A TDS reading in ppm is a conductivity multiplied by a number the meter’s manufacturer chose. Three such numbers are in circulation, and they are forty per cent apart.
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Try:
1000 µS/cm
1,000 µS/cm
Closest reference water: Hard tap water at about 800.0 µS/cm.
The ppm figure depends on the meter, not the water
1,000 µS/cm on each conversion convention
Scale
Factor
Reads as
What it is
NaCl (the “500 scale”)
0.5
500.0 ppm
US practice, most hydroponics meters, and most aquarium meters. Calibrated against sodium chloride.
442 (the “700 scale”)
0.64
640.0 ppm
A blend of sulphate, bicarbonate and chloride intended to resemble natural fresh water.
KCl (the “700 scale”)
0.7
700.0 ppm
European and Australian practice, and the standard for laboratory calibration solutions.
A spread of 40% from one reading. The factor is a property of what the meter was calibrated against, not of your water — so two meters in the same tank showing 500 and 700 are usually both correct on different conventions. Before comparing a ppm figure with anyone else’s, find out which scale each meter uses; if the documentation does not say, the conductivity in µS/cm is the figure that travels.
The conductivity units, which do agree
The same conductivity in every unit
Unit
Value
Used by
microsiemens per centimetre (µS/cm)
1,000
The unit aquarium and drinking-water meters read in.
millisiemens per centimetre (mS/cm)
1.0
What hydroponics calls simply “EC”. A thousand µS/cm.
decisiemens per metre (dS/m)
1.0
Exactly the same as mS/cm — the metre and the centimetre cancel against the deci and the milli. Agriculture and soil science use this one.
siemens per metre (S/m)
0.1000
The SI form. Rarely used for water because the numbers are inconveniently small.
A decisiemens per metre and a millisiemens per centimetre are exactly the same unit — the deci cancels against the milli and the metre against the centimetre. Agriculture writes dS/m, aquaria write µS/cm, and hydroponics says “EC 1.8” meaning mS/cm. Unlike the ppm scales, these three always agree. This water’s resistivity is 1.00e-3 MΩ·cm, which is simply the reciprocal — the way ultrapure water is specified.
A reading without a temperature is not a measurement
If 1,000 µS/cm was read uncompensated at 18 °C1,163 µS/cmcorrected to 25 °C
What an uncompensated meter would show for this water
Sample at
Meter reads
Against 25 °C
5 °C
600.0 µS/cm
-40%
10 °C
700.0 µS/cm
-30%
15 °C
800.0 µS/cm
-20%
20 °C
900.0 µS/cm
-10%
25 °C
1,000 µS/cm
0%
30 °C
1,100 µS/cm
10%
35 °C
1,200 µS/cm
20%
Conductivity rises about 2% per degree, so identical water reads half again higher at 15 °C than at 35 °C before compensation. Every modern meter corrects to 25 °C automatically, which is why the figure it shows is not the figure its electrodes measured — and why a reading copied from a probe without compensation, or taken immediately after topping up with cold water, is not comparable with anything.
Where this water sits
Reference conductivities at 25 °C, with ppm on the NaCl scale
Water
µS/cm
ppm (0.5)
Note
Ultrapure (18.2 MΩ·cm)
0.0550
0.0275
The theoretical limit for water with nothing in it but its own ions.
Distilled or RO permeate
10.0
5.0
Varies widely with how recently it was made — it absorbs carbon dioxide from the air.
Rainwater
50.0
25.0
Higher near coasts and cities.
Soft tap water
200.0
100.0
Upland and reservoir supplies.
Hard tap water
800.0
400.0
Chalk and limestone aquifers.
Hydroponic nutrient solution
1,800
900.0
A typical mid-strength feed. Crop and stage dependent.
Brackish water
8,000
4,000
Estuaries and some groundwater.
Seawater
53,000
26,500
About 35 g/L of salts, and the reason a TDS scale factor matters so little here.
Six decades from ultrapure water to seawater. Worth knowing what the meter does not see: it measures how well the water carries current, which only ionic solutes do. Sugar, alcohol, urea and most organics dissolve completely and contribute almost nothing, so a “TDS” figure can be near zero in water that is full of them. The real measurement of total dissolved solids is gravimetric — evaporate a known volume and weigh what is left — and a conductivity meter is an inference from it, which is exactly why the inference needs a stated convention.
What this converter covers
Every conversion on all three scales at once, with temperature compensation and a ladder from ultrapure water to seawater.
µS/cm, mS/cm, dS/m and S/m, which always agree
ppm on the NaCl, 442 and KCl scales, which do not
What the same water reads as on a different meter
Temperature compensation to 25 °C, in both directions
Resistivity in MΩ·cm, and reference waters across six decades
Three ppm scales 2% per degree dS/m = mS/cm Ionic only
1000 µS/cm is 500, 640 or 700 ppm · 700 ppm on KCl is 500 ppm on NaCl
Educational estimate
Planning support from the values you enter — not professional advice.
