LPM to GPM

Two Units, One Flow Rate

LPM to GPM is the conversion from litres per minute (L/min) to gallons per minute (gal/min) — the two most widely used volumetric flow rate units in the world. Countries using the International System of Units (SI) measure liquid flow in LPM, while the United States uses GPM across pump engineering, residential plumbing, agricultural irrigation, and industrial fluid systems.

Why This Conversion Is Needed

When a European centrifugal pump rated at 50 LPM must interface with a North American system rated in GPM, or when comparing metric HVAC chiller specifications against US equipment datasheets, this conversion bridges the two measurement standards. The exact factor is 1 LPM = 0.264172 US GPM.

Example: 10 LPM × 0.264172 = 2.642 GPM

LPM — Litres Per Minute (L/min)

LPM (litres per minute, symbol L/min) is an SI-derived unit of volumetric flow rate. It expresses how many litres of liquid pass through a cross-section of a pipe or system every minute. LPM is the standard flow rate unit across the European Union, Australia, India, China, Japan, Brazil, and most countries that use the International System of Units (SI).

Common Applications of LPM

LPM appears in centrifugal pump performance curves, submersible borehole pump datasheets, HVAC chilled water and hot water circuits, reverse osmosis (RO) water treatment plants, hydraulic power units (HPU), medical oxygen delivery systems, and industrial process fluid control. A domestic kitchen tap typically flows at 6–9 LPM; a municipal fire hydrant can exceed 1,500 LPM.

Example: A pump rated 60 LPM delivers 60 litres every minute.

GPM — Gallons Per Minute (gal/min)

GPM (gallons per minute, symbol gal/min) is a United States customary unit of volumetric flow rate. It is the dominant flow rate standard for pump ratings, plumbing fixtures, irrigation controllers, pressure washers, fire suppression sprinkler systems, and industrial fluid equipment in North America.

US GPM vs. Imperial GPM

Two gallon definitions exist. The US liquid gallon equals exactly 3.785411784 litres as defined by the National Institute of Standards and Technology (NIST). The Imperial gallon equals exactly 4.54609 litres under the UK Weights and Measures Act 1985. The two standards differ by approximately 20%, so always confirm which applies before converting pump or irrigation specifications.

Example: A WaterSense‑certified showerhead at 2.0 GPM uses 7.571 litres per minute.

Unit System & Origin

LPM belongs to the International System of Units (SI), derived from the litre — a unit of volume equal to one cubic decimetre (1 dm³). GPM belongs to the United States customary system, derived from the US liquid gallon defined under federal law. The two units measure identical physical quantities but are rooted in incompatible measurement traditions.

Where Does 0.264172 Come From?

The conversion factor 0.264172 is derived directly from the NIST definition of the US liquid gallon (3.785411784 L). Dividing 1 ÷ 3.785411784 gives 0.26417205235815. Rounded to six significant figures: 0.264172. This factor is constant for all LPM→US GPM conversions.

Imperial GPM Formula

For Imperial GPM (used in the United Kingdom and Commonwealth countries): divide LPM by 4.54609 (the Imperial gallon in litres), giving the factor 0.219969. The two formulas are not interchangeable.

Step‑by‑Step Calculation

Apply GPM = LPM × 0.264172. Multiply the known litres-per-minute value by the US gallon conversion factor.

Worked example — centrifugal pump: A Grundfos CM5‑5 pump runs at 75 LPM.
75 × 0.264172 = 19.813 US GPM.

Reverse Calculation: GPM to LPM

Multiply GPM by 3.785411784 (the US gallon in litres).
Example: 5 GPM × 3.785411784 = 18.927 LPM.

Formula: GPM = LPM × 0.264172

The Exact Value

1 LPM = 0.264172 US GPM — roughly one‑quarter of a US gallon per minute. This is the value defined by the NIST gallon standard (3.785411784 litres per US gallon).

US vs. Imperial

1 LPM = 0.219969 Imperial GPM based on the UK Imperial gallon (4.54609 litres). Imperial GPM is used in United Kingdom pump standards, British Standards Institution (BSI) specifications, and older Commonwealth engineering documents. In all other contexts, assume US GPM unless a document explicitly states otherwise.

LPM to US GPM & Imperial GPM Table

All US GPM values use the NIST definition (1 US gallon = 3.785411784 L, factor 0.264172). Imperial GPM values use the UK definition (1 Imperial gallon = 4.54609 L, factor 0.219969).

The US Gallon Standard (NIST)

The US liquid gallon is defined as exactly 3.785411784 litres by the National Institute of Standards and Technology (NIST). Multiplying any LPM value by 0.264172 gives US GPM. This factor governs pump ratings published in ANSI/HI standards, fixture flow limits in the Uniform Plumbing Code (UPC), WaterSense‑label thresholds, and National Fire Protection Association (NFPA) sprinkler design flows.

When US GPM Applies

Any pump datasheet, faucet label, irrigation controller spec, or pressure washer rating produced in the United States refers to US GPM unless explicitly marked “Imperial.” All conversions on this site default to US GPM.

Formula: US GPM = LPM × 0.264172

The Imperial Gallon (UK Weights & Measures)

The Imperial gallon is defined as exactly 4.54609 litres under the UK Weights and Measures Act 1985 — approximately 20% larger than the US gallon. Multiplying LPM by 0.219969 converts to Imperial GPM. This standard is referenced in British Standards Institution (BSI) specifications, older Canadian pump catalogues, and engineering documents from Commonwealth countries.

Why the Difference Matters

Confusing US GPM with Imperial GPM introduces a systematic 20% error. A pump spec reading “10 GPM” in a BSI document delivers 45.46 L/min, not 37.85 L/min as a US GPM reading would imply. Always verify the gallon standard before sizing pipes, valves, or pressure vessels.

Formula: Imperial GPM = LPM × 0.219969

Municipal & Domestic Water Supply

Water supply networks, potable water treatment plants, and municipal distribution mains are designed in LPM (or m³/h) in metric countries and in GPM in the United States. When an ISO‑rated Grundfos or Xylem water pump must be matched to an American Waterworks Association (AWWA) system specification, LPM→GPM conversion aligns pipe sizing, pressure ratings, and electromagnetic flow meter calibration.

Unit Cross‑Reference

Domestic utility meters often record in cubic metres per hour (m³/h). Cross‑conversion: 1 m³/h = 16.667 LPM = 4.403 US GPM. Apply the same 0.264172 factor for any volume in the same time base.

Example: 30 LPM water flow = 7.925 US GPM

Cross‑Market Pump Sizing

Pump manufacturers in Europe (Grundfos, KSB, Wilo, DAB) and Asia (Ebara, Leo) publish performance curves in LPM or m³/h. US manufacturers (Goulds, Bell & Gossett, Armstrong, Pentair) publish in GPM. Converting LPM to GPM is essential when specifying a metric‑rated centrifugal pump, submersible pump, or end‑suction pump for a North American facility.

Reading the Pump Curve

The LPM→GPM ratio is constant (× 0.264172), but the pump duty point — the intersection of the pump curve and system resistance curve — shifts with pipe diameter, head loss, and static lift. A centrifugal pump rated at 120 LPM at 20 m head delivers 31.7 GPM at the same duty point.

Example: 120 LPM pump = 31.7 GPM capacity

Irrigation Zone Flow Calculation

Irrigation controllers from Rain Bird, Hunter Industries, and Toro use GPM for zone flow budgeting in North America. Netafim, Irritec, and European equipment rate emitters and drip lines in LPM. When mixing equipment or programming a multi‑brand system, LPM→GPM conversion ensures accurate zone totals and prevents pipe undersizing.

Typical Flow Rates

Pressure‑compensating drip emitters output 1–8 LPM (0.26–2.1 GPM). Hunter MP Rotator heads run at 10–40 LPM (2.6–10.6 GPM). Sum all emitter flow rates in a zone in consistent units before comparing against the rated capacity of the supply valve or lateral pipe.

Example: 20 LPM irrigation zone = 5.283 GPM demand

Plumbing Codes & Fixture Flow Limits

The Uniform Plumbing Code (UPC) and International Plumbing Code (IPC) specify maximum fixture flow rates in GPM. The US Environmental Protection Agency (EPA) WaterSense programme sets showerhead limits at 2.0 GPM (7.57 LPM) and lavatory faucets at 1.2 GPM (4.54 LPM). Converting LPM→GPM allows cross‑verification of imported fixtures against these code thresholds.

Common Plumbing Flow Rates

A standard kitchen faucet certified under ASME A112.18.1 runs at 1.8 GPM (6.81 LPM). A low‑flow showerhead runs at 1.5 GPM (5.68 LPM). Rinnai and Navien tankless water heaters specify minimum activation flow at 1.5–3 LPM (0.4–0.8 GPM) on metric models.

Example: 8 LPM faucet flow = 2.113 GPM

Cleaning Units & Flow Rate

Pressure washers are rated by operating pressure (PSI or bar) and volumetric flow rate (GPM or LPM). Cleaning power is quantified as Cleaning Units (CU) = PSI × GPM. US brands (Simpson, Sun Joe, DeWalt) publish in GPM; European brands (Kärcher, Nilfisk, Bosch) publish in LPM. Converting LPM→GPM enables direct CU comparison between models.

Practical Examples

A consumer Kärcher K5 running at 8 LPM delivers 2.113 GPM. A professional cold‑water unit at 15 LPM delivers 3.963 GPM. Chemical injection ratios specified in GPM terms must be re‑calculated in LPM to ensure correct detergent dosing on metric machines.

Example: 8 LPM pressure washer = 2.113 GPM flow rate

Hydraulic Power Units & Component Matching

Hydraulic pumps (gear, vane, piston), hydraulic motors, and double‑acting cylinders manufactured outside North America are rated in LPM according to ISO 4391 and ISO 4413. Parker Hannifin, Bosch Rexroth, and Eaton publish US datasheets in GPM. When integrating a Danfoss or Linde hydraulic motor into a US‑designed system, LPM→GPM conversion prevents flow mismatches that overpressure directional control valves and damage hydraulic seals.

Cylinder Fill Time & Motor Speed

Hydraulic cylinder extension velocity (mm/s) and motor shaft speed (RPM) are both functions of flow rate. A 40 LPM (10.567 GPM) pump driving a 50 mm bore cylinder produces a specific rod velocity calculable only if flow is in consistent units. Always confirm rated flow at the test pressure specified in the pump datasheet.

Example: 40 LPM hydraulic pump = 10.567 GPM

Converting US GPM Back to Litres Per Minute

Multiply GPM by 3.785411784 — the exact NIST value of the US liquid gallon in litres. This reverse conversion is required when a US‑rated pump, valve, or flow meter must be re‑specified in SI units for international project documentation, EU CE declarations, or ISO 9001 quality records.

Common Reverse Conversions

5 GPM × 3.785411784 = 18.927 LPM. A Goulds 3196 pump at 100 GPM delivers 378.54 LPM. Use the GPM → LPM toggle on the calculator above to convert any value instantly.

GPM to LPM Reference Table

US GPM values converted to LPM using the factor 3.785411784 (NIST US gallon definition).

Volumetric Flow Rate Units

Volumetric flow rate measures how much fluid volume passes a cross‑section per unit time. Common SI and customary units include litres per minute (L/min), gallons per minute (gal/min), cubic metres per hour (m³/h), cubic feet per minute (CFM), litres per second (L/s), and cubic feet per second (CFS). All measure the same physical quantity — only the scale differs.

Choosing the Right Conversion Factor

The most critical step in any flow rate conversion is identifying which gallon standard or cubic unit the source document uses. The US liquid gallon (NIST, 3.785411784 L) and the Imperial gallon (UK Weights & Measures, 4.54609 L) differ by 20.1%. Using the wrong factor introduces a systematic error in every pipe size, valve rating, and pump selection downstream. All conversions on this site use the US liquid gallon unless a section explicitly states Imperial.

Use the live calculator at the top of this page for instant LPM ↔ GPM conversion. For other unit combinations — CFM, m³/h, LPS, CFS — visit the calculator library below.