How Many Samples Do I Need?

The calculator below is provided as guidance only.

The number of Legionella samples required depends on water system design, complexity, and risk factors — not simply the number of outlets or taps. 

Factors such as storage tanks, dead legs, usage patterns, system complexity, and occupancy type all influence how many samples are needed to monitor risk effectively.

Accurate sampling requirements should always be based on a site-specific Legionella Risk Assessment, which considers outlet numbers, risk level, system layout, and operational conditions.

Use this Legionella sampling requirements calculator to estimate the number of Legionella and TVC water samples typically required for routine monitoring, based on your system characteristics and risk profile.

Samples Calculator

Do you have a Risk Assessment?

Important: Risk Assessments normally identify 2 hot + 2 cold sentinel outlets. This calculator enforces that minimum.

If you have a risk assessment, select sentinel outlets and tanks.

Typically nearest and furthest on the hot system. Typically nearest and furthest on the cold system.

Minimum: 2 samples normally taken from representative hot/cold outlets.

Your Sample Requirements:

Total samples needed: —
Legionella: — (Outlets —, Tanks —)
TVC (tanks): —

Ongoing compliance?
Set up regular testing

Understanding the Factors

Testing is one part of effective Legionella control and should be used alongside appropriate risk assessment, monitoring, and management.


System Complexity: Storage tanks, dead legs, and multiple interconnected systems require more sampling points to ensure comprehensive coverage.

Usage Patterns: Rarely used outlets and seasonal properties may have different water quality issues than regularly used systems.

Premises Type and Occupancy: Residential, commercial, and industrial properties have different risk profiles and sampling requirements.

Risk Factors: Properties with vulnerable occupants or previous water quality issues require more frequent and comprehensive sampling.

Who Should Carry Out Water Sampling

Why water safety is critical in care settings

Care environments accommodate people who are often:

  • Elderly
  • Immunocompromised
  • Living with chronic or complex health conditions

These individuals are significantly more vulnerable to waterborne infections, including Legionnaires’ disease. In these settings, failures in water management can have serious — and sometimes fatal — consequences.

Unlike many commercial buildings, care homes experience:

  • Frequent changes in room occupancy
  • Periods of low water usage
  • Assisted bathing and showering
  • Complex pipework and stored water

All of these factors increase risk if not carefully managed.

Care Homes & Healthcare Settings

Modern classroom with blue chairs, wooden tables, and educational posters on green walls.

Why water systems in schools and nurseries can present risk.

Education and childcare settings often include:

  • Large buildings with long pipe runs
  • Seasonal closures and holiday periods
  • Low-use outlets in classrooms or unused wings
  • Shared washrooms and specialist facilities

Risk increases when:

  • Buildings are partially occupied
  • Outlets are rarely used
  • Water temperatures are poorly controlled
  • Systems contain scale, biofilm, or stagnant sections

Holiday closures, in particular, can allow water to sit unused for extended periods, increasing the potential for bacterial growth.

Schools, Nurseries & Educational Settings

Hotel bathroom tap

Why water systems in hotels are higher risk

Hotels and leisure facilities often include:

  • Large hot and cold water storage systems
  • Long pipe runs serving multiple floors
  • Showers, spa pools, steam rooms, and leisure features
  • Guest rooms that may be vacant for extended periods

These conditions can create an environment where Legionella bacteria can grow if systems are not properly managed.

Risk increases when:

  • Rooms are unoccupied
  • Seasonal closures occur
  • Water temperatures are poorly controlled
  • Systems contain scale, biofilm, or low-flow areas

Hotels, Leisure & Hospitality

Why Water Sampling Matters — No Matter the Size of Your Business

Water safety risks are not limited to large buildings or complex sites. Even small premises can develop conditions that allow harmful bacteria to grow.

Legionella bacteria thrive where water becomes stagnant, temperatures are poorly controlled, or outlets are used infrequently — issues that can affect any workplace, regardless of size.

Small offices, shops, salons, cafés, and studios often assume their systems are “low risk”. In reality, reduced water use, seasonal closures, and unnoticed changes in occupancy can increase risk, not reduce it.

Routine water sampling provides evidence, not assumptions. It confirms whether control measures are working and helps identify issues before they become a health risk.

For businesses of all sizes, water sampling is a simple, proportionate way to:

  • Protect staff, customers, and visitors
  • Demonstrate legal compliance
  • Reduce the risk of illness, enforcement action, or reputational damage

When it comes to water safety, size does not remove responsibility.

Testing ensures safety is proven — not presumed.