GPM to LPM Calculator

Two Measurement Systems, One Flow Rate

GPM to LPM converts gallons per minute (gal/min) to litres per minute (L/min). The United States uses GPM for pump ratings, irrigation equipment, plumbing codes, and industrial fluid systems, while the International System of Units (SI) uses LPM. This conversion is required whenever a North American flow specification must be expressed in metric units.

When You Need This Conversion

Common situations include re-specifying a US centrifugal pump for a European project, comparing a North American irrigation controller against metric drip line ratings, or converting NFPA sprinkler flow requirements into SI units for international engineering. The exact factor is 1 GPM = 3.785411784 LPM.

Example: 10 GPM × 3.785411784 = 37.854 LPM

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 in the United States for pump engineering, residential plumbing, agricultural irrigation, fire suppression systems, and industrial process equipment.

US GPM vs. Imperial GPM

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. They differ by 20.1%. This calculator uses US GPM unless stated otherwise.

Example: A Goulds 3196 centrifugal pump at 100 GPM delivers 378.541 LPM.

LPM — Litres Per Minute (L/min)

LPM (litres per minute, symbol L/min) is an SI-derived unit of volumetric flow rate. It is the standard unit in the European Union, Australia, India, China, Japan, Brazil, and most countries using the International System of Units (SI).

Where LPM Is Used

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

Example: A Grundfos CM5‑5 pump rated 60 LPM delivers 60 litres every minute.

Unit System & Origin

GPM belongs to the United States customary system, derived from the US liquid gallon defined by NIST. LPM belongs to the International System of Units (SI), derived from the litre — equal to one cubic decimetre (1 dm³).

Step‑by‑Step Worked Example

Apply LPM = GPM × 3.785411784. Multiply the GPM value by the US gallon conversion factor to get litres per minute.

Example — residential booster pump: A Pentair Intelliflo runs at 15 GPM.
15 × 3.785411784 = 56.781 LPM.

Multiple Worked Examples

5 GPM × 3.785411784 = 18.927 LPM.
25 GPM × 3.785411784 = 94.635 LPM.
100 GPM × 3.785411784 = 378.541 LPM.

Formula: LPM = GPM × 3.785411784

The Exact Value

1 US GPM = 3.785411784 LPM — defined by NIST SP 811 (2008). One US gallon per minute equals approximately 3.785 litres per minute.

US vs. Imperial

1 Imperial GPM = 4.54609 LPM based on the UK Imperial gallon defined under the UK Weights and Measures Act 1985. Imperial GPM is referenced in British Standards Institution (BSI) pump specs and older Commonwealth documents. Always confirm which gallon standard the source document uses before converting.

LPM to GPM Formula

Multiply LPM by 0.264172 (the reciprocal of 3.785411784):
GPM = LPM × 0.264172.

Example: 50 LPM × 0.264172 = 13.209 GPM.

When You Need LPM to GPM

Use this when a metric pump from Grundfos, KSB, or Wilo must match a US system rated in GPM, or when comparing European HVAC chiller flow rates against ANSI/HI pump curves expressed in gallons per minute.

Formula: GPM = LPM × 0.264172

Reading US Flow Rate Labels

In the United States, flow rate is expressed in GPM on pump nameplates, faucet aerators, irrigation controllers (Rain Bird, Hunter Industries, Toro), pressure washer labels, and ANSI/HI pump curves. A label reading “25 GPM” means 25 US gallons (94.635 litres) pass through the system every minute.

Reading Metric Flow Rate Labels

Outside the United States, flow rate is expressed in L/min or m³/h. On Grundfos, KSB, or Wilo nameplates, “60 L/min” means 60 litres per minute — equivalent to 15.850 GPM. Cross-reference: 1 m³/h = 16.667 LPM = 4.403 GPM.

From Volume and Time

The direct method: LPM = Volume (litres) ÷ Time (minutes). Fill a calibrated container, measure elapsed time, then divide. Example: 30 litres in 2 minutes = 15 LPM.

