EC (Electrical Conductivity) and PPM (Parts Per Million) measure the concentration of dissolved nutrients in your water. Together with run-off testing — collecting and measuring what drains from the bottom of your pot — they give you a real-time picture of what your plants are actually eating, so you can catch problems before they become visible damage. If you’re not testing both input and run-off, you’re flying blind.

What Is EC? (And Why It Matters More Than PPM)

EC stands for Electrical Conductivity. It measures how well your water conducts electricity, which is a direct reflection of how many dissolved salts and mineral ions are present. More dissolved nutrients = better conductivity = higher EC reading. The unit is millisiemens per centimeter, written as mS/cm or just EC.

Pure, distilled water conducts almost no electricity, so its EC is effectively zero. The moment you add nutrients, pH buffers, or any dissolved mineral, that number climbs. A well-mixed nutrient solution at peak flower might read EC 2.2. Tap water straight from the faucet typically reads EC 0.2–0.5, depending on your local water quality — which is worth knowing before you even start mixing nutrients.

We prefer working in EC over PPM at IWantClones.com for one simple reason: EC is a universal standard. Every meter measures in the same millisiemens per centimeter unit, with no scale confusion. PPM meters introduce a variable that trips up growers constantly — more on that next.

What Is PPM?

PPM stands for Parts Per Million. It’s a measure of dissolved solid concentration — how many milligrams of dissolved solids exist per liter of solution. Unlike EC, PPM is not a universal measurement. It’s a conversion of EC, and the conversion factor your meter uses depends entirely on which standard it was built around.

In practical terms, PPM and EC are measuring the same physical reality — dissolved solids concentration — just through different lenses. Most growers encounter both because nutrient manufacturer feeding charts often express targets in PPM, while meters display EC. Knowing how to translate between them is non-negotiable.

The 500 vs. 700 Scale Confusion (Read This Twice)

This is the single most common source of confusion we see among growers making the jump from casual to data-driven feeding. Two PPM scales exist, and your meter uses one of them — usually without making it obvious on the display.

The 500 scale (also called the NaCl or Hanna scale) multiplies the EC reading by 500. It’s used by Hanna Instruments and many European hydro companies.

The 700 scale (also called the KCl scale) multiplies the EC reading by 700. It’s used by Bluelab, Truncheon, and many North American nutrient companies.

Here’s why this matters in practice. Take a nutrient solution with an EC of 2.0 mS/cm:

• On a 500-scale meter: 2.0 EC × 500 = 1000 PPM
• On a 700-scale meter: 2.0 EC × 700 = 1400 PPM

That’s a 400 PPM difference on the exact same solution. If a grower with a 500-scale meter reads a forum post from someone running a 700-scale meter — both saying “I run 1200 PPM in peak flower” — they’ll end up at completely different actual nutrient concentrations. The 700-scale grower is actually running EC 1.71. The 500-scale grower chasing “1200 PPM” lands at EC 2.4, which is overfeeding territory for most strains.

Before you do anything else, check which scale your meter uses. It’s usually noted in the manual or on the manufacturer’s website. If you’re not sure, test a known EC solution and see whether the PPM display is closer to EC × 500 or EC × 700. When in doubt, skip PPM entirely and just run EC. Every meter reads mS/cm the same way.

Our recommendation: Buy a meter that displays both EC and PPM. Use EC as your primary number. Reference PPM only when comparing against a nutrient company’s feeding chart — and confirm which scale that chart uses before you mix anything.

Why Bother Measuring EC and PPM at All?

Because your plants can’t tell you what they’re eating — but the numbers can. Testing input EC and run-off EC together gives you four critical pieces of information:

1. Overfeeding detection — If your run-off EC is significantly higher than your input, you’re pushing in more than roots are using. Salts are piling up at the root zone.
2. Underfeeding detection — If your run-off EC drops well below your input, roots are consuming nutrients aggressively. It may be time to bump up feed strength.
3. Salt buildup monitoring — Even if input and run-off match today, tracking run-off EC over time reveals gradual accumulation before it becomes a lockout problem.
4. Nutrient lockout diagnosis — When you see deficiency symptoms but the EC looks right, that’s often a pH issue rather than a nutrient shortage. EC tells you nutrients are present; pH tells you whether roots can access them.

