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TDS Calculator

Ready to calculate
EC ↔ TDS Conversion.
All 3 K-Factors.
Hydroponic Reference.
100% Free.
No Data Stored.

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How it Works

01Enter EC or TDS

Input either electrical conductivity or TDS reading.

02Choose K-Factor

0.5 (NaCl), 0.64 (442), or 0.7 (KCl) per meter.

03Bidirectional Convert

EC → TDS and TDS → EC across all factors.

04Reference Targets

Compare against drinking water and hydroponic ranges.

What is a TDS Calculator?

Total Dissolved Solids (TDS) measures the total mass of dissolved substances in water — minerals, salts, organics — expressed as parts per million (ppm) or mg/L. TDS meters actually measure electrical conductivity (EC) in microsiemens/cm and convert to ppm using a k-factor (typically 0.5, 0.64, or 0.7 depending on calibration standard).


The calculator converts between EC (μS/cm) and TDS (ppm) using your meter's k-factor — essential for hydroponics, drinking water testing, aquariums, and pool/spa management. Different industries use different conversion factors, and getting the wrong one means your "500 ppm" reading could actually be 700 ppm.

How to Use the Calculator

Pick conversion direction: EC → TDS or TDS → EC.
Enter measured value: EC in μS/cm or TDS in ppm.
Set k-factor: 0.5 (NaCl, US standard), 0.64 (442 mixed standard), 0.7 (KCl, European). Check your meter's spec sheet.
Calculate: Returns the converted value plus reference levels for drinking water, hydroponics, and pool standards.

The Math Behind It

The fundamental relationship:

TDS (ppm) = EC (μS/cm) × k

where k is the conversion factor:

  • k = 0.50: calibrated against NaCl (US "ppm-500" scale, common in hydroponics)
  • k = 0.64: calibrated against the "442" mixed standard (40% Na₂SO₄, 40% NaHCO₃, 20% NaCl) — natural water proxy
  • k = 0.70: calibrated against KCl (European/lab standard, "ppm-700" scale)

The reverse: EC (μS/cm) = TDS (ppm) / k. A reading of "500 ppm" on a 0.5-scale meter = 1000 μS/cm = "700 ppm" on a 0.7-scale meter — same water, different number.

Real-World Example

Worked Example

Hydroponic nutrient solution measured at 1.4 mS/cm = 1400 μS/cm:

  • k = 0.5: TDS = 1400 × 0.5 = 700 ppm (US scale)
  • k = 0.64: TDS = 1400 × 0.64 = 896 ppm (442 scale)
  • k = 0.7: TDS = 1400 × 0.7 = 980 ppm (KCl scale)

Same nutrient strength, three different "ppm" numbers — always quote EC alongside TDS for unambiguous communication.

Who Uses It

1
🌱 Hydroponic Growers: Manage nutrient strength across crops and growth stages.
2
🚰 Drinking Water Testing: Compare against EPA secondary standard (500 ppm).
3
🐠 Aquarists: Track salinity in marine/brackish tanks; monitor TDS rise from waste.
4
🏊 Pool/Spa Managers: Maintain TDS below 1500 ppm above source water.
5
💧 RO/DI System Operators: Verify membrane and resin performance.
6
🧪 Lab Technicians: Cross-reference EC and TDS measurements across instruments.

Technical Reference

Reference levels:

  • RO/DI water: <10 ppm (k=0.5)
  • Distilled water: <5 ppm
  • Tap water (US average): 100–400 ppm
  • EPA secondary standard: 500 ppm max for drinking water
  • Hydroponic seedlings: 400–700 ppm (EC 0.8–1.4)
  • Hydroponic veg: 800–1200 ppm (EC 1.6–2.4)
  • Hydroponic fruiting/flower: 1200–2000 ppm (EC 2.4–4.0)
  • Brackish water: 1000–10,000 ppm
  • Seawater: ~35,000 ppm (EC ~50,000 μS/cm)

Key Takeaways

EC is the actual measurement; TDS is a calibration-dependent estimate. Always note your meter's k-factor (0.5, 0.64, or 0.7) when sharing readings — "1000 ppm" is meaningless without it. For hydroponics and analytical comparison, quote EC in μS/cm or mS/cm directly.

Frequently Asked Questions

Which k-factor should I use?
Match your meter's factory calibration — usually printed on the meter or in its manual. US-made hydroponic meters (BlueLab, HM Digital) often default to 0.5 or 0.7. European meters typically 0.64 or 0.7. When in doubt, just use EC and ignore the ppm conversion.
Why is my TDS rising over time in a hydroponic reservoir?
Plants absorb water faster than nutrients (transpiration concentrates the solution). Top off with plain water (or weakened nutrient) when EC rises 10–20% above target. Dump and refresh weekly to prevent salt buildup.
Can I measure TDS without a meter?
Gravimetric: evaporate a known volume of water, weigh the residue. Lab-grade but slow (24+ hours at 180°C). EC meters are the practical standard.
What's the difference between TDS and salinity?
Salinity (PSU or ppt) measures dissolved salts only — primarily NaCl. TDS includes everything dissolved (organics, hardness ions, etc.). For seawater/aquarium use, the two are nearly identical; for natural fresh water, TDS > salinity.
How does temperature affect EC readings?
EC rises ~2% per °C. Most meters auto-temperature-compensate (ATC) to a 25 °C reference. If yours doesn't, multiply or divide accordingly.
Is high TDS in drinking water dangerous?
TDS is a "secondary" (aesthetic) parameter — high TDS affects taste and may indicate hardness or contamination but isn't directly toxic. WHO finds no health-based guideline for TDS. Consistent values >1000 ppm warrant a full mineral analysis.

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Disclaimer

TDS values are calibration-dependent estimates. For regulatory or analytical purposes, use gravimetric TDS (EPA Method 160.1) or report EC directly. The k-factor varies with the dominant ions in your specific water.