Air Compressor CFM Calculator — HP, PSI & Air Tool Demand

Two directions. To size a compressor, add each tool's CFM at 90 PSI multiplied by its duty cycle (a die grinder is continuous work; a framing nailer fires seconds at a time), then apply a simultaneity factor if several people share the line. To measure a compressor you already own, use the tank-fill test: CFM = (tank gallons × ΔP) ÷ (7.48 × 14.7 × fill minutes). The calculator above runs both — the demand tab builds the shopping number, the test tab audits the machine in the garage.

Drain the tank, run the compressor from cut-in to cut-out pressure, and time it. A 60-gallon tank filling from 40 to 120 PSI (ΔP = 80) in 5 minutes delivers (60 × 80) ÷ (7.48 × 14.7 × 5) ≈ 8.7 CFM at that mean pressure — roughly the pump's free-air rating with real-world losses included. Within 10% of the nameplate means healthy; substantially lower points to intake filter clogging, belt slip, or worn rings and valves.

The working heuristic is 1 HP ≈ 3.5–4 CFM delivered at 90 PSI for single-stage shop compressors, so 7 CFM at 80 PSI maps to roughly 2–2.5 HP — a common small-shop pancake or twin-stack rating. Two-stage industrial pumps do better (4–5 CFM per HP), and 'peak HP' marketing numbers do far worse; always compare delivered CFM at a stated PSI, never the horsepower sticker.

Use absolute-pressure ratios: CFM₂ = CFM₁ × (P₁_abs ÷ P₂_abs) with P_abs = gauge PSI + 14.7. A tool that moves 8.75 CFM at 90 PSI (104.7 PSIA) moves 8.75 × (104.7 ÷ 54.7) ≈ 16.7 CFM at 40 PSI (54.7 PSIA) — because lower discharge pressure means less compression work per cubic foot. The same formula in reverse tells you what a 40 PSI spec is worth at 90.

Multiply each tool's CFM at working pressure by its duty cycle measured on the floor (a stopwatch over ten minutes beats any datasheet), sum the results per zone, apply a simultaneity factor — typically 0.6–0.8 because not every station cycles at once — and add 10–25% for leaks. A healthy line leaks 10%; an ignored one loses 30% of compressor output through fittings and cracked hoses before a single tool fires.

Cylinder volume = π × (bore/2)² × stroke, in cubic inches; air used per cycle = volume × (working PSIA ÷ 14.7) for both directions if the rod side is included. A 2-inch bore, 10-inch stroke cylinder at 90 PSI: π × 1² × 10 = 31.4 in³ = 0.136 ft³ per extension, ×(104.7 ÷ 14.7) ≈ 0.97 ft³ of free air per stroke. At 30 cycles per minute that is about 29 CFM — before the return stroke.

Dryers are rated in CFM at inlet conditions, and the inlet is hot and saturated — derate the compressor rating by about 20% when matching: a 25 CFM compressor wants a dryer rated 30+ CFM at its rated inlet temperature and pressure. Refrigerated dryers hit dewpoints of 35–40°F (fine for shop tools); desiccant dryers reach −40°F for painting and instrumentation, at the cost of 10–15% purge air drawn from your own supply.

Vacuum pump CFM is displacement flow at the pump inlet, and it falls as the vessel evacuates because each stroke removes a fixed fraction of what remains: t = (V ÷ S) × ln(P₀ ÷ P₁), with V in ft³, S in ft³/min and pressures absolute. A 6 ft³ tank from atmospheric (14.7 PSIA) to 3 PSIA with a 10 CFM pump: t = 0.6 × ln(4.9) ≈ 0.95 min. Air watts — the suction-power metric on vacuum cleaners — divide inlet vacuum (inches of water) by airflow (CFM) with unit constants; they rate suction work, not volume moved.

No — and the reason is dimensional, not regulatory. CFM is a flow rate (volume per time); bar, kPa and kg/cm² are pressure units. Asking for a CFM-to-bar conversion is like asking how many gallons per minute a tire has: any 'converter' online is either converting between flow units (CFM↔m³/h↔L/min) or assumes a hidden pressure to do a specific task. What you can convert: a given compressor's CFM at one pressure to its CFM at another (the ratio method above) — pressure changes the flow, but never equals it.

The homepage keeps the bare tank-fill formula for quick health checks. This page builds the full demand picture: tool stacking with duty cycles, HP matching, pressure-ratio conversions, cylinder consumption, dryer and line sizing. For converting a rating between reference conditions — SCFM vs CFM vs Nm³/h — the SCFM to CFM calculator handles the temperature, pressure and baseline bookkeeping; this page stays on sizing and measuring.