# TDS

> TDS (total dissolved solids) is the mass of dissolved material in water; grow meters estimate it from EC with a conversion factor and display it as ppm.

- Canonical: https://genetixbay.com/knowledge/glossary/tds.html
- Also written: total dissolved solids, TDS meter, TDS reading, ppm reading, dissolved solids
- Glossary category: pH, EC and water · Evidence: Established
- Status: Draft: awaiting expert review · Updated: 2026-10-31
- Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay cannabis glossary"

TDS (total dissolved solids) is the total mass of dissolved substances in water, properly measured in a lab by evaporating a filtered sample and weighing the residue, in mg/L (ppm). Handheld "TDS meters" do not do this. They measure [EC](https://genetixbay.com/knowledge/glossary/ec.md) and multiply it by a [conversion factor](https://genetixbay.com/knowledge/glossary/conversion-factor.md) chosen by the maker, usually 0.5 or 0.7, sometimes 0.64.

## In practice

Because different ions conduct differently, no single factor is accurate for every solution. A nutrient mix rich in [nitrate](https://genetixbay.com/knowledge/glossary/nitrate.md) and potassium and a [tap water](https://genetixbay.com/knowledge/glossary/tap-water.md) rich in bicarbonate can have the same EC but very different true TDS. That is why two meters can read "800 ppm" and "1,120 ppm" for the same reservoir, and why ppm figures from someone else's chart are meaningless without the scale.

TDS meters are still useful for checking [RO](https://genetixbay.com/knowledge/glossary/ro-water.md) output, comparing your water to its usual baseline, and tracking reservoir changes over time with one meter. For sharing or following nutrient charts, use [EC in](https://genetixbay.com/knowledge/glossary/input-ec.md) mS/cm.

## Units and formula

- TDS (ppm) ≈ EC (µS/cm) × conversion factor
- [500 scale](https://genetixbay.com/knowledge/glossary/ppm500.md): × 0.5; [700 scale](https://genetixbay.com/knowledge/glossary/ppm700.md): × 0.7; some agronomy sources use 640 ppm per dS/m

## Don't confuse with

- **EC**: the real measurement; TDS on a meter is derived from it.
- **ppm of a single element**: e.g. 150 ppm N in a recipe; unrelated to meter TDS.

## Related terms

[EC](https://genetixbay.com/knowledge/glossary/ec.md), [ppm500](https://genetixbay.com/knowledge/glossary/ppm500.md), [ppm700](https://genetixbay.com/knowledge/glossary/ppm700.md), [Conversion factor](https://genetixbay.com/knowledge/glossary/conversion-factor.md), [mS/cm](https://genetixbay.com/knowledge/glossary/ms-cm.md)

## Read more

- [EC vs PPM: the 500 and 700 scales, CF, and how to convert](https://genetixbay.com/knowledge/ph-ec/ec-ppm-scales.md)
- [Water quality for cannabis: RO vs tap, alkalinity, bicarbonates and chloramine](https://genetixbay.com/knowledge/ph-ec/water-quality.md)

## Sources

1. Prasad L, Meehan M. Using electrical conductivity and total dissolved solids meters to field test water quality. NDSU Extension WQ1923 (rev. 2025). https://www.ndsu.edu/agriculture/extension/publications/using-electrical-conductivity-and-total-dissolved-solids-meters-field-test-water-quality
2. University of California Cooperative Extension, Ventura County. Salinity units and conversions (640 ppm = 1 dS/m). https://ucanr.edu/node/125832