From GPM

If you have a GPM value from a US pump datasheet or flow meter: LPM = GPM × 3.785411784. Example: 8 GPM from a Taco comfort circulator = 30.283 LPM.

Formula: LPM = Volume (L) ÷ Time (min)

The Single‑Step Method

Multiply gallons per minute by 3.785411784 to get litres per minute. This factor is the exact NIST definition of the US liquid gallon. No intermediate steps are required — the conversion is a single multiplication.

Verify Your Input Standard

If your source uses Imperial gallons (UK, older Canadian specs), multiply by 4.54609 instead. Mixing the two standards introduces a 20% systematic error in all downstream pipe sizing, pump selection, and flow meter calibration.

US formula: LPM = GPM × 3.785411784

Deriving the Formula

The US liquid gallon is defined as 231 cubic inches. Since 1 cubic inch = 16.387064 mL exactly (NIST SP 811), 1 US gallon = 231 × 16.387064 = 3,785.411784 mL = 3.785411784 litres. Therefore: LPM = GPM × 3.785411784. For Imperial GPM: LPM = GPM × 4.54609.

Common Volumetric Flow Rate Units

Common units: LPM (litres per minute), GPM (US gallons per minute), m³/h (cubic metres per hour), CFM (cubic feet per minute), L/s (litres per second), CFS (cubic feet per second). All describe the same physical quantity.

Key Conversion Relationships

From GPM: 1 GPM = 3.785 LPM = 0.2271 m³/h = 0.06309 L/s = 0.1337 CFM.
From LPM: 1 LPM = 0.2642 GPM = 0.06 m³/h = 0.01667 L/s.
Confirm the gallon standard (US vs. Imperial) before applying any GPM conversion. Other unit conversions are purely mathematical.

Bucket and Timer Method

Direct a flow into a calibrated container and time the fill. Divide volume (litres) by time (minutes). A 10-litre bucket filled in 45 seconds (0.75 min) = 13.33 LPM. Accuracy: ±2–5% for low-flow applications.

Electromagnetic & Ultrasonic Flow Meters

For permanently installed systems, electromagnetic meters (Endress+Hauser Promag, Siemens Sitrans FM) and ultrasonic clamp‑on meters (Keyence FD‑Q) output continuous LPM readings for SCADA, building management systems (BMS), and PLC integration. When a US meter outputs GPM, multiply by 3.785411784 to convert to LPM.

Reading the Performance Curve

Every centrifugal pump ships with an H‑Q performance curve. European and Asian manufacturers (Grundfos, KSB, Wilo, Ebara) publish in LPM or m³/h. US manufacturers (Goulds, Bell & Gossett, Armstrong, Pentair) publish in GPM. Locate the duty point, read the flow value, then multiply GPM by 3.785411784 to get LPM.

Pressure Differential Method

For in-service verification, measure differential pressure across a calibrated orifice plate or Venturi tube. Differential pressure transducers (Yokogawa EJX series) achieve ±0.1% accuracy. Convert the resulting flow value using this calculator.

Example: Pump curve shows 120 LPM at duty = 31.700 GPM

Zone Flow Demand Calculation

Sum the rated flow of every emitter in a zone. Pressure‑compensating drip emitters (Netafim PC, Raindrip) are rated in LPM or L/hr; rotary sprinkler heads (Hunter MP Rotator, Rain Bird R‑50) are rated in LPM or GPM. Convert all values to LPM before summing. Zone demand must not exceed the rated capacity of the lateral pipe or supply valve.

Converting GPM Controller Readings to LPM

If a Rain Bird ESP‑Me or Hunter Pro‑C controller shows zone flow in GPM, multiply by 3.785411784 to get the LPM demand. Example: a zone reading 5 GPM demands 18.927 LPM from the pump.

