What ppm should my hydroponic nutrients be?

It depends on the crop and stage. Leafy greens and lettuce run lean (about 400–900 ppm on the 500 scale), herbs and strawberries in the middle, and fruiting tomato, pepper and cucumber higher (roughly 900–1400). Use the target lookup above for per-element N-P-K-Ca-Mg figures and the EC band for each crop.

How do I convert K₂O and P₂O₅ on a fertilizer label to actual K and P?

Fertilizer labels report potassium as K₂O and phosphorus as P₂O₅, which overstate the element. Multiply K₂O% by 0.8301 for elemental K, and P₂O₅% by 0.4364 for elemental P (CaO×0.7147 for Ca, MgO×0.6030 for Mg). A 4-18-38 label is really about 4% N, 7.9% P, 31.5% K.

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Hydroponic nutrient calculator

Q: How do I convert EC to ppm?

Multiply EC in mS/cm by the scale factor: 500 (Hanna, Bluelab), 640 (European) or 700 (US hydroponic default). So EC 2.0 mS/cm is 1000 ppm on the 500 scale, 1280 on the 640, or 1400 on the 700 — the same solution, three numbers. Always check which scale your meter uses.

Convert your meter reading, decode a fertilizer label, and look up the N-P-K-Ca-Mg target for your crop — all cited, all free.

Three things every hydroponic grower needs to pin down: what your EC reading means in ppm, what a fertilizer label's oxide numbers actually contain, and what the target is for the crop you're growing. All three are below. The conversions are exact; the crop targets are published starting points from Cornell CEA, Penn State Extension and UF/IFAS.

1. EC ↔ ppm converter

Meter scale

EC (mS/cm)

…or ppm (on the scale above)

2. Fertilizer label converter (oxide → element)

Labels report K as K₂O and P as P₂O₅ — both overstate the real element. Enter the guaranteed-analysis percentages to get elemental N-P-K-Ca-Mg.

N %

P₂O₅ %

K₂O %

CaO % (opt.)

MgO % (opt.)

3. Nutrient target by crop & stage

Crop & stage

Mixing your own salts?

This page gives you the target. Crop does the last step — enter your reservoir volume and it solves the exact grams of calcium nitrate, KNO₃, MKP, K₂SO₄ and Epsom to hit any target, with a reverse calculator for MasterBlend, Jack's and General Hydroponics, plus overshoot warnings. The mixing solver is the paid part; everything on this page stays free.

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How to use these numbers

Read EC and ppm with an EC / ppm meter — confirm whether it's a 500- or 700-scale model, then use converter 1.
Pick your crop in lookup 3 to get the per-element target and EC band.
Hit the target with hydroponic nutrients — use converter 2 on the label to know what you're really dosing.

FAQ

How do I convert EC to ppm?
Multiply EC (mS/cm) by 500, 640 or 700 depending on your meter's scale. EC 1.4 is 700 ppm on the 500 scale or 980 ppm on the 700. The same solution shows different ppm on different meters — the EC is the fixed value.

What ppm should my nutrients be?
Crop- and stage-dependent. Leafy greens run lean, fruiting crops higher. Use lookup 3 for the per-element figures; the EC band there is the fastest daily check.

Why doesn't the label's NPK match what I'm dosing?
Because the P and K on a label are oxides. Converter 2 turns K₂O and P₂O₅ into the real elemental K and P — a 4-18-38 is closer to 4-7.9-31.5 elemental.

Sources

Cornell CEA (Mattson & Lieth) — A Recipe for Hydroponic Success (lettuce, pepper targets)
Penn State Extension — Hydroponics nutrient solutions (tomato, basil, leafy herbs/greens)
UF/IFAS — hydroponic tomato, cucumber & strawberry guidance; Produce Grower — microgreens range
Oxide→element factors from IUPAC 2021 atomic weights (K₂O 0.8301, P₂O₅ 0.4364, CaO 0.7147, MgO 0.6030)

Conversions are exact; crop ppm ranges are starting points — adjust to your cultivar, system, and measurements.