A pump catalog from Germany lists capacity in m³/h. The US fire-protection code calls for a flow density in GPM/ft². The HVAC duct designer next door spec’d it in CFM. They’re all volumetric flow rates, all describing the same physical thing — but the unit choice depends on industry and continent. This guide gives you the full conversion matrix between the units you’ll actually meet, the standard formulas, and a cheat sheet of which industry uses which.
Jump to a section
- The six units that cover 95% of cases
- Which industry uses which unit
- Full conversion matrix (any unit to any unit)
- Worked example: matching a German pump to a US system
- Common engineering benchmarks
- Quick converter links
- FAQ
The six units that cover 95% of cases
You’ll see dozens of variants in spec sheets (gallons per second, liters per day, cubic decimeters per minute), but six units cover the vast majority of real engineering work:
| Unit | Symbol | Where it dominates |
|---|---|---|
| Gallons per minute | GPM (gal/min) | US plumbing, irrigation, fire protection |
| Liters per minute | LPM (L/min) | International plumbing, automotive cooling |
| Cubic meters per hour | m³/h (or CMH) | European pumps, industrial water, water treatment |
| Liters per second | L/s | SI engineering, drainage, large-scale water |
| Cubic feet per minute | CFM (ft³/min) | HVAC airflow, compressed air, ventilation |
| Cubic meters per second | m³/s | Hydrology, civil engineering, river flow |
For lighter use, gallons per hour (GPH) for slow trickle systems, and CFM-on-water (rare but it appears in legacy US plumbing code) round out the long tail.
Which industry uses which unit
Knowing the dominant unit in your industry saves you from converting unnecessarily — and from errors when an off-region spec sheet ambushes you.
| Industry | Primary unit | Secondary | Notes |
|---|---|---|---|
| US residential plumbing | GPM | — | Faucets, showers, water heaters all rated in GPM |
| US fire protection (NFPA) | GPM | GPM/ft² (density) | NFPA 13 sprinkler design uses density figures |
| US irrigation | GPM | gallons per hour for drip | Turf design uses GPM at the head |
| EU plumbing | LPM | L/s | Bathroom fittings standardized in LPM |
| EU pump industry | m³/h | LPM | Manufacturer catalogs (Grundfos, Wilo) lead with m³/h |
| Water treatment (global) | m³/h | L/s | Plant capacity always in m³/h or m³/d |
| HVAC (air, all regions) | CFM (US) / L/s (metric) | m³/h | Air flow only — never use GPM for air |
| HVAC (chilled/condenser water) | GPM (US) / L/s (metric) | m³/h | Hydronic systems only |
| Compressed air | CFM | L/min, m³/h | SCFM is “standard” CFM (corrected for pressure/temp) |
| Hydrology / civil | m³/s (cumecs) | ft³/s (cusec) | Rivers, drainage, stormwater |
A quiet trap: in mixed buildings (a US-built chiller plant with imported European pumps), the GPM-vs-m³/h boundary moves from system to system. Always verify which units the data sheet you’re reading uses before converting.
Full conversion matrix (any unit to any unit)
To convert FROM the row unit TO the column unit, multiply by the value in the cell. All conversions use US gallons (UK Imperial gallons need a separate factor — see GPM to LPM article).
| FROM ↓ / TO → | GPM | LPM | m³/h | L/s | CFM | m³/s |
|---|---|---|---|---|---|---|
| GPM | 1 | 3.78541 | 0.227125 | 0.0630902 | 0.133681 | 6.3090×10⁻⁵ |
| LPM | 0.264172 | 1 | 0.06 | 0.016667 | 0.0353147 | 1.6667×10⁻⁵ |
| m³/h | 4.40287 | 16.6667 | 1 | 0.277778 | 0.588578 | 2.7778×10⁻⁴ |
| L/s | 15.8503 | 60 | 3.6 | 1 | 2.11888 | 0.001 |
| CFM | 7.48052 | 28.3168 | 1.69901 | 0.471947 | 1 | 4.7195×10⁻⁴ |
| m³/s | 15850.3 | 60000 | 3600 | 1000 | 2118.88 | 1 |
Reading the matrix: to convert 100 m³/h into GPM, find row m³/h × column GPM = 4.40287. So 100 × 4.40287 ≈ 440 GPM.
For higher precision and direct conversion, use the unit-specific converters (linked in the Quick converter links section below) — they carry 13 decimal places and handle the swap automatically.
Worked example: matching a German pump to a US system
A US contractor needs to replace a hydronic-loop circulator in a chiller plant. The original is a US-built Bell & Gossett rated at 80 GPM at 30 ft of head. The replacement is a Grundfos UPS rated at 18 m³/h at 9 m of head. Are they equivalent?
