The total dissolved solids calculator above handles the two routes to a TDS figure. The quick one converts an electrical conductivity reading into milligrams per litre using a conversion factor, with a correction back to the 25 °C reference temperature that conductivity is defined at. The rigorous one takes the weight of residue left after a filtered sample has been evaporated and dried, which is what the standard analytical method actually measures.
Arb Digital publishes free calculators that state the limits of what they produce. The two routes here do not measure the same thing, and the gap between them is the whole story of this page. One is an inference from an electrical property; the other is a gravimetric determination. Understanding why they disagree is more useful than either number on its own.
What This Total Dissolved Solids Calculator Does
Total dissolved solids is the mass of dissolved material remaining after a water sample has been filtered and the water evaporated. It covers everything in solution: the major ions, dissolved silica, dissolved organic matter, and anything else small enough to pass the filter. It is one of the most widely reported water quality parameters and one of the most widely misunderstood, because the handheld meters that report it do not measure it.
In conductivity mode this page corrects your reading to 25 °C, applies the conversion factor you choose, and reports TDS in mg/L and g/L. In gravimetric mode it subtracts the dish tare from the final weight and divides by the sample volume, which is the arithmetic of the standard method. The fourth grid item converts TDS into an approximate ionic strength using the empirical correlation used in water chemistry, so the figure can be carried into equilibrium work.
A boundary worth stating: the live water hardness calculator deals only with calcium and magnesium, which are one part of the dissolved load and often not the largest. A softened supply has had its calcium exchanged for sodium, so its hardness collapses while its TDS barely moves. Separately, the conductivity to resistivity calculator handles the electrical properties of conductors as materials and does not convert to a water quality parameter at all.
How to Use It
- Choose the method that matches what you actually have: a meter reading, or a weighed residue.
- Enter the sample temperature. If your meter applies automatic temperature compensation, leave this at 25 so no second correction is applied.
- Set the conversion factor deliberately. The default of 0.65 is a common middle value, not a universal one, and changing it changes the answer proportionally.
- For the gravimetric route, enter the dish tare, the dish plus dried residue, and the volume of filtered sample evaporated.
- Record which method produced your number whenever you report it. The two are not interchangeable.
The Formulas and How They Are Calculated
Conductivity is referenced to 25 °C because it rises with temperature. The correction is EC25 = ECt / (1 + α(t − 25)) with α expressed as a fraction per degree. TDS then follows as TDS (mg/L) = k × EC25 (µS/cm), where k is the conversion factor.
Worked through: a reading of 520 µS/cm taken at 18 °C with a 2 percent per degree coefficient corrects to 520 / (1 − 0.14) = 604.7 µS/cm at 25 °C. Cool water conducts less, so the reference value is higher than the raw reading. Applying k = 0.65 gives 393 mg/L. Had the sample been at 25 °C to begin with, the same raw reading would have given 338 mg/L, so a seven degree temperature difference moves the answer by about fourteen percent.
The gravimetric route is TDS (mg/L) = (mfinal − mtare) in mg / volume in litres. A dish going from 1.2100 g to 1.2345 g after evaporating 100 mL of filtered sample gained 0.0245 g, which is 24.5 mg in 0.1 L, so 245 mg/L. That determination is standardised as Total Dissolved Solids Dried at 180 °C, method 2540 C in Standard Methods for the Examination of Water and Wastewater, with the equivalent EPA method numbered 160.1. Both specify the filter, the drying temperature and the requirement to dry to constant weight.
The ionic strength figure uses the empirical relation I ≈ 2.5 × 10−5 × TDS in mg/L, a correlation fitted to typical natural waters and reliable only for waters with an ordinary ion mix.
Why the Conversion Factor Is Not a Constant
This is the most important thing on the page. A conductivity meter measures how readily current passes between two electrodes. That depends on how many ions are present, how much charge each carries, and how fast each moves through water. It does not depend on mass, which is what TDS is.
Different ions conduct very differently per unit mass. Hydrogen and hydroxide ions are exceptionally mobile and conduct far better than their mass suggests. Sulfate carries two charges and contributes more per mole than chloride. And a great deal of dissolved material carries no charge at all: dissolved silica, sugars, alcohols and much dissolved organic carbon are entirely invisible to a conductivity meter while contributing fully to a gravimetric TDS.
The factor k therefore encodes an assumption about the ion mix. Values between roughly 0.55 and 0.8 are quoted for different water types, and a meter set to one value used on a water it was not intended for will be systematically wrong. Meters sold with a fixed internal factor and a TDS display are the worst case, because the assumption is hidden. If two laboratories report different TDS values for the same water, the first thing to compare is which method and which factor each used, and if you are quantifying that gap the percent error calculator is the straightforward tool.
What the Gravimetric Method Includes and Excludes
The standard method is precise about its own boundaries, and the boundaries are not intuitive. Dissolved is defined operationally as whatever passes the specified filter; anything retained is total suspended solids and is measured separately. Change the filter and you change the answer, which is why the method specifies it.
Drying at 180 °C is also a definition rather than a physical truth. At that temperature most occluded water is driven off, but bicarbonate is converted to carbonate with loss of carbon dioxide and water, so a bicarbonate-dominated water loses part of its dissolved load in the oven. Volatile organic material is driven off too. The number that results is the mass of what remains after a specified procedure, which is why the method name states the drying temperature.
