You've found a pump spec sheet that reads in cubic feet per hour. Your project plans call for litres per second. Both numbers describe the same thing—how much fluid moves through a system—but they speak different languages. If you guess at the translation, you risk undersized pipes, underperforming equipment, or costly redesigns.

This guide clears up the confusion. We'll define both units, explain how they relate, show you where each one appears in real projects, and give you a quick-reference table you can bookmark.

First Things First: What Is Volume Flow Rate?

Volume flow rate measures how much space a fluid occupies as it moves past a point over a set period of time. Think of water filling a bucket from a tap. The flow rate tells you how quickly that bucket fills—not just how much water exists, but how fast it arrives.

Every volume flow rate measurement combines two ideas: a volume (like cubic feet or litres) and a time interval (like hours or seconds). Change either one, and the number changes—even if the physical flow stays identical. That's exactly why conversions like cubic foot per hour to litre per second exist.

Cubic Foot Per Hour (ft³/h): The Imperial Workhorse

The cubic foot per hour expresses how many cubic feet of fluid pass a given point every hour. One cubic foot is the volume of a cube measuring one foot on each side—roughly the size of a medium basketball sphere's worth of space, or about 12 inches by 12 inches by 12 inches.

This unit is common in North America, where imperial measurements still dominate engineering documents, fuel gas sizing, and legacy equipment ratings.

Where ft³/h Shows Up in the Real World

• Natural gas appliances: Furnaces, boilers, and water heaters in the U.S. are often rated by gas consumption in cubic feet per hour.
• HVAC documentation: Older American specifications and manuals frequently list air or gas movement in ft³/h rather than the more familiar CFM (cubic feet per minute).
• Industrial process lines: Chemical dosing, steam tracing, and low-flow transfer systems may reference hourly volumes.
• Legacy pump curves: Equipment manufactured decades ago often carries original ratings in imperial hourly units.

Litre Per Second (l/s): The Metric Standard

The litre per second describes how many litres of fluid pass a point each second. A litre is the volume of a cube measuring 10 centimetres on each side—about the size of a large water bottle.

Because it uses seconds instead of hours, l/s delivers immediate, intuitive readings. If a system flows at 2 l/s, you know instantly that two litres arrive every single second. That immediacy makes it a favorite among engineers who need fast mental math.

Where l/s Dominates

• Municipal water systems: Pipe capacity, fire hydrant output, and treatment plant throughput are commonly specified in l/s.
• Plumbing codes: Many countries outside the U.S. rate fixture flow rates and drainage capacity in litres per second.
• Irrigation design: Pump selection and sprinkler performance data often use l/s.
• Fire protection engineering: Sprinkler and standpipe calculations frequently rely on this unit.

The Relationship Between ft³/h and l/s

Converting between these units means reconciling two differences at once: the volume unit and the time unit.

Two Differences, One Conversion

1. The volume gap: A cubic foot holds more than a litre. Based on the international definition of the foot (exactly 0.3048 metres), one cubic foot equals approximately 28.317 litres.
2. The time gap: An hour contains 3,600 seconds. So a flow measured per hour spreads across far more time than the same volume measured per second.

Put together, one cubic foot per hour works out to roughly 0.0079 litres per second. In other words, a flow of 1 ft³/h is quite small when expressed in l/s—because an hour is a long window and a cubic foot is a modest volume.

Going the other direction? One litre per second equals roughly 127.13 cubic feet per hour. That asymmetry surprises people: the same physical flow looks like a big number in ft³/h and a small number in l/s.

A Quick Worked Example

Suppose a gas appliance consumes 40 ft³/h. To express that in litres per second, multiply 40 by 0.0079. The result is approximately 0.32 l/s. Now a European engineer reviewing the spec can compare it directly against metric-rated equipment.

Practical Applications: When You'll Need This Conversion

Sourcing Equipment Across Borders

A facility in Germany might import a compressor rated in ft³/h while its piping was designed around l/s. Converting the rating ensures the pipe diameter matches actual demand—too narrow, and you get pressure losses; too wide, and you've wasted money on oversized components.

Bridging Legacy and Modern Systems

Retrofit projects often pair decades-old equipment (imperial ratings) with new infrastructure (metric ratings). Accurate conversion keeps the old and new talking the same language.

Compliance and Reporting

Environmental permits, water usage reports, and energy audits may require figures in specific units. Converting correctly keeps your submissions consistent and defensible.

Common Mistakes to Avoid

• Confusing ft³/h with CFM: Cubic feet per minute is 60 times larger than cubic feet per hour. Mixing them up throws your result off dramatically.
• Assuming litres per minute equals litres per second: Always confirm the time unit before comparing figures.
• Rounding too early: Keep extra decimal places through multi-step calculations, then round only your final answer.
• Forgetting direction: Multiplying when you should divide (or vice versa) produces errors by orders of magnitude. A sanity check helps: ft³/h values should shrink when converted to l/s.

Quick Reference Conversion Table

The table below converts common cubic-foot-per-hour values into litres per second. Figures are rounded to four decimal places for everyday use.

Notice the pattern: ft³/h values always produce much smaller l/s figures. If your converted number looks bigger than the original, double-check your math.

The Bottom Line

Cubic feet per hour and litres per second measure the same physical reality—volume flow rate—through different volume and time units. One cubic foot per hour equals roughly 0.0079 litres per second, and one litre per second equals roughly 127.13 cubic feet per hour. Master that relationship, and cross-border specs, retrofits, and compliance reports become straightforward instead of stressful.

Need another pairing handled? Our conversion tool covers the full range of volume flow rate units, so you can translate any spec sheet in seconds.