EC unit
mS/cm (same as dS/m)
PPM 500 scale
EC × 500
PPM 700 scale
EC × 700
Leafy greens
Often 0.8–1.4 mS/cm
Fruiting crops
Often 2.0–3.5 mS/cm
Calibrate
Use EC standard solution
What EC actually measures
Electrical conductivity (EC) measures how easily electricity passes through your nutrient solution. Dissolved fertilizer salts (ions) increase conductivity. Higher EC generally means a stronger nutrient solution; lower EC means a weaker one. EC does not tell you the exact amount of nitrogen versus potassium — it is a bulk strength meter.
In hydroponics, EC is reported in mS/cm (millisiemens per centimeter), which equals dS/m (deciSiemens per meter). You may also see µS/cm: 1.0 mS/cm = 1000 µS/cm. Use one unit consistently in your logbook.
Pure water conducts poorly. RO water starts near zero. Tap water carries some EC from minerals. Always measure source water so you know how much of your reading is fertilizer versus hardness.
EC vs PPM vs TDS — stop the confusion
PPM (parts per million) and TDS (total dissolved solids) on hobby meters are usually estimated from EC with a conversion factor — they are not a separate direct elemental assay. A 500-scale meter approximates PPM ≈ EC(mS/cm) × 500. A 700-scale meter uses × 700. The same solution reads different PPM on different meters.
That is why two growers can argue “800 PPM vs 1120 PPM” while both have the same 1.6 mS/cm solution. Prefer EC in mS/cm for communication, or always state your scale. Use a PPM-to-EC converter when a nutrient chart only lists PPM.
Laboratory TDS methods differ from pen estimates. For daily hydroponics, treat your calibrated pen as a relative tool: same meter, same scale, consistent targets.
Target ranges by crop type
Leafy greens such as lettuce and spinach often grow well around 0.8–1.4 mS/cm. Basil may sit near the middle to high end of that leafy range. Seedlings frequently prefer ~0.4–0.8 mS/cm or half-strength mix.
Fruiting crops like tomatoes and peppers commonly run higher in mid-to-late production — roughly 2.0–3.5 mS/cm depending on cultivar, light intensity, and stage. High light and CO₂ environments may support higher EC; low light does not.
Manufacturer charts beat generic internet tables for your exact fertilizer. Start at the low end of a range and raise if plants are pale and hungry with no tip burn; lower if margins crisp and burn.
How to measure EC correctly
Rinse the probe in clean water, measure in the reservoir or a cup of mixed solution at roughly room/reservoir temperature, and wait for the reading to stabilize. Calibrate periodically with a standard solution (often 1.413 mS/cm or similar — match your meter’s instructions).
Dirty probes read wrong. Film from nutrients and hard water coats sensors. Clean per manufacturer guidance. Store pens as directed — some must stay moist.
Measure after mixing and stirring thoroughly. Stratified tanks lie. In recirculating systems, measure in the control reservoir once solution is well mixed.
Using EC to drive daily decisions
If EC rises over days while volume falls, plants (and evaporation) are removing water faster than nutrients — top up with plain water. If EC falls while plants grow vigorously, they may be consuming minerals quickly — add nutrient to return to target.
Sudden EC spikes after topping with fertilizer mean you overdosed. Dilute immediately. Chronic tip burn at “chart strength” under weak light means your environment cannot support that osmotic load — lower EC.
Log EC alongside pH and temperature. Patterns diagnose problems faster than memory. A heat wave that concentrates tanks shows up as climbing EC before leaves scream.
Limits of EC — what it cannot see
EC cannot detect whether calcium is low while sodium is high, or whether you precipitated phosphorus out of solution. Balanced nutrients plus correct mixing order still matter. Weird deficiencies at “perfect EC” point to pH lockout, missing Cal-Mag, or bad water.
Organic additives and some microbial products may not move EC the way mineral salts do. Interpret readings in context of what you added.
Replace reservoirs on a schedule even if EC looks fine — bicarbonate buildup, uneven ion uptake, and organic load accumulate beyond what conductivity shows.
Practical EC workflow for beginners
1) Measure source water EC. 2) Mix nutrients into known volume. 3) Stir and read EC. 4) Adjust dose to hit crop target. 5) Set pH. 6) Log values. 7) Recheck daily for two weeks.
Keep a written target on the reservoir lid: e.g., “Lettuce 1.1–1.3 mS/cm, pH 5.8–6.2.” Decision-making becomes mechanical and calmer.
When buying meters, a decent EC pen and pH pen outperform a fancy light you cannot feed correctly. Calibrate both — an uncalibrated EC pen is false confidence.
Frequently asked questions
Is higher EC always better for faster growth?
No. Too-high EC burns roots and leaves and can slow growth. Match EC to crop, stage, and light level. More salts are not more nutrition if the plant cannot use them safely.
Why does my PPM meter disagree with an online chart?
Charts may use a different PPM scale (500 vs 700) or different nutrient lines. Convert both sides to EC in mS/cm to compare accurately.
Should I measure EC before or after pH adjust?
Measure EC after nutrients are fully mixed. Small pH adjustments usually change EC only slightly, but always do a final check before putting plants back on a fresh reservoir.
What EC should seedlings have?
Often roughly half of mid-veg strength — commonly around 0.4–0.8 mS/cm for many leafy starts. Raise gradually after roots establish.
Related tools & plant guides
- EC ↔ PPM Converter — Convert between Electrical Conductivity (EC) and PPM across 500, 640, and 700 TDS scales.
- Nutrient Mixing Calculator — Calculate exact nutrient amounts for your reservoir based on volume and concentration ratios.
- pH Adjustment Calculator — Estimate how much pH Up or pH Down solution is needed to reach your target pH.
- Hydroponic Lettuce guide — pH 5.5–6.5 (aim 5.8–6.2), EC 0.8–1.2 mS/cm
- Hydroponic Basil guide — pH 5.5–6.5, EC 1.0–1.6 mS/cm
- Hydroponic Tomatoes guide — pH 5.5–6.5, EC 2.0–3.5 mS/cm (stage-dependent)
- Hydroponic Peppers guide — pH 5.5–6.5, EC 1.8–2.8 mS/cm
Keep measuring
Pair this guide with free hydroponic calculators, crop targets in plant guides, and meter picks in product reviews.