Liters per second is the SI-friendly cousin of cubic meters per hour. It’s what shows up on water-treatment skids, wastewater plants, hydraulic equipment, and process control instrumentation when the numbers are big enough that LPM gets unwieldy but small enough that m³/h sounds overblown. If you’re working with an EU- or ISO-standard spec sheet that lists flow in L/s and need to compare it to a US pump rated in GPM, this is the page.
Jump to a section
- The formula and conversion factor
- Why L/s shows up where it does
- Quick reference table
- Worked example: water-treatment plant capacity
- Common engineering benchmarks
- FAQ
The formula and conversion factor
The conversion is a single multiplication:
For higher precision, use 15.8503231415 (the exact factor from US gallons being defined as 231 in³ = 3.78541178… L). xconvert’s L/s to GPM converter carries 13 decimal places.
The formula derivation is straightforward: 1 L/s × (60 s / 1 min) × (1 gal / 3.78541 L) = 15.8503 gal/min. Multiplying by 60 (seconds in a minute) and dividing by 3.78541 (litres in a US gallon) gives the factor.
If you’ve memorized other flow conversions, here’s how L/s relates:
Why L/s shows up where it does
The choice of unit is a tell about who designed the equipment:
- Water-treatment plants publish capacity in L/s (for small) or m³/h (for large). 1 L/s plant ≈ a small village; 100 L/s ≈ a mid-size town.
- Wastewater pumps are commonly rated in L/s in EU and ANZ standards, GPM in the US.
- Hydraulic systems (industrial machinery, presses) use L/s for fluid power because the flows are small (1–20 L/s) and L/s gives readable numbers.
- Drainage and stormwater design use L/s for sub-watershed catchments, m³/s for entire watersheds.
- Process instrumentation (flowmeters with 4-20 mA outputs) is often spanned in L/s because it’s an SI-derived unit and matches PLC engineering culture.
When a US plant operator inherits a European-spec instrument, the L/s readout is a constant friction. Either you re-span the instrument to GPM (if the protocol allows) or you train the operator to do the conversion mentally. The mental version: divide by 16 for a rough GPM, then add 1% (since the actual factor is 15.85, not 16).
Quick reference table
| L/s | GPM | Typical context |
|---|---|---|
| 0.1 | 1.59 | Small dosing pump |
| 0.5 | 7.93 | Hydraulic system fluid flow |
| 1 | 15.85 | Domestic water main at peak |
| 2 | 31.70 | Light commercial well pump |
| 5 | 79.25 | Small water-treatment skid |
| 10 | 158.50 | Industrial process feed |
| 25 | 396.26 | Wastewater lift station (small) |
| 50 | 792.52 | Mid-size water booster |
| 100 | 1,585.03 | Municipal water-treatment basin |
| 250 | 3,962.58 | Large industrial cooling water |
| 500 | 7,925.16 | Major pump station |
| 1,000 | 15,850.32 | Watershed-scale pump (large) |
Worked example: water-treatment plant capacity
A specification calls for a small water-treatment skid rated 8 L/s at 5 bar discharge. The available US-built pump on hand is a Goulds 3656 SP at 130 GPM at 70 psi. Will it work?
Step 1 — Convert flow:
The required is 126.8 GPM; the pump delivers 130 GPM. Within 3% — the system curve will shift the operating point slightly to compensate, and modulating valves can absorb the rest.
Step 2 — Convert pressure to verify head margin:
The pump is 2.5 psi short on head. That’s marginal. At the operating point, the pump curve might cross 70 psi at 126 GPM — barely meeting the requirement. Either confirm the operating-point head against the pump curve (not just rated head) or oversize the pump by going to a bigger frame.
This is where unit conversion isn’t just bookkeeping — it lets you make engineering judgements with confidence. Without converting, “8 L/s at 5 bar” and “130 GPM at 70 psi” feel completely different. Once converted, the comparison is direct.
