EC to TDS Converter
Convert an electrical conductivity (EC) reading in µS/cm, mS/cm or S/m into an estimated TDS value in ppm. Choose the conversion factor your meter uses (0.5, 0.64 or 0.7) and get an instant result.
Convert Conductivity to Total Dissolved Solids
EC to TDS Formula
Where:
- \(\text{TDS}\) — total dissolved solids, in ppm (mg/L for dilute water solutions)
- \(\text{EC}\) — electrical conductivity of the water, expressed in µS/cm
- \(k\) — the TDS conversion factor: 0.5 (NaCl scale), 0.64 (KCl scale) or 0.7 (natural water scale)
The formula converts the conductivity reading into µS/cm if needed, then multiplies it by the scale factor used by your meter. Because different salts conduct electricity differently, the factor is only an approximation of the true dissolved-solids content.
EC to TDS – Calculation Example
Example: a conductivity meter reads 500 µS/cm in a hydroponic nutrient solution, and the meter uses the NaCl scale (factor 0.5).
\[ \text{TDS} = 500\ \mu\text{S/cm} \times 0.5 = 250\ \text{ppm} \]
Result: the estimated TDS is 250 ppm (≈ 250 mg/L).
How to use the calculator
- Enter your Electrical conductivity (EC) reading in the number field.
- Select the unit of the reading — µS/cm, mS/cm or S/m — in the dropdown next to the value. Most hobby meters report µS/cm; many laboratory instruments use mS/cm.
- Choose the TDS conversion factor that matches your meter or application: 0.5 for the NaCl scale (common in the US and in hydroponics), 0.64 for the KCl scale, or 0.7 for the natural water scale (common in Europe). If you are unsure, check the meter's manual.
- Read the Total dissolved solids (TDS) result in ppm (mg/L), together with a note showing the factor that was applied.
What are EC and TDS?
Electrical conductivity (EC) measures how well a water sample conducts electricity. Pure water is a very poor conductor, but dissolved ions — mainly calcium, magnesium, sodium, potassium, chloride, sulfate and bicarbonate — carry charge and raise the conductivity. The more dissolved ions there are, the higher the EC, so conductivity is a quick and reliable proxy for the total amount of dissolved material in water.
Total dissolved solids (TDS) is the actual mass of dissolved substances per unit volume, usually expressed in mg/L. In dilute aqueous solutions — drinking water, hydroponic solutions, aquariums — 1 ppm is essentially equal to 1 mg/L. Measuring TDS directly requires evaporating and weighing a sample, which is slow and impractical for routine checks. Instead, inexpensive meters measure EC and convert it to an estimated TDS using a fixed multiplication factor.
The catch is that no single factor fits all waters. A chloride ion conducts more current than a bicarbonate ion of the same mass, so the TDS/EC ratio depends on the ionic mix. Meter makers standardise on three common scales: 0.5 (sodium chloride solutions), 0.64 (potassium chloride solutions) and 0.7 (typical natural waters). Using the wrong factor can shift the reading by 40%, which is why comparing EC values directly is often safer than comparing TDS values from different meters.
Typical EC and TDS values
| Water type | EC (µS/cm) | TDS at 0.5 (ppm) |
|---|---|---|
| Distilled / RO water | 0–50 | 0–25 |
| Fresh drinking water | 50–500 | 25–250 |
| Average tap water | 250–750 | 125–375 |
| Hydroponic nutrient solution | 1000–2500 | 500–1250 |
| Sea water | ≈ 50,000 | ≈ 25,000 |
Assumptions and limitations
- The result is an estimate. The true TDS/EC ratio of your water may differ from the selected scale factor. For precise work, calibrate against a gravimetric or laboratory TDS measurement.
- ppm ≈ mg/L holds only for dilute solutions. For brines and very concentrated liquids the density deviates from 1 kg/L and the equivalence breaks down.
- EC says nothing about which solids are present. Two waters with identical TDS can contain completely different minerals — harmful contaminants like heavy metals can raise EC just like benign salts. If you also work with concentration units, the PPM Converter helps translate the result.
- Always match the meter's factor. If your TDS meter uses the 0.7 scale and you convert an EC reading with 0.5, the two numbers will disagree even for the same water.
- Temperature matters. EC rises by roughly 2% per °C. Good meters apply automatic temperature compensation (ATC); check whether yours does before comparing readings taken at different temperatures.
Practical tips
Hydroponic and aquarium growers usually track EC rather than TDS precisely because it avoids the factor ambiguity — an EC of 1.4 mS/cm means the same thing everywhere. If you follow feeding charts, note whether they quote EC or TDS and at which scale before converting. For brewing and gardening applications that involve sugar solutions rather than salts, a refractometer-based tool such as the Brix to Specific Gravity Calculator is the more appropriate instrument, since sugars barely affect conductivity.
When monitoring water quality over time, keep the meter, the factor and the temperature conditions consistent so the trend is meaningful. A slowly rising TDS in a boiler, cooling loop or irrigation line is often the first warning of scale formation or contamination long before visible deposits appear.
Frequently Asked Questions
How do I convert EC to TDS without a calculator?
First convert the reading to µS/cm: multiply mS/cm by 1000, or S/m by 10,000. Then multiply by your meter's factor. For example, 1.2 mS/cm = 1200 µS/cm; at the 0.5 scale that is 1200 × 0.5 = 600 ppm. A quick mental shortcut: 1 mS/cm equals 500, 640 or 700 ppm depending on the chosen scale.
How do I convert TDS back to EC (ppm to µS/cm)?
Divide the TDS value by the same factor that was used to produce it. If a TDS meter reads 350 ppm on the 0.7 scale, the underlying conductivity is 350 ÷ 0.7 = 500 µS/cm. You must know the scale the meter used — applying the wrong factor in reverse introduces the same 40% error as in the forward direction.
What is a good TDS level for drinking water?
Most guidelines consider water below 300 ppm excellent and 300–600 ppm good. The WHO suggests palatability degrades above about 600 ppm, while levels over 1000 ppm are generally considered unacceptable for taste. These are aesthetic benchmarks, not safety limits — TDS cannot reveal whether specific harmful contaminants are present. For health-related concerns, have the water tested in a laboratory.
Which TDS conversion factor should I choose?
Match the scale printed in your meter's manual or documentation. US-made meters and hydroponics equipment commonly use the 0.5 (NaCl) scale, European instruments often use 0.7 (natural water), and 0.64 (KCl) appears on some laboratory devices. If you are comparing readings with someone else, both sides must use the same factor — otherwise convert everything to EC in µS/cm instead.
Why do my EC meter and TDS meter give inconsistent numbers?
Most likely the two devices use different conversion factors. A 1000 µS/cm solution reads 500 ppm on a 0.5-scale meter but 700 ppm on a 0.7-scale meter — a 40% difference for identical water. Check both manuals, pick one factor, and convert all readings consistently. Comparing raw EC values in µS/cm avoids the problem entirely.
Are ppm and mg/L the same thing?
For dilute fresh water, yes: 1 ppm ≈ 1 mg/L because the solution's density is close to 1 kg/L. The equivalence breaks down for brines and very concentrated liquids where the density is significantly higher than water. For drinking water, hydroponics and aquarium use, the two units can be treated as interchangeable.
Does water temperature affect the EC to TDS conversion?
Yes. Conductivity rises by roughly 2% per °C, so a cold sample will read lower than the same water when warm. Meters with automatic temperature compensation (ATC) correct this to a reference of 25 °C. Without ATC, always note the temperature when taking readings, and avoid comparing values measured at very different temperatures.