How to Calculate Flow Rate (Drainage Capacity) in Floor Drains and Shower Channels

How to Calculate Flow Rate (Drainage Capacity) in Floor Drains and Shower Channels

One of the most common problems in bathrooms and wet areas is water pooling. In most cases, this is not caused by poor installation but by incorrect flow rate (drainage capacity) calculation.

A stylish shower channel or a stainless steel floor drain alone is not enough. If the drainage capacity does not match the water input, water accumulation is inevitable.

What Is Flow Rate?

Flow rate refers to the amount of water a drain can discharge per unit of time.
It is usually expressed in liters per minute (L/min).

Example:
A drain that discharges 30 liters of water in one minute has a flow rate of 30 L/min.

Why Is Flow Rate Calculation Important?

Incorrect flow rate selection leads to:

  • Water accumulation during showering

  • Floor flooding

  • Odor problems

  • Customer complaints and renovation costs

Flow rate calculation is especially critical for:

  • Rain shower systems

  • Multi-outlet spa showers

  • Hotels, gyms, and public showers

  • Bathrooms with shower channels

How to Calculate Required Flow Rate for Showers

1. Determine Water Consumption of the Shower System

Approximate values:

  • Standard shower head: 8–12 L/min

  • Rain shower: 15–25 L/min

  • Spa / multi-outlet systems: 25–40 L/min

If multiple outlets are used simultaneously, total consumption must be calculated.

2. Add a Safety Margin

In practice, a 20–30% safety margin should always be added.

Example:

  • Shower consumption: 20 L/min

  • Safety margin (25%): 5 L/min

  • Required minimum flow rate: 25 L/min

Flow Rate Calculation for Floor Drains

The flow rate of floor drains depends on:

  • Outlet diameter

  • Siphon (trap) design

  • Grating open area

  • Drain height

Average Flow Rates by Outlet Diameter

Outlet Diameter Average Flow Rate
Ø50 mm 25–30 L/min
Ø70 mm 35–45 L/min
Ø100 mm 50–60 L/min

These values apply to properly designed, high-quality products.

Flow Rate Calculation for Shower Channels

In shower channels, flow rate is influenced not only by outlet diameter but also by:

  • Channel length

  • Internal body design

  • Grating open area

  • Vertical or horizontal outlet type

Average shower channel capacities:

  • Standard models: 30–40 L/min

  • Wide-body or dual-outlet models: 45–60 L/min

Effect of Siphon Design on Flow Rate

Not all siphons perform the same.
S-shaped trap designs ensure:

  • Continuous water flow

  • Reduced back pressure

  • Odor prevention without reducing flow rate

Poor siphon design may reduce actual drainage capacity, even if theoretical values look sufficient.

Conclusion

In floor drains and shower channels, flow rate is more important than appearance.
Correct calculation ensures:

  • No water accumulation

  • No odor problems

  • Long-lasting and trouble-free use