The ppm factor is a convention
Two meters in the same tank showing 500 and 700 are, almost always, both correct. A TDS meter does not measure dissolved solids — it measures conductivity and multiplies by a factor, and three factors are in common use.
1000 µS/cm, on each convention
Scale
Factor
Reads
Common in
NaCl (“500 scale”)
0.50
500 ppm
US practice, most hydroponics and aquarium meters
442
0.64
640 ppm
Meters aiming at natural fresh water
KCl (“700 scale”)
0.70
700 ppm
Europe, Australia, laboratory calibration
A 40% spread on one reading. The factor describes what the meter was calibrated against — sodium chloride, a blend called 442, or potassium chloride — and answers the question “what would this conductivity be if the dissolved solid were that salt?”. Your water is not that salt, so none of the three is more correct than the others.
This is why a nutrient recipe from one forum and a meter from another shop can disagree by a third with nobody at fault. It is also why experienced growers talk in EC rather than ppm: the conductivity in µS/cm or mS/cm is the measurement, and the ppm figure is an interpretation of it.
The practical rule is short. Before comparing a ppm figure with anyone else’s, find out which scale each meter uses — it is usually in the manual, sometimes switchable in the settings, and occasionally printed on the device. If you cannot find out, convert to conductivity and compare that.
A conductivity meter cannot see sugar
The deeper limitation is not which factor is used but what the measurement is. Conductivity is how well water carries an electric current, and only ions carry current. Anything that dissolves without splitting into charged particles is invisible.
Sugar, alcohol, urea, most organic acids, oils and glycols all dissolve completely and contribute almost nothing to a reading. A sugar solution can be syrupy and read close to distilled water. Meanwhile a pinch of salt in a bucket moves the number a great deal, because it is entirely ionic.
The reference method for total dissolved solids is gravimetric: filter a known volume, evaporate it, and weigh what is left. That measures everything dissolved, ionic or not, and it is what “TDS” actually means. A conductivity meter reports an inference from a related quantity — which is exactly why the inference needs a stated convention, and why the two can differ substantially in water with a lot of organic content.
It also cannot see suspended matter, since a filter removes that before the gravimetric test and particles do not conduct. So a reading says nothing about cloudiness, algae or sediment, and nothing at all about bacteria.
Two per cent per degree
Conductivity is strongly temperature dependent — ions move more freely when warm — at roughly two per cent per degree Celsius for natural waters. That is large enough to swamp most of what people are trying to detect.
Identical water, read by an uncompensated meter
Sample at
Meter reads
Against 25 °C
5 °C
600 µS/cm
−40%
15 °C
800 µS/cm
−20%
25 °C
1000 µS/cm
—
35 °C
1200 µS/cm
+20%
End to end that is half again across an ordinary range. Every modern meter compensates automatically to 25 °C, which means the number on the screen is deliberately not the number its electrodes measured — and that is the correct behaviour, not a fudge.
Where it bites: a reservoir topped up with cold tap water reads low until it equilibrates, a probe left in the sun reads high, and a reading taken with a cheap meter that lacks compensation is not comparable with one that has it. Before concluding that something has changed, check that the temperature has not.
The two per cent figure is a convention rather than a constant. The true coefficient depends on which ions are present and drifts at the extremes of the range, so compensation is an approximation — a good one near room temperature and a rougher one near freezing.
dS/m and mS/cm are the same unit
The conductivity units look unrelated and three of them are trivially interchangeable — which is a relief after the ppm scales.
One conductivity, four ways
Unit
Value
Who uses it
µS/cm
1800
Aquaria, drinking water
mS/cm
1.8
Hydroponics — “EC 1.8”
dS/m
1.8
Agriculture, soil science
S/m
0.18
The SI form, rarely used for water
Rows two and three are the same number because they are the same unit: the deci cancels against the milli and the metre against the centimetre. A soil report in dS/m and a nutrient chart in mS/cm can be read straight across, which is worth knowing because the two literatures rarely mention each other.
At the very clean end the convention flips to resistivity, the reciprocal, in megohm-centimetres. Ultrapure water is specified as 18.2 MΩ·cm, which is 0.055 µS/cm — the limit set by water’s own self-ionisation, since even perfectly pure water contains some hydrogen and hydroxide ions. Nothing can be cleaner than that, which is why the number is a fixed target rather than an aspiration.
Using a reading properly
Four habits cover most of what goes wrong, and none of them requires a better meter.
Record conductivity, not ppm. The µS/cm or mS/cm figure is the measurement and travels between people and instruments. A ppm figure carries a hidden convention and does not.
Note the temperature, or trust the compensation. If the meter compensates, the reading is already at 25 °C and can be compared with any other compensated reading. If it does not, the temperature is part of the measurement and a reading without it is not usable later.
Calibrate against a standard solution. Probes drift as they foul and age, and a drifted meter is confidently wrong in a way that no amount of unit conversion fixes. Calibration solutions are cheap and the interval is usually stated in the manual.
Do not ask the reading what is in the water. It reports total ionic content and nothing about composition, so it cannot distinguish harmless minerals from anything else, and it is blind to whatever is not ionic. For water that has to be safe, or a feed that has to be right, the reading is a monitoring tool between proper analyses rather than a substitute for one.