Hygroscopic residues complicate the weighing, since a dried dish will pick up moisture from the air on the way to the balance, which is why the method requires cooling in a desiccator and drying to constant weight rather than for a fixed time. None of this makes the gravimetric method unreliable. It makes it a defined measurement rather than an absolute one, which is the same status every analytical result has.
What the Regulations Say, and What the Number Cannot Tell You
In the United States, total dissolved solids appears in the EPA's secondary drinking water standards, which set a secondary maximum contaminant level of 500 mg/L. Those standards are explicitly non-enforceable federal guidelines concerned with taste, odour and appearance rather than with health, and states may adopt them or not. Other jurisdictions publish their own values under their own regulations, and the applicable figure for any particular supply is the one set by the authority with jurisdiction over it.
Nothing in a TDS number identifies what the dissolved solids are. A water at 300 mg/L of dissolved calcium bicarbonate and a water at 300 mg/L containing a dissolved contaminant produce the same reading. TDS is a bulk parameter and a screening indicator; it is not a test for anything specific and it says nothing about microbiological quality, which is measured by entirely different methods. Whether any particular water is suitable for any particular use is a determination for the water authority with jurisdiction and an accredited laboratory, not something this or any calculator can supply.
Where TDS is genuinely useful is as a trend. Dissolved solids in surface water come overwhelmingly from natural weathering, with road deicers, agriculture and urban runoff contributing the remainder, as the USGS national assessment of dissolved-solids sources, loads, yields and concentrations in streams sets out. A step change in a monitored supply is a signal worth investigating, and that is what routine conductivity monitoring is for.
Reading TDS Into the Rest of Your Water Chemistry
TDS is an input to several other calculations rather than an endpoint. It correlates with ionic strength, which governs activity coefficients and therefore every equilibrium constant that matters in a real water; the ionic strength calculator does that properly from an ion list when you have a full analysis, and the correlation printed here is a stopgap when you do not.
It also underpins a charge balance check on a laboratory report. If you convert every reported ion from mg/L to millimoles per litre and the cations and anions do not balance, something is missing or wrong, and comparing the sum of the reported ions against the measured TDS is a second, independent check on the same thing. The molar mass calculator handles that conversion, and the concentration converter covers the unit changes between mg/L, ppm and molar units.
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Browse All Free Tools Suggest a ToolCommon Mistakes to Avoid
- Applying a temperature correction twice — if the meter already compensates to 25 °C, entering the sample temperature here corrects an already corrected value.
- Treating the conversion factor as universal — it encodes an assumed ion mix and ranges from about 0.55 to 0.8 across different water types.
- Assuming conductivity sees everything dissolved — silica, sugars and much dissolved organic carbon carry no charge and are invisible to the meter.
- Comparing a meter figure against a laboratory figure — they are different determinations, and reporting which was used matters more than the difference between them.
- Reading TDS as a safety indicator — it is a bulk parameter that identifies nothing specific and says nothing about microbiological quality.
Related Free Tools From Arb Digital
Use the water hardness calculator for the calcium and magnesium fraction specifically, and the ionic strength calculator when you have a full ion list. The molar mass calculator and concentration converter handle unit changes on a laboratory report, the pH calculator covers the acid-base side of the same sample, and the percent error calculator compares two determinations. The full free online tools hub lists everything else.
Frequently Asked Questions
TDS is the mass of dissolved material left after a water sample has been filtered and evaporated to dryness, reported in milligrams per litre. It includes the major ions plus dissolved silica and dissolved organic matter, and excludes anything the filter retains.
Correct the reading to 25 degrees Celsius, then multiply by a conversion factor. The factor is commonly between about 0.55 and 0.8 and depends on the ion mix, so the result is an estimate rather than a measurement.
Because conductivity depends on how many ions there are, what charge they carry and how fast they move, while TDS depends on mass. Uncharged dissolved material such as silica and many organics contributes to TDS while conducting nothing at all.
Ions move faster in warmer water, so conductivity rises with temperature by roughly 2 percent per degree. Readings are referenced to 25 degrees Celsius, and many meters apply that correction internally, in which case it should not be applied again.
Total Dissolved Solids Dried at 180 degrees Celsius, method 2540 C in Standard Methods for the Examination of Water and Wastewater, with the equivalent EPA method numbered 160.1. It specifies the filter, the drying temperature and drying to constant weight.
The United States EPA lists TDS in its secondary drinking water standards at a secondary maximum contaminant level of 500 mg/L. Those standards are non-enforceable federal guidelines addressing taste, odour and appearance, and other jurisdictions set their own values.
No. Hardness counts only calcium and magnesium, while TDS counts everything dissolved. A softened water has had its calcium exchanged for sodium, so its hardness falls sharply while its total dissolved solids stay much the same.
No. It is a bulk parameter that identifies nothing specific and says nothing about microbiological quality. Suitability for any particular use is determined by the water authority with jurisdiction, on the basis of testing by an accredited laboratory.
This calculator is provided for education and general reference. It describes how a total dissolved solids figure is computed and is not water quality, treatment or public health guidance, and it makes no assessment of whether any water is fit for any purpose. Drinking water and wastewater are regulated; the water authority with jurisdiction over your supply and an accredited laboratory are the authorities on any real sample.