Common engineering benchmarks
A few L/s numbers worth knowing:
- Small water service (residential): 0.5–1.0 L/s = 8–16 GPM
- Medium commercial water service: 5–10 L/s = 80–160 GPM
- Building chilled-water distribution: ~0.043 L/s per kW of cooling load (= 2.4 GPM/ton, ASHRAE rule of thumb)
- Storm-sewer design (urban catchment): 5–50 L/s per hectare during 10-year storm
- Wastewater lift-station typical: 10–50 L/s = 160–800 GPM
- Water-treatment plant typical: 100–1,000 L/s = 1,600–16,000 GPM
- Hydraulic press circuit: 0.5–5 L/s per actuator
- Fire pump (EU spec): 30–60 L/s = 475–950 GPM
- River flow (small creek): 1–10 L/s baseflow
- Sewage treatment per capita: 200–400 L/day per person ≈ 0.0023–0.0046 L/s per person
Frequently Asked Questions
Why isn’t L/s the SI unit for flow?
The SI unit for volumetric flow is m³/s. L/s is a derived unit (1 L = 0.001 m³, so 1 L/s = 0.001 m³/s) but allowed under the SI brochure. It survives because the numbers are friendlier in everyday engineering: a 50 L/s plant is more legible than 0.05 m³/s. For very large flows (rivers, storm sewers) m³/s starts to win.
What’s the difference between L/s and L/min?
Just a factor of 60. 1 L/s = 60 L/min = 60 LPM. The unit choice tracks the size of the system: LPM for residential and small commercial (where flows are 0.1–10 L/s), L/s for industrial and treatment (where flows are 1–1000 L/s). For very small flows (drip irrigation, single faucets), GPH or LPM gives more readable numbers.
Is there an Imperial version of L/s?
The litre is metric, so L/s is metric only. The Imperial-system equivalents are GPM (US gallon) or imp gpm (Imperial gallon). 1 L/s = 15.85 US GPM = 13.20 imp GPM. UK water-industry specifications use either L/s or m³/h depending on the era of the document.
How do I convert L/s to mass flow rate?
Multiply by fluid density. For water at 20 °C: 1 L/s × 998.2 kg/m³ = 0.998 kg/s ≈ 1 kg/s. The “1 L/s ≈ 1 kg/s” approximation is good to within 0.5% across the typical operating range of clean water systems. For other fluids (oil, ammonia, brine), use the actual density from the fluid datasheet.
Why does my flow meter read in L/s but the controller works in GPM?
Industrial flow meters often output 4–20 mA spanned in L/s (an EU-style configuration), but US-built PLCs and SCADA systems typically scale display values in GPM. The fix is in the PLC scaling: change the analog input scale factor to convert the 4–20 mA range to GPM before storing/displaying. Or change the meter’s span to GPM if the protocol allows it. Don’t do the conversion in the operator’s head — that’s where errors compound.
What’s the relationship between L/s and pipe size?
For full-pipe flow at typical velocities (1–3 m/s for water mains), the rough relationship is:
- DN50 (2") pipe at 2 m/s: ~4 L/s
- DN100 (4") at 2 m/s: ~16 L/s
- DN150 (6") at 2 m/s: ~35 L/s
- DN200 (8") at 2 m/s: ~63 L/s
- DN300 (12") at 2 m/s: ~141 L/s
These are rough — actual flow depends on pipe internal diameter and velocity, and you should use the Hazen-Williams or Darcy-Weisbach equations for friction loss in design. Use them for sanity-checks: if a spec says “DN100 at 200 L/s,” somebody made a mistake (that would require 12+ m/s velocity, which is far above the usual design ceiling).
Try it now
Convert any L/s value with the xconvert L/s to GPM converter — full precision, instant. For other flow-rate pairs (LPM, m³/h, CFM, m³/s), see the Volume Flow Rate catalog. For the broader picture of which industries use which units, our Flow Rate Conversion for HVAC and Plumbing overview is the place to start.