Step 1 — Convert flow:
Step 2 — Convert head:
Step 3 — Compare:
The Grundfos pump is within 1% on flow and 1.5% on head — close enough to substitute directly for most balancing applications. If the numbers came out 5–10% apart, you’d revisit the system curve and confirm the replacement isn’t undersized at part-load operating points.
Common engineering benchmarks
A few flow rates worth memorizing because they show up everywhere:
- Residential cold-water service: 8–15 GPM (30–57 LPM, 1.8–3.4 m³/h)
- NFPA 13 light-hazard sprinkler design density: 0.10 GPM/ft² (= 4.07 LPM/m²) over 1500 ft² (NFPA 13D residential is the lower 0.05 GPM/ft²)
- Standard US shower head: 2.5 GPM = 9.46 LPM
- WaterSense low-flow shower: 2.0 GPM = 7.57 LPM
- HVAC chilled-water rule of thumb: 2.4 GPM per ton of cooling (= 0.043 L/s per kW)
- HVAC condenser-water rule of thumb: 3 GPM per ton (= 0.054 L/s per kW)
- Residential well pump (typical): 8–25 GPM = 30–95 LPM
- Pool pump (residential inground): 30–80 GPM = 6.8–18.2 m³/h
- Air change rate (typical office): 4–6 ACH ≈ 1 CFM/ft² of floor area
- Compressed air shop air gun: 2–5 SCFM at 90 psi
- Storm sewer design (residential street): 1–10 ft³/s = 0.028–0.28 m³/s
Quick converter links
xconvert has dedicated calculators for every common pair. Bookmark the ones that match your industry:
Plumbing / pumps (US ↔ metric):
HVAC (air):
Industrial / water treatment:
Hydrology / civil:
The full Volume Flow Rate catalog has every other pair you might need.
Frequently Asked Questions
Why do US and metric pumps use such different units?
Historical convention. The US plumbing industry standardized on gallons per minute in the early 20th century when most fixtures were sized in gallons. Continental Europe never adopted the gallon, and instead built engineering culture around the litre and the cubic meter. International standards (ISO, IEC) prefer SI units — m³/h, L/s, m³/s — so newer global standards lean metric even when individual markets still write specs in legacy units.
What’s the difference between m³/h and CMH?
Nothing. m³/h, m^3/h, CMH, and “cubic meters per hour” all mean the same unit. CMH is mostly seen in HVAC and air-handler datasheets; m³/h is the SI-style notation. Both are 1000 LPH or 16.667 LPM.
When should I use L/s instead of m³/h?
L/s is more useful when you’re working with small flows or process control, because the numbers stay manageable (a 5 L/s flow vs 18 m³/h — same thing, but 5 reads more naturally). m³/h is dominant in pump catalogs and large water-system specifications because typical flows there are bigger and the m³/h numbers stay reasonable. The conversion is exact: 1 L/s = 3.6 m³/h.
Is CFM the same for water as for air?
Numerically yes — a cubic foot is a cubic foot regardless of fluid. But practically, you almost never see CFM applied to water. US plumbing uses GPM; HVAC uses CFM only for air. If you find a water spec in CFM (rare, mostly in older industrial documentation), convert it to GPM (× 7.48052) for clarity.
What does SCFM mean?
“Standard CFM” — a CFM value corrected to a reference temperature and pressure (commonly 60 °F and 14.696 psia in US compressed-air practice). It exists because gas volume changes with pressure: a compressor delivering 100 SCFM moves the same mass of air no matter what the operating pressure is, even though the actual cubic feet per minute varies. For incompressible fluids (water, oil), the “S” is meaningless.
How do I convert flow rate to mass flow rate?
Multiply by density. For water at 4 °C: 1 m³ = 1000 kg, so 1 L/s ≈ 1 kg/s. At 20 °C, water density is 998.2 kg/m³, so the difference is under 0.2%. For air, oil, or gases, density varies enough that the conversion needs the actual fluid density and operating conditions. Mass flow stays constant through a system; volumetric flow doesn’t (it changes with temperature and, for compressible flows, with pressure).
Does pump head convert the same way as flow?
No. Pump head is conventionally given in feet of water column (US) or meters (metric). The conversion is 1 ft = 0.3048 m, regardless of fluid — but the pressure that head represents depends on the fluid’s density. For water, 1 ft head ≈ 0.433 psi. For diesel, kerosene, or oil, the same head represents a smaller pressure because the fluid is less dense.
Try it now
If you have a spec in one unit and need it in another, jump straight to the right xconvert flow-rate converter — no math, full precision. For a deeper dive on the GPM-vs-LPM conversion specifically (including the US-vs-UK gallon trap), see GPM to LPM Conversion: Formula, Calculator, and Reference Table.