Growers who skip this step tend to chase deficiencies visually — by the time you see yellowing or tip burn, the problem has been building for days. Growers who test both input and run-off EC routinely catch problems at the number stage, before they show up on the leaves. That’s a significant advantage, especially for high-value genetics from a provider like IWantClones.com where you want every plant performing at its ceiling.

Target EC and PPM by Growth Stage

Cannabis has meaningfully different nutritional demands at each stage of development. Clones and seedlings are the most sensitive — their root systems are small, their vascular tissue is still developing, and they’re easily burned by full-strength nutrients. As plants mature into veg and then flower, they can handle and benefit from progressively higher EC levels. Late flower calls for a taper back down, and a final flush clears accumulated salts before harvest.

The table below uses realistic ranges based on soil and coco grows. Hydro growers can often push the upper end of each range — but start conservative and let the plant’s run-off guide your adjustments.

These are ranges, not targets to hit exactly. Different cultivars and different growing media respond differently. A more salt-sensitive strain like some sativas might show tip burn at the top of the peak flower range, while a heavy-feeding indica hybrid might thrive there and want more. Use these numbers as your starting point, then adjust based on what your run-off tells you. For a deeper look at how nutritional needs map to cannabis clone growth stages, we’ve covered that separately.

How to Measure: Input EC vs. Run-Off EC

The process is straightforward once you build it into your watering routine. Here’s exactly how we do it:

Step 1: Test Your Input Water First

Before you mix nutrients, test your source water. Tap water EC varies significantly by region — some areas have EC 0.1–0.2 (nearly pure), while others come out at 0.5–0.8 due to minerals in the municipal supply. That background EC matters because it counts toward your total nutrient load. If your tap water is EC 0.4 and you mix nutrients to hit EC 1.8, your actual nutrient contribution is only 1.4 EC.

Mix your nutrients, pH-correct the solution, then test the final input EC before watering. Write that number down.

Step 2: Water Until You Get Run-Off

Proper watering means watering to run-off — always. Aim for 10–20% of the water volume to drain out the bottom of the pot. Place a collection tray or small container underneath and let it collect. This serves two purposes: it ensures the entire root zone gets wet (no dry pockets), and it gives you the run-off sample you need to test.

For more on proper watering technique, check our guide on how to water cannabis clones.

Step 3: Test the Run-Off

Dip your EC meter into the collected run-off water and note the reading. Compare it to your input EC. The run-off should land within 0.2–0.5 EC of your input number. Within that band, things are running smoothly — the root zone is in balance and the medium isn’t accumulating excessive salts.

Step 4: Log Both Numbers

Keep a simple grow journal or spreadsheet with the date, input EC, run-off EC, and the difference (delta). Tracking these numbers over multiple waterings reveals trends. A slowly climbing run-off EC over several weeks tells you salts are accumulating even if each individual reading looks “okay.” Catching that trend early prevents problems.

What Your Run-Off EC Is Telling You

The run-off EC reading is the most diagnostic number in your feeding program. Here’s how to interpret what you’re seeing:

Run-Off EC Is 0.2–0.5 Higher Than Input

This is the sweet spot. Nutrients are cycling through the root zone without major accumulation. The slight increase is normal — some salts always linger in the growing medium. Keep doing what you’re doing.

Run-Off EC Is More Than 0.5 Above Input

Salt is building up in the medium faster than roots are consuming it. This is a warning sign. In soil, it might mean you’re watering too frequently without enough run-off volume. In coco or hydro, it usually means the EC is too high for the current uptake rate. Left unchecked, the root zone EC climbs to the point where it creates reverse osmosis pressure — literally pulling water out of root cells rather than letting them absorb it. The result is nutrient burn and lockout even when your input solution looks fine.

Solution: flush the medium (see the flush section below) and dial back your input EC by 0.3–0.5 before resuming normal feeding.

Run-Off EC Is Significantly Below Input

Roots are drinking hard. The plant is consuming nutrients at a high rate, and the medium has relatively little salt reserve. This can be a sign of healthy, vigorous growth — but if the plant looks stressed or shows deficiency symptoms, it may be telling you to bump up the feed. Gradually increase input EC by 0.2–0.3 and monitor the next run-off.

