the hose off the floor. 15m reaches the corners of most farm workshop bays.
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