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NTU vs. FTU — What's the Difference?

By Maham Liaqat & Urooj Arif — Published on February 19, 2024
NTU stands for Nephelometric Turbidity Units, a measure of water turbidity, and FTU stands for Formazin Turbidity Units, also related to measuring water turbidity.
NTU vs. FTU — What's the Difference?

Difference Between NTU and FTU


Key Differences

NTU (Nephelometric Turbidity Units) and FTU (Formazin Turbidity Units) are both measures of water turbidity, which is a key indicator of water quality. Turbidity refers to the cloudiness or haziness of a fluid caused by large numbers of individual particles that are generally invisible to the naked eye, similar to smoke in the air.
NTU is the unit of measure used in nephelometry, which is based on the amount of light that is scattered by the suspended particles in the water at a 90-degree angle to the light source. Nephelometric turbidity measurements are widely accepted and used for monitoring drinking water and other types of water quality assessments. The NTU scale is used by environmental agencies and water treatment facilities to ensure water clarity and safety.
FTU, referring to Formazin Turbidity Units, is based on a calibration standard created by mixing hydrazine sulfate with hexamethylenetetramine to produce a solution with a specific turbidity. While FTU is less commonly cited in modern standards, it historically served as a benchmark for measuring and comparing turbidity levels. FTU units were used to calibrate instruments that measure turbidity and to ensure consistency across measurements.
The transition from FTU to NTU in many contexts reflects advances in measurement technologies and standardization efforts to ensure consistency and accuracy in water quality testing. While both units serve to measure the same property—water turbidity—they originate from different calibration standards and measurement methodologies.
Modern turbidity sensors and analyzers are typically calibrated in NTU, following EPA and international standards for water testing. This standardization facilitates comparability of data and ensures that water treatment processes meet health and safety requirements.

Comparison Chart

Measurement Method

Based on light scatter at a 90-degree angle.
Based on a formazin standard solution.


Widely used in drinking water quality assessment.
Historical calibration standard for turbidity.


Accepted by EPA and international standards.
Less commonly used in modern standards.


Calibrated using formazin or alternative standards.
Original calibration solution for turbidity meters.


Current standard for water treatment and analysis.
Primarily of historical significance in calibration.

Compare with Definitions


NTU measures water turbidity via light scatter.
The water sample showed a clarity of 5 NTU, indicating low turbidity.


Transitioned to NTU in modern standards.
We converted historical FTU data to NTU for consistency.


Used in environmental water testing.
The river's NTU levels increased after the storm.


Based on formazin turbidity standard.
Instruments were once calibrated with a formazin solution to measure in FTU.


Reflects particulate content in water.
A high NTU value suggests more particles in suspension.


FTU is a historical turbidity measurement unit.
Older turbidity records were expressed in FTU.


Essential for water treatment monitoring.
The treatment plant achieved an NTU reduction to acceptable levels.


Reflects calibration practices.
The FTU scale was foundational in developing turbidity measurement.


Indicator of water quality in NTU.
Drinking water should have an NTU value less than 1 for safety.


Used in comparison of turbidity sensors.
FTU values helped standardize early turbidity sensors.

Common Curiosities

How do modern instruments measure turbidity?

Most now use NTU, calibrated against formazin or alternative stable standards.

How is NTU measured?

By assessing the light scattered by particles in water at a 90-degree angle.

Are NTU and FTU interchangeable?

They measure the same property but are based on different standards and methodologies.

What role does turbidity play in water treatment?

Monitoring turbidity is essential for ensuring the effectiveness of water purification processes.

What is the significance of FTU?

Formazin Turbidity Units, historically used to calibrate turbidity measurements.

Why is water turbidity measurement important?

It's critical for assessing water quality, particularly for drinking water safety.

What does NTU stand for?

Nephelometric Turbidity Units, a measure of water turbidity.

What is considered a safe NTU level for drinking water?

Generally, NTU values below 1 are safe for drinking water, but standards may vary.

How can one convert FTU to NTU?

Direct conversion may not be straightforward due to different calibration standards; instrument calibration is key.

What advancements have improved turbidity measurement accuracy?

Technological advancements in sensor design and calibration standards have enhanced accuracy and reliability.

Can high NTU levels affect water safety?

Yes, high NTU can indicate contamination that may pose health risks.

How have turbidity measurement standards evolved?

From using FTU and formazin solutions to widespread adoption of NTU in line with EPA guidelines.

Is specialized equipment required to measure NTU?

Yes, turbidimeters or nephelometers calibrated in NTU are used for precise measurements.

Do NTU levels fluctuate with environmental changes?

Yes, events like storms can stir up sediments, increasing water turbidity.

Why is historical turbidity data important?

It helps in understanding long-term changes in water bodies and the effectiveness of environmental policies.

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Author Spotlight

Written by
Maham Liaqat
Co-written by
Urooj Arif
Urooj is a skilled content writer at Ask Difference, known for her exceptional ability to simplify complex topics into engaging and informative content. With a passion for research and a flair for clear, concise writing, she consistently delivers articles that resonate with our diverse audience.

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