What Is TDS in Water? Safe Level, Normal Ranges, and Myths Explained

In recent years, “TDS” has become a popular buzzword in water quality discussions. Many believe a lower TDS reading automatically means cleaner, safer water—and often use TDS meters to judge their water filters.

But what exactly does TDS measure, and is a lower number always better? Here is an expert breakdown of TDS standards, normal drinking ranges, and how to properly evaluate your tap water.

What Is the Meaning of TDS (Total Dissolved Solids) in Water?

TDS refers to Total Dissolved Solids, meaning the aggregate weight of all inorganic and organic substances dissolved in a given volume of water. These dissolved solids primarily consist of:

• Inorganic salts: Calcium, magnesium, potassium, sodium, bicarbonates, chlorides, and sulphates.
• Trace metals: Iron, copper, zinc, and lead (from ageing pipes).
• Organic matter: Microscopic particles dissolved in the water supply.

TDS directly influences the taste and mouthfeel of drinking water. However, a TDS reading measures quantity, not quality. It indicates how much is dissolved in the water, but cannot distinguish between healthy essential minerals (like calcium and magnesium) and harmful contaminants (like lead or nitrate).

What Is a Safe TDS Level for Drinking Water?

Neither the World Health Organization (WHO) nor local health agencies set a strict safety limit for TDS, because dissolved solids themselves do not pose a direct health hazard. Instead, TDS is evaluated based on palatability and taste aesthetics:

WHO TDS Taste Guidelines

• Under 300 mg/L (ppm): Excellent
• 300 – 600 mg/L: Good
• 600 – 900 mg/L: Fair
• 900 – 1,200 mg/L: Poor
• Above 1,200 mg/L: Unacceptable (Unpalatable)

Normal TDS Level in Drinking Water in Hong Kong

According to the Water Supplies Department (WSD), tap water in Hong Kong generally ranges between 30 and 150 mg/L (ppm). This places Hong Kong’s municipal water comfortably within the WHO’s “Excellent” category.

Note: While local water sources start pure, actual TDS levels at your tap can spike due to corroded building pipes, unwashed rooftop water storage tanks, or ageing plumbing infrastructure.

How to Accurately Measure TDS Levels in Drinking Water?

The simplest method is to use a household TDS testing pen. Simply place the probe in the water, and the testing pen will estimate the total dissolved solids content using electrical conductivity, displaying the TDS value within a few seconds. However, TDS testing pens are for daily reference only and cannot directly determine whether water contains bacteria, heavy metals, or other contaminants.

What Is the Relationship Between TDS and Hard or Soft Water?

Water hardness specifically measures the concentration of calcium and magnesium ions.

Hard water usually has a higher TDS because of these minerals.

However, a high TDS does not always mean hard water. Water rich in sodium or chloride will yield a high TDS reading while remaining technically “soft.

What Is a Normal TDS Level in Drinking Water? What Do Different Values Represent?

TDS values can serve as a reference for determining the content of dissolved substances in water, generally categorised into the following ranges:

• 0–50 ppm: Close to pure water, commonly found in laboratories or industrial uses, and not recommended as long-term daily drinking water.
• 50–150 ppm: Low TDS water, with a lighter taste and relatively lower mineral content.
• 150–300 ppm: Common drinking water range, containing an appropriate amount of minerals with a more natural taste.
• 300–600 ppm: Still within an acceptable range, but the taste may be heavier.
• 600–1000 ppm: High TDS, it is recommended to pay attention to the water source and water quality conditions.
• Above 1000 ppm: Not recommended for direct drinking; further water quality testing should be conducted.

What Do TDS In and TDS Out Mean?

TDS In refers to the value of untreated raw water, while TDS Out refers to the value of filtered water. The difference between the two can provide a preliminary understanding of a water filter’s ability to reduce dissolved solids.

How Filter Systems Affect TDS In vs. TDS Out

Comparing TDS In (unfiltered raw water) and TDS Out (filtered water) is a common way to measure filter performance. However, the result depends entirely on the filtration technology used:

• Reverse Osmosis (RO) Systems: RO membranes filter particles down to 0.0001 microns, removing up to 95–99% of all dissolved solids. A dramatic drop in TDS is expected and indicates the system is working properly.
• Activated Carbon & Ultrafiltration (UF): These systems target chlorine, odours, VOCs, sediment, and bacteria while retaining beneficial minerals. As a result, TDS readings will remain virtually unchanged—this is completely normal and intended.

Elevating Drinking Water Quality with Life Solutions

Relying solely on a TDS number can be misleading. To ensure water is both pure and mineral-balanced, Life Solutions offers commercial and residential filtration systems tailored to your specific infrastructure.

Whether utilising precision Reverse Osmosis for high-purity demands or multi-stage carbon filtration with Firewall® UVC purification to eliminate 99.9999% of bacteria while preserving mineral taste, Life Solutions delivers safe, crisp, and sustainable drinking water for homes and businesses across Hong Kong and Macau.

Frequently Asked Questions About TDS Levels

What TDS Level Is Safe for Drinking Water?

TDS cannot be used to directly determine whether drinking water can be consumed raw. It only reflects the amount of dissolved substances in the water and cannot detect bacteria, heavy metals, or other contaminants. To determine whether water is suitable for direct consumption, one must still look at the water source, disinfection treatment, and overall water safety.

What TDS Level Indicates Soft Water?

Generally, 0–60 ppm can be regarded as soft water, while 60–120 ppm is mostly slightly hard or moderately hard water. Soft water usually has a gentler mouthfeel and is less likely to form obvious limescale in electric kettles.

Does boiling water lower the TDS?

No. Boiling water evaporates pure H₂O as steam, leaving dissolved solids behind. As a result, boiling tap water actually slightly increases the TDS concentration.

Is a Lower TDS Always Better?

No. Ultra-low TDS (under 10 ppm) means the water is almost entirely demineralised, which can result in a flat, acidic taste. For daily drinking, water containing moderate, healthy mineral content (50–200 ppm) is generally preferred.

What causes a sudden rise in tap water TDS?

A sudden spike in TDS often points to local plumbing issues—such as rusting pipes, dissolved sediment from recently cleaned water tanks, or changes in municipal water sourcing. If accompanied by discolouration or odour, inspect your building’s supply line immediately.

References:

Water Purification Knowledge Base – Hong Kong Water Quality Analysis