Last updated: July 3, 2026
TDS Calculator
Creators
Dharmendra SinghReviewers
Creators
Dharmendra SinghReviewers
Quick Answer
TDS, or total dissolved solids, is commonly estimated from electrical conductivity using TDS = ke × EC, where EC is in µS/cm and ke is usually about 0.5–0.7. It can also be measured gravimetrically as dry residue mass in milligrams divided by sample volume in litres. For dilute water, mg/L is approximately equal to ppm.
Total dissolved solids can be estimated by multiplying electrical conductivity in microsiemens per centimetre by a conversion factor, usually about 0.5 to 0.7, or measured as dry residue mass divided by sample volume.
Key Takeaways
- Conductivity-based TDS is estimated as TDS = ke × EC, with EC in µS/cm.
- The ke conversion factor depends on dissolved ion composition and commonly ranges from 0.5 to 0.7.
- Gravimetric TDS equals dry residue mass in milligrams divided by sample volume in litres.
- For dilute water, TDS in mg/L is numerically close to ppm.
- TDS is a screening measurement; it does not identify which dissolved substances are present.
Creators
Dharmendra SinghReviewers
Creators
Dharmendra SinghReviewers
Formula
TDS = ke × EC; gravimetric TDS = dry residue mass / sample volume
Where:
- TDS=Total dissolved solids(mg/L or ppm)
- k_e=Conductivity-to-TDS conversion factor(mg·L⁻¹ per µS·cm⁻¹)
- EC=Electrical conductivity(µS/cm)
- m_residue=Mass of dried filtrate residue(mg)
- V_sample=Water sample volume(L)
Worked Examples
Conductivity estimate with ke = 0.65
A water sample has EC = 1000 µS/cm and a typical mixed-salt conversion factor.
- 1Use the conductivity estimate TDS = ke × EC.
- 2Substitute ke = 0.65 and EC = 1000 µS/cm.
- 3TDS = 0.65 × 1000 = 650 mg/L, approximately 650 ppm.
Low-factor conductivity estimate
A sodium-chloride-like calibration uses ke = 0.5 for EC = 500 µS/cm.
- 1Apply TDS = ke × EC.
- 2Substitute ke = 0.5 and EC = 500 µS/cm.
- 3TDS = 0.5 × 500 = 250 mg/L, approximately 250 ppm.
Gravimetric dry-residue result
Drying a 0.1 L filtered sample leaves 40 mg of residue.
- 1Use gravimetric TDS = dry residue mass ÷ sample volume.
- 2Substitute 40 mg and 0.1 L.
- 3TDS = 40 ÷ 0.1 = 400 mg/L, approximately 400 ppm.
Introduction
Total dissolved solids (TDS) is the mass concentration of dissolved inorganic salts and small amounts of dissolved organic matter in water. This calculator estimates TDS from electrical conductivity using TDS = ke × EC or calculates a gravimetric TDS from dried residue mass divided by sample volume. It is useful for drinking-water screening, boilers, aquariums, hydroponics, and process water checks. For solution concentration context, compare it with the mass percent calculator and molarity calculator. Laboratory TDS methods are described by USGS water-quality guidance and Standard Methods.
What is TDS?
TDS means total dissolved solids: minerals, salts, metals, cations, anions, and other dissolved material small enough to pass through a standard filter. It is usually reported as mg/L. In dilute water, 1 mg/L is numerically close to 1 ppm because the density of water is close to 1 kg/L.
TDS is not a single chemical; it is a combined mass concentration.
Common contributors include calcium, magnesium, sodium, chloride, sulfate, bicarbonate, nitrate, and potassium.
A conductivity meter responds to ions, so non-ionic dissolved organics may not be represented well.
For organic pollution indicators, compare with the chemical oxygen demand calculator.
Conductivity to TDS formula
Electrical conductivity (EC) measures how easily ions carry current in water. A meter-estimated TDS multiplies EC in µS/cm by an empirical conversion factor ke. The factor depends on ionic composition: sodium chloride solutions often use about 0.5, while natural waters with mixed ions often use 0.55–0.7. This calculator defaults to 0.65 because it is a common field estimate, not a universal constant.
Always record the conversion factor with a conductivity-based TDS value; two meters can display different TDS for the same EC if their factors differ.
Gravimetric TDS formula
The gravimetric method filters a measured water volume, evaporates the filtrate, dries the residue at a specified temperature, and weighs the remaining solids. The calculation is TDS = residue mass in milligrams divided by sample volume in litres. This method directly measures dissolved residue and is the reference approach when accurate TDS is required.
Use a clean, dried, weighed dish or capsule.