Run-Off EC Is Wildly Different From Input

A very large discrepancy in either direction — more than 1.0 EC difference — usually signals a deeper problem. Either the medium has been allowed to dry out severely (concentrating salts) or there’s been significant salt buildup over time that’s now spiking the run-off. This situation calls for an immediate flush and a reset of your feeding program. Check out our cannabis clone troubleshooting guide for a full symptom-by-symptom breakdown of what extreme EC readings can indicate.

How to Flush Your Medium Correctly

Flushing is not complicated, but it needs to be done correctly to actually clear accumulated salts. A half-measure flush — not enough water volume, wrong pH — won’t do the job and will leave you wondering why the run-off EC didn’t come down.

The Volume Rule

Use 2–3 times the volume of your pot in plain, pH-corrected water. A 3-gallon pot needs 6–9 gallons of flush water. A 5-gallon pot needs 10–15 gallons. That volume is what it takes to physically push accumulated salts out of the medium and through the drainage holes.

pH Your Flush Water

Flush water must be pH-corrected even though it’s plain water. Use:

• Soil: pH 6.0–6.5
• Coco or hydro: pH 5.5–6.0

Off-pH flush water can lock the medium’s pH into a bad range, which defeats the purpose of the flush. The goal is to run the medium’s pH and EC back toward a neutral baseline.

Monitor the Run-Off During the Flush

As you flush, test the run-off EC every gallon or two. You’ll see it start high and gradually come down. Keep flushing until the run-off EC is approaching 0.3–0.5 or ideally lower. At that point, the medium is effectively cleared. Let the pot drain thoroughly — at least 30 minutes — before resuming feeding at a reduced EC. Start 0.3–0.5 below your pre-flush input and let the plant’s run-off guide you back up.

For additional context on how environment interacts with feeding — high temperatures accelerate evaporation and concentrate root-zone salts faster — read our notes on climate and environment for cannabis clones.

EC vs. pH: Two Numbers You Always Need Together

This is a point that can’t be overstated: EC and pH measure completely different things, and you need both to diagnose feeding problems accurately.

EC tells you how much nutrient is dissolved in your water. pH tells you whether your plant can access what’s dissolved. Nutrient ions — iron, calcium, magnesium, phosphorus — each have a solubility window. Outside that pH window, they lock out of solution or become unavailable to root uptake even though they’re chemically present. You can have perfect EC and still see deficiency symptoms if pH is off.

Target pH ranges by medium:

• Soil: 6.0–7.0 (optimal 6.3–6.8)
• Coco coir / hydro: 5.5–6.5 (optimal 5.8–6.2)

When you see a deficiency and your EC looks normal, the first thing to check is pH — both input and run-off. If your run-off pH is drifting outside the target range, that’s often more urgent than the EC reading. Fix pH first, then reassess EC. Our complete nutrient feeding guide covers the pH-EC interaction in full detail, including which nutrients lock out at which pH levels.

Tools You Need

You don’t need expensive equipment to test EC and run-off properly, but cheap, uncalibrated meters will give you unreliable readings — and unreliable readings are worse than no readings at all because they give you false confidence.

EC/TDS Meter

Budget pick: Apera Instruments EC60. It reads EC and TDS, has a temperature compensation feature, and is accurate enough for serious grows. Mid-range: Bluelab Truncheon — built for commercial cultivation, durable, no calibration required. The Truncheon uses the 700 scale, which is worth knowing before you start comparing PPM numbers with other growers.

Whatever meter you buy, calibrate it with a known EC calibration solution at the start of each grow. A meter that reads EC 1.4 on a solution that should be EC 1.4 is working correctly. One that drifts even 0.2 off can skew your entire feeding program.

pH Pen

Apera PH20, Bluelab pH Pen, or Vivosun pH meter are all solid choices at different price points. Calibrate with pH 7.0 buffer solution, and store the probe tip in storage solution (not water). A dry or improperly stored pH probe drifts fast and gives you garbage readings.

Collection Tray or Cup

You need to catch run-off to test it. A simple plastic saucer works. Some growers use a turkey baster to pull a sample from the tray without disturbing the plant. Just make sure what you’re testing is the fresh run-off, not old standing water that’s been sitting in the tray — that will read artificially high as water evaporates and concentrates the dissolved solids.