Related calculators
Other water and concentration tools:
Water Hardnessppm, gpg, °dH, °fH and °e — every factor derived from molar masses, plus the calcium and magnesium a lab actually reports.
PPMppm to percent, mg/L and µg/m³ — asking which liquid or which gas, because without that the conversion has no answer.
Moles to GramsGrams, moles, molarity, molality and normality — asking for the molar mass and the density, because neither conversion exists without them.
TemperatureCelsius, Fahrenheit, kelvin and Rankine — and temperature CHANGES, which convert differently from readings.
Densitykg/m³, lb/ft³, lb/in³ and lb/gal, with specific gravity against any water reference and the API scale that runs backwards.
VolumeLitres, gallons, pints and cubic units — with US and imperial named apart, because a UK gallon is 20% larger than a US one.
The two standard methods here say the thing this page is built on: total dissolved solids is defined gravimetrically — evaporate and weigh — while conductivity is a separate measurement that can be used to estimate it against a chosen reference salt. The 0.7 convention comes from the potassium chloride calibration solutions in ISO 7888, and the 2% per degree correction is the conventional figure for natural waters in Standard Methods 2510.
Standard Methods 2510 — ConductivityAmerican Public Health Association / AWWA / WEF · verified 2026-09-08 · That conductivity is reported at 25 °C with a temperature correction of about 2% per degree, and that a conductivity-derived “TDS” is an empirical estimate against a chosen reference salt rather than a measurement of dissolved solids
Standard Methods 2540 C — Total Dissolved Solids Dried at 180 °CAmerican Public Health Association / AWWA / WEF · verified 2026-09-08 · That the reference method for total dissolved solids is gravimetric — evaporate a filtered sample and weigh the residue — which is why a conductivity meter reports an inference and not the quantity itself
ISO 7888 — Water quality: determination of electrical conductivityInternational Organization for Standardization · verified 2026-09-08 · The definition of electrical conductivity for water, its reference temperature, and the potassium chloride calibration solutions that underlie the 0.7 convention
Conversion note
This converts readings and does not assess water. A conductivity or TDS figure says nothing about what is dissolved: the same reading can come from harmless minerals or from something that matters, and the meter cannot tell them apart. It is also blind to anything non-ionic, so it will not detect sugars, alcohols, most organics, oils, bacteria or particulates. Nothing here indicates whether water is safe to drink, suitable for a fish species, or correctly fed for a crop — those are questions for a laboratory analysis, a local water report or an agronomist, and the right target depends on the species, the growth stage, the substrate and the source water. The temperature correction is the conventional linear 2% per degree, which is an approximation that degrades at the extremes and differs between waters. Meters drift and need calibration against a standard solution; a reading from an uncalibrated probe is not a measurement whatever units it is in.
Published the EC to TDS Converter: microsiemens and millisiemens per centimetre, decisiemens per metre and siemens per metre, with parts per million on all three conversion conventions at once.
Establishes that the ppm factor is a convention set by the meter's calibration standard rather than a property of the water: 0.5 for sodium chloride, 0.64 for the 442 blend and 0.7 for potassium chloride, so 1000 microsiemens per centimetre reads as 500, 640 or 700 ppm.
Resolves the question that brings most people here -- two meters in the same tank showing 500 and 700 are usually both correct, on different conventions, and 700 ppm on a KCl meter is exactly 500 on a NaCl one.
Recommends recording conductivity rather than ppm, since the microsiemens figure is the measurement and travels between instruments while a ppm figure carries a hidden convention.
States the deeper limitation: a conductivity meter measures how well water carries current, which only ionic solutes do, so sugar, alcohol, urea and most organics dissolve completely and contribute almost nothing to a reading.
Notes that the reference method for total dissolved solids is gravimetric -- filter, evaporate and weigh -- so a conductivity meter reports an inference from a related quantity, which is exactly why the inference needs a stated convention.
Handles temperature explicitly, at the conventional two percent per degree: identical water reads 600 microsiemens at 5 degrees and 1200 at 35 before compensation, half again across an ordinary range.
Points out that a compensated meter deliberately shows a figure its electrodes did not measure, and names where that bites -- a reservoir topped up with cold water, a probe left in the sun, and a cheap meter without compensation.
Records that decisiemens per metre and millisiemens per centimetre are exactly the same unit, since the deci cancels against the milli and the metre against the centimetre, so a soil report and a nutrient chart can be read straight across.
Covers the resistivity convention at the clean end, where ultrapure water is specified as 18.2 megohm-centimetres -- the reciprocal of 0.055 microsiemens per centimetre, and the limit set by water's own self-ionisation.
States what a reading cannot tell you: nothing about composition, nothing non-ionic, nothing about particulates or bacteria, and nothing about whether water is safe to drink or correctly fed for a crop.
Verified by 47 automated cases, asserting the spread across the three scales rather than any one factor, that decisiemens per metre equals millisiemens per centimetre at zero tolerance, and that compensation moves cold readings up and warm ones down at every degree.
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