Filter to remove suspended solids before evaporation.
Report drying conditions because volatile components can be lost.
Use adequate sample volume so the residue mass is measurable.
mg/L and ppm for water
For dilute aqueous samples, mg/L and ppm are treated as approximately equal because one litre of water has a mass close to one kilogram. The approximation becomes less exact for dense brines, hot process streams, or non-water solvents. If density is far from 1 kg/L, convert ppm by mass using the actual solution density instead of assuming equality.
| Water type | Typical TDS range | Comment |
|---|---|---|
| Rainwater | < 50 mg/L | Very low dissolved mineral content |
| Fresh surface water | 50–500 mg/L | Varies with geology and runoff |
| Drinking water palatability guideline | < 500 mg/L | Often considered acceptable taste |
| Seawater | ≈ 35,000 mg/L | High dissolved salt content |
How to calculate TDS step by step
For a quick field estimate, measure EC with a calibrated conductivity meter, choose the appropriate ke factor, and multiply. For gravimetric work, weigh the dry residue and divide by the sample volume in litres. The calculator uses a valid gravimetric result when residue mass and volume are supplied; otherwise it uses the conductivity estimate.
Measure EC in µS/cm or enter residue mass and sample volume.
Use ke = 0.65 unless your meter, standard, or water type specifies another factor.
Compute TDS = ke × EC for conductivity estimates.
Compute TDS = residue mg ÷ sample L for gravimetric measurements.
Treat mg/L and ppm as equal only for dilute water.
Limitations and interpretation
TDS is a screening parameter, not a complete water analysis. It does not identify which ions are present, whether a contaminant exceeds a health-based limit, or whether suspended solids are present. Temperature compensation, calibration standards, evaporation losses, and filter choice can all affect results. For health or compliance decisions, use certified laboratory analysis and consult authoritative drinking-water guidance such as the WHO drinking-water guidelines.
If TDS changes suddenly in a process or well, investigate individual ions, pH, alkalinity, hardness, and possible contamination sources rather than relying on TDS alone.
Quick Reference Card
TDS — Quick Reference
Quick reference • TDS Calculator
TDS (mg/L) = ke × EC (µS/cm); gravimetric TDS = residue mg ÷ sample LValid range: Fresh waters often range from <50 to 1500 mg/L; seawater is about 35,000 mg/L
Common Values
⚠ Watch Out
- •Do not treat ke as universal; ionic composition changes the EC-to-TDS relationship.
- •Use µS/cm for the EC input; mS/cm must be multiplied by 1000 first.
- •TDS does not identify individual contaminants or prove water is safe to drink.
- •For brines or dense solutions, mg/L and ppm are not exactly equal.
- •Gravimetric results depend on filtration, drying temperature, and residue handling.
Pro Tips
- →Calibrate conductivity meters with standards near your expected range.
- →Record temperature compensation settings and the ke factor with each TDS reading.
- →Use gravimetric TDS for confirmatory laboratory measurements.
- →Track trends over time; sudden TDS shifts can signal intrusion, leaks, or treatment changes.
- →Pair TDS with hardness, alkalinity, pH, and ion-specific tests for diagnosis.
FAQs
How do I estimate TDS from conductivity?
Multiply electrical conductivity in µS/cm by a conversion factor ke. For example, EC = 1000 µS/cm and ke = 0.65 gives TDS = 650 mg/L. Choose the factor that matches your meter calibration or water type.
Is TDS in ppm the same as mg/L?
For dilute water, ppm and mg/L are approximately equal because water density is close to 1 kg/L. The equality is an approximation and becomes less reliable for concentrated brines or liquids with density far from water.
What conversion factor should I use?
Common EC-to-TDS factors are about 0.5 to 0
- 1Sodium chloride standards often use roughly 0.5, while mixed natural waters often use 0.55 to 0
- 2The default 0.65 is a practical estimate, not a universal constant
What is the difference between TDS and hardness?
Hardness mainly describes calcium and magnesium ions, while TDS includes all dissolved solids. A water sample can have high TDS from sodium chloride but relatively low hardness, or high hardness with moderate TDS.
Why can a TDS meter differ from a lab result?
A handheld TDS meter estimates dissolved solids from conductivity and an assumed factor. A laboratory gravimetric result measures dry residue directly. Differences arise from ionic composition, temperature compensation, calibration, volatile solids, and non-ionic dissolved material.
What TDS level is safe for drinking water?
TDS by itself is usually an aesthetic and palatability indicator rather than a direct toxicity measure. Many guidelines discuss taste concerns above about 500 mg/L, but safety depends on the specific dissolved substances present.