Common Mistakes Growers Make with EC and Run-Off

Not Testing Run-Off at All

Testing input EC only tells you what went in. It tells you nothing about what’s happening at the root zone. This is the single biggest gap we see in grower feeding programs. Make run-off testing part of every watering session — at minimum, every other watering.

Confusing PPM Scales Mid-Grow

You switch meters, a friend loans you theirs, you find a used one at a swap meet. Different meter, different scale. If you’ve been running 1000 PPM on a 500-scale meter (EC 2.0) and switch to a 700-scale meter without realizing it, your “1000 PPM” is now EC 1.43. You’ll underfeed and wonder why your plants went light green in week 4 of flower. Always confirm the scale before you use a new meter.

Mixing Nutrients Without Accounting for Tap Water EC

If your tap water comes in at EC 0.4 and you mix nutrients aiming for EC 2.0 total, you only need to add nutrients worth EC 1.6. Growers who ignore source water EC consistently overfeed. Test your tap before every grow — municipal water EC changes seasonally in many areas.

Flushing Without Enough Water Volume

A quick pour-through doesn’t flush salts — it pushes the top layer down and leaves a salt concentration band in the middle of the root zone. Two to three times pot volume is the rule. Don’t shortcut it.

Testing Old Run-Off Water

If that saucer has been sitting under your plant for 24 hours, the water in it has been evaporating. Evaporation concentrates dissolved solids and raises EC. Always test run-off fresh — within a few minutes of collecting it — for accurate numbers.

Skipping EC Testing During the Flush Period

The late flower flush isn’t “just use plain water and forget it.” Run-off EC should be dropping steadily as you flush. Testing it during the flush period tells you whether the accumulated salts are actually clearing. Some strains or over-fertilized plants take longer to flush — you’ll know by watching the run-off EC trend down toward 0.3–0.5.

A Note on Starting Clones Right

Clones and seedlings are the most vulnerable to EC problems because their root systems haven’t established the buffering capacity that mature plants have. At IWantClones.com, our clones ship already rooted and acclimated, but they still need a gentle transition into their final feeding program.

Start new arrivals at EC 0.4–0.6. Give the root system a few days to establish in the new medium before pushing toward the early veg range of EC 0.8–1.2. Rushing the EC ramp is one of the fastest ways to stress a healthy clone into a nutrient lockout scenario. The clones in our shop are selected for genetic vigor, but even vigorous genetics need a measured start.

Frequently Asked Questions

What should my run-off EC be compared to my input EC?

Run-off EC should be within 0.2–0.5 mS/cm of your input EC. A run-off reading 0.2 above input is perfectly normal — some salt always lingers in the medium. If the gap is larger than 0.5, salt is accumulating and you should flush. If run-off EC is significantly below input, roots are feeding hard and may need a higher feed concentration.

My PPM meter reads differently than my friend’s. Why?

You’re almost certainly using different scales. The 500-scale meters (common with Hanna Instruments) multiply EC by 500. The 700-scale meters (Bluelab, Truncheon) multiply EC by 700. An EC of 2.0 reads 1000 PPM on the 500 scale and 1400 PPM on the 700 scale. Check which scale each meter uses before comparing numbers — or just compare EC directly, since that’s universal.

How often should I test run-off EC?

At minimum, test run-off every other watering during veg and every watering during peak flower. Flower is when nutrient demand is highest and when salt accumulation happens fastest. During the flush period, test every watering to confirm the EC is trending down toward your target of 0.2–0.5 run-off EC before harvest.

Can high EC cause nutrient lockout?

Yes. When root-zone EC gets too high — usually above 3.0–3.5 mS/cm — osmotic pressure reverses. Instead of water and nutrients flowing into root cells, the high salt concentration outside the cells draws moisture out of the roots. Plants show symptoms of drought stress and deficiency simultaneously despite being watered and fed. This is why monitoring run-off EC, not just input EC, is essential — root-zone EC is what actually matters to the plant.

Do I need to measure EC if I’m using organic soil?

EC testing is less critical in a well-amended, living organic soil where microbial activity regulates nutrient availability, but it’s still useful. If you’re top-dressing with dry amendments or using bottled organics, EC testing helps you avoid overfeeding even in organic systems. Run-off EC in heavily amended organic soil often reads higher than synthetic systems — trending run-off EC over time matters more than any single reading in that context.