TDS Calculator - Total Dissolved Solids
Calculate total dissolved solids (TDS) two ways: enter an EC meter reading and a conversion factor to get TDS in mg/L (ppm), or switch to ion-sum mode and enter individual ion concentrations from a lab water report. Both modes display a WHO/EPA quality rating, a visual quality gauge, and a suitability guide for drinking, brewing, hydroponics, and aquariums.
What is TDS and why does it matter?
Total dissolved solids (TDS) is the combined mass of all inorganic and organic substances dissolved in water, expressed in milligrams per liter (mg/L) or parts per million (ppm) - numerically identical for water. These dissolved substances include calcium, magnesium, sodium, potassium, bicarbonates, sulfates, chlorides, nitrates, and trace metals. TDS is one of the most widely used indicators of water quality because elevated levels can affect taste, scaling in pipes and appliances, crop yields, fish health, and the performance of industrial processes. The US Environmental Protection Agency sets a secondary (non-enforceable) maximum contaminant level of 500 ppm for aesthetic quality in drinking water, while the World Health Organization considers below 300 ppm good and below 600 ppm acceptable. TDS alone does not tell you what is dissolved - a low TDS reading can coexist with a specific harmful contaminant - so it is a screening tool rather than a full safety guarantee.
EC to TDS conversion: how the factor works
Most pocket TDS meters do not measure dissolved solids directly - they measure electrical conductivity (EC) and multiply by a built-in conversion factor, typically 0.64. EC measures how easily electricity flows through the water: more dissolved ions means higher conductivity. The conversion factor k depends on the dominant salts present. A solution of pure sodium chloride has k around 0.55; typical tap water (a mix of calcium, magnesium, bicarbonate, sulfate, and chloride) has k between 0.64 and 0.70; seawater and brine solutions are closer to 0.80. If you know your water source or the dominant salt type, choosing the correct k gives a more accurate TDS estimate. Using the default 0.64 or 0.67 is appropriate for most freshwater supplies when the exact composition is unknown.
Ion summation: reading a water analysis report
Laboratories report water chemistry as individual ion concentrations in mg/L. TDS is the sum of all those concentrations: TDS = (all anions) + (all cations). Common anions include chloride, sulfate, bicarbonate, nitrate, and fluoride. Common cations include sodium, calcium, magnesium, potassium, iron, and manganese. A complete water report should have its anion total and cation total within about 10% of each other - a larger imbalance suggests an ion is missing from the report or measurement error. Ion summation generally gives a more accurate TDS figure than EC conversion because it avoids assumptions about which salts are present.
TDS and use-case suitability
Different applications have very different TDS requirements. Specialty coffee roasters and the Specialty Coffee Association recommend source water of 75-250 ppm, with 150 ppm often cited as optimal for extraction balance. Hydroponics growers track TDS (or its electrical conductivity equivalent) because nutrients are added as salts: seedlings typically need 200-400 ppm, leafy greens 800-1600 ppm, and fruiting plants 1400-2800 ppm. Freshwater aquariums generally perform best below 500 ppm, while marine tanks may require specific ion ratios rather than just a TDS target. For irrigation, most field crops tolerate up to about 1000-2000 ppm, though sensitive crops such as strawberries or lettuce prefer below 640 ppm. Boilers and cooling towers have strict TDS limits to prevent scale and corrosion - typically 1000-3500 ppm depending on pressure and design.
TDS water quality and suitability guide
| TDS (ppm) | Quality | Drinking water | Brewing / coffee | Hydroponics | Aquarium |
|---|---|---|---|---|---|
| 0-50 | Excellent | Very pure, may taste flat | Too soft for most styles | Ideal - add nutrients | Depends on species |
| 50-150 | Good | WHO ideal range | Ideal for specialty | Good with nutrients | Most freshwater |
| 150-300 | Fair | WHO good quality | Good for most styles | Acceptable | Hardy freshwater |
| 300-500 | Acceptable | WHO max preferred | Acceptable | Marginal | Tolerant species only |
| 500-1000 | Poor | Exceeds EPA 500 limit | Not recommended | Damaging to many plants | Not suitable |
| 1000-10000 | Very poor | Not suitable | Not suitable | Not suitable | Marine / brackish only |
Based on WHO drinking-water quality guidelines (4th edition, 2011) and US EPA secondary standards. Suitability for non-drinking uses is indicative.
Frequently asked questions
What TDS level is safe for drinking water?
The US EPA secondary standard sets 500 ppm (mg/L) as the upper limit for taste and aesthetic quality, while the WHO considers below 300 ppm good and below 600 ppm acceptable. Water below 1000 ppm is generally safe from a TDS perspective, though individual ions such as nitrate, lead, or arsenic may be harmful at much lower concentrations that a TDS meter cannot detect separately.
Is TDS the same as ppm?
For water, yes. One ppm means one milligram of solute per liter of water (one milligram per kilogram, given that water weighs approximately 1 kg/L). So TDS in mg/L and TDS in ppm are numerically the same value for dilute aqueous solutions. Only at very high salinity (seawater and brine) does the density correction become meaningful.
What is the conversion factor between EC and TDS?
The factor k typically ranges from 0.55 to 0.80 depending on the dissolved salt type. TDS (mg/L) = k * EC (µS/cm). Most general-purpose TDS meters use 0.64 internally, which works well for typical freshwater supplies. NaCl-based solutions use 0.55; mixed tap water is usually 0.64-0.67; brine and seawater are closer to 0.80. You can enter the appropriate factor in EC mode to get a more accurate result for your specific water source.
Does a zero TDS reading mean the water is pure?
Not exactly. A TDS of zero (or near zero) means there are no measurable dissolved ionic solids, which is the case for distilled or high-quality reverse-osmosis water. However, some non-ionizing organic compounds, dissolved gases like CO2, and some pathogens do not contribute to TDS. Very low TDS water can also be corrosive because it lacks the mineral buffering that prevents leaching of metals from pipes.
Why is my anion total much higher than my cation total?
A significant imbalance (more than 10-15%) between anion and cation totals usually means the water analysis is incomplete - one or more major ions are missing from the report. In natural water, the sum of positive and negative charges must balance (electroneutrality). Common missing ions include sodium, which labs sometimes omit from basic hardness panels, or silica, which is neutral and not counted in the ion charge balance but does contribute to TDS mass.
What TDS is best for hydroponics?
It depends on the growth stage and crop. As a rough guide: seedlings and clones prefer 100-400 ppm; leafy greens and herbs do well at 600-1200 ppm; fruiting crops like tomatoes or peppers typically need 1200-2400 ppm at peak growth. These values represent nutrient solution TDS - the TDS of the source water is subtracted first to calculate the actual nutrient contribution.
Can I use a TDS meter to check if my water filter is working?
Yes, comparing TDS before and after the filter is a quick way to estimate filter performance. Reverse osmosis filters typically achieve 90-99% TDS rejection. Activated carbon filters remove chlorine and some organics but change TDS very little, so a large TDS drop before and after a carbon filter is not expected. For an RO system, TDS rejection (%) = (source TDS - permeate TDS) / source TDS * 100.