How do I fix my nutrient mix and size my reservoir?
Mix to the EC meter, not the bottle. Too strong? Add plain water: strength times volume stays the same. Too weak? Add nutrient at your own "EC per ml/L" rate. Make the reservoir hold 3–5 days of what the plants drink, and read labels in elemental ppm.
Your result
How it works, in plain words
Too strong: dilute
Water to add = volume × (EC now − target) ÷ (target − your water's EC). 100 L at EC 2.4, target 1.8, tap water 0.2: 100 × 0.6 ÷ 1.6 = 37.5 L. No room? Drain that much first.
Too weak: add nutrient
ml/L to add = (target − EC now) ÷ your EC per ml/L. Raising 100 L from 1.4 to 1.8 at 0.4 per ml/L takes 1.0 ml/L, so 100 ml of each part.
Reservoir size
Daily use = canopy m² × 3–6 L. A 4 × 4 ft tent at 4 L/m² drinks about 6 L a day, so a 18–30 L reservoir holds 3–5 days. Change it after you've topped up about one reservoir's worth, or every 1–2 weeks.
Label to ppm
ppm = g/L × % × 10. Labels give phosphorus as P₂O₅ and potassium as K₂O: × 0.436 for P, × 0.830 for K. 1 g/L of 15-5-15 is 150 ppm N, 22 ppm P, 125 ppm K.
EC and ppm
Meters measure EC and multiply: × 500 or × 700. EC 1.8 is 900 on one and 1,260 on the other.
Every number this calculator uses
The same constants feed the Calculate it properly guides, so the tools and the articles agree. Rules of thumb are labelled as such.
| Constant | Value | What it is | Evidence |
|---|---|---|---|
WATER_USE_M2 | 4 L per m² canopy per day | Planning water use in flower (range 3–6 L/m²/day; 1 mm a day = 1 L/m²) | Grower practice |
RES_DAYS_LO | 3 days | Reservoir holds at least 3 days of water use, so EC and pH swing less | Grower practice |
RES_DAYS_HI | 5 days | …and up to 5 days | Grower practice |
P_FROM_P2O5 | 0.436 × | Elemental P from a label's P₂O₅ (2 × 30.974 ÷ 141.94) | Established |
K_FROM_K2O | 0.83 × | Elemental K from a label's K₂O (2 × 39.098 ÷ 94.20) | Established |
L_PER_GAL | 3.78541 L per US gal | Water conversion | Established |
GAL_IMP | 4.54609 L per imperial gallon | Imperial (UK) gallon; most North American labels mean the US gallon (3.785 L) | Established |
PPM500 | 500 ppm per EC 1.0 | Hanna/NaCl '500' scale (TDS meters) | Established |
PPM700 | 700 ppm per EC 1.0 | '700' (KCl) scale | Established |
Sources
- Allen RG, Pereira LS, Raes D, Smith M. 1998. Crop evapotranspiration (FAO Irrigation and Drainage Paper 56), Ch. 1 (1 mm of water over 1 m² = 1 litre). https://www.fao.org/4/x0490e/x0490e04.htm
- Mattson NS, Lieth JH. 2019. Liquid culture hydroponic system operation. In: Raviv M, Lieth JH, Bar-Tal A (eds), Soilless Culture: Theory and Practice, 2nd ed. Elsevier, pp. 567–585 (EC monitoring, replenishment and replacement). https://doi.org/10.1016/B978-0-444-63696-6.00012-8
- NIST. Atomic weights and isotopic compositions (P 30.974, K 39.098, O 15