Example: 5 GPM zone demand = 18.927 LPM pump requirement

Where 3.785411784 Comes From

The US liquid gallon is defined as exactly 231 cubic inches. 1 cubic inch = 16.387064 mL (NIST SP 811, 2008). Therefore: 231 × 16.387064 = 3,785.411784 mL = 3.785411784 litres per US gallon. Dividing 1 by 3.785411784 gives 0.264172 — the LPM→GPM factor.

Imperial GPM Formula

For Imperial GPM (United Kingdom, some Commonwealth countries): LPM = GPM × 4.54609. The Imperial gallon is defined in the UK Weights and Measures Act 1985 as 4.54609 litres exactly.

GPM to LPM Table — US & Imperial

US LPM uses factor 3.785411784 (NIST). Imperial LPM uses factor 4.54609 (UK Weights & Measures Act 1985).

The NIST US Gallon Definition

When converting US GPM to LPM, the factor is 3.785411784 — the exact US liquid gallon volume codified in NIST SP 811 (2008). This governs all US commercial and industrial flow specifications: ANSI/HI pump standards, UL fire suppression listings, AWWA water main flow requirements, and EPA WaterSense fixture ratings.

When US GPM Applies

Any pump datasheet, plumbing code limit, irrigation controller flow budget, or pressure washer spec originating in the United States uses US GPM. Converting to LPM using 3.785411784 gives the SI‑equivalent flow rate for metric engineering documents, EU CE declarations, ISO 9001 quality records, and international project submissions.

Formula: LPM = US GPM × 3.785411784

The Imperial Gallon (UK Weights & Measures)

The Imperial gallon equals exactly 4.54609 litres under the UK Weights and Measures Act 1985. To convert Imperial GPM to LPM, multiply by 4.54609. This applies to British Standards Institution (BSI) pump specifications, older Canadian catalogues, UK water authority flow records, and Commonwealth construction documents.

Why the 20% Difference Matters

The Imperial gallon is 20.1% larger than the US gallon. Using the US factor on an Imperial value (or vice versa) introduces a 20% systematic error across all pipe sizing, valve selection, pump matching, and pressure drop calculations. Always confirm the gallon standard before converting.

Formula: LPM = Imperial GPM × 4.54609

Municipal & Potable Water Systems

Water supply networks and potable water treatment plants designed under American Water Works Association (AWWA) standards express flow in GPM. Converting to LPM is required for ISO-standard documentation, WHO water quality assessments, or when integrating US-rated Xylem or Grundfos pumping stations into European or Asian water treatment facilities.

Cross‑Reference with m³/h

Large system flow meters often display in m³/h. Cross-reference: 1 GPM = 0.2271 m³/h = 3.785 LPM. For large flows: 1,000 GPM = 3,785.41 LPM = 227.12 m³/h.

Example: 30 GPM water flow = 113.562 LPM

Cross‑Market Pump Specification

US pump manufacturers — Goulds Pumps, Bell & Gossett, Armstrong Fluid Technology, Pentair, Taco Comfort Solutions — rate equipment in GPM per ANSI/HI 1.1‑1.5 standards. When this equipment is imported for metric-standard facilities or documented under ISO 9906, GPM must be converted to LPM using 3.785411784.

Reading the Pump Curve

Multiply any point on the GPM axis by 3.785411784 to get the LPM equivalent. A Goulds 3196 rated at 50 GPM at 20 m head delivers 189.271 LPM. The conversion ratio is constant across the entire flow range.

Example: 50 GPM pump = 189.271 LPM capacity

Irrigation Controller & Emitter Matching

Rain Bird, Hunter Industries, and Toro publish zone flow data in GPM. Netafim, Irritec, and European drip system manufacturers rate emitters in LPM or L/hr. Converting the controller's GPM zone budget to LPM enables direct comparison with metric emitter catalogs and ensures the supply pump is correctly sized.

Typical Zone Flow Rates

A Rain Bird 5000 rotary head runs at 0.97–1.97 GPM (3.67–7.46 LPM). A Hunter MP2000 Rotator runs at 0.20–0.49 GPM (0.76–1.85 LPM). Multiply all GPM values by 3.785411784 before totalling zone demand.

Example: 10 GPM irrigation zone = 37.854 LPM demand

Plumbing Code Flow Limits in LPM

The Uniform Plumbing Code (UPC) and International Plumbing Code (IPC) specify maximum fixture flow rates in GPM. EPA WaterSense limits showerheads to 2.0 GPM (7.571 LPM) and lavatory faucets to 1.2 GPM (4.542 LPM). Converting GPM to LPM is needed for metric project documentation and EU CE declarations.

Tankless Water Heater Activation

Rinnai, Navien, and Noritz tankless heaters specify minimum activation flow in GPM. A Rinnai RU180iN activates at 0.26 GPM = 0.984 LPM. Plumbers specifying these units for metric-standard projects need the LPM equivalent for pipe sizing and minimum flow verification.

Example: 2.0 GPM showerhead = 7.571 LPM

Cleaning Units & Flow Comparison

Pressure washers are rated by pressure (PSI or bar) and flow (GPM or LPM). Cleaning power: Cleaning Units (CU) = PSI × GPM. US brands — Simpson, Sun Joe, DeWalt, Generac — publish in GPM. European brands — Kärcher, Nilfisk, Bosch — publish in LPM. Converting GPM to LPM enables CU comparison and chemical dosing calculation for metric machines.

Practical Examples

A DeWalt DWPW2400 at 1.1 GPM delivers 4.164 LPM. A Simpson MegaShot at 2.5 GPM delivers 9.464 LPM. Detergent injection rates given in GPM must be multiplied by 3.785411784 for correct dosing on metric machines.

Example: 2.5 GPM pressure washer = 9.464 LPM

Hydraulic Component Matching

Parker Hannifin, Eaton (Vickers), and Bosch Rexroth publish US hydraulic system data in GPM per ANSI/NFPA T3.6.7 standards. Converting to LPM is required when integrating US hydraulic pumps or directional control valves into ISO 4413‑compliant systems, or when specifying replacements from European manufacturers such as Bosch Rexroth, Danfoss Power Solutions, or Linde Hydraulics.

Cylinder Fill Time & Motor Speed

A Parker P1‑075 piston pump rated at 20 GPM delivers 75.708 LPM. Cylinder fill time, motor displacement matching, and system heat dissipation all require flow in consistent units. Multiply any GPM value from a US hydraulic datasheet by 3.785411784 to get LPM.

Example: 20 GPM hydraulic pump = 75.708 LPM

When to Use the Reverse Conversion

Use LPM → GPM when a metric pump (Grundfos, KSB, Wilo) rated in LPM must be specified for a North American project under ANSI/HI or NFPA standards, or when comparing a metric HVAC chiller datasheet against US equipment in GPM. Example: 100 LPM × 0.264172 = 26.417 GPM.

Round‑Trip Verification

Converting GPM → LPM then LPM → GPM should recover the original value: 10 GPM × 3.785411784 = 37.854 LPM; 37.854 × 0.264172 = 10.000 GPM. Any deviation beyond rounding tolerance indicates a mixed gallon standard was applied.

GPM to LPM Reference Table

US GPM converted to LPM using 3.785411784 (NIST). Common engineering contexts shown for quick reference.

Volumetric Flow Rate Units in Engineering

Volumetric flow rate measures how much fluid volume passes a cross‑section per unit time. The International System of Units (SI) expresses this as litres per second (L/s) or cubic metres per hour (m³/h); litres per minute (L/min) is widely used for convenience. United States customary units express flow as gallons per minute (GPM) or cubic feet per minute (CFM). All measure the same physical property — only the scale differs.

Choosing the Correct Conversion Factor

The most critical step is confirming which volume 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 20% error in every pipe size, valve rating, pump selection, and pressure drop calculation. For CFM: 1 CFM = 28.317 LPM = 7.481 GPM. All conversions on this page default to US liquid gallons unless explicitly stated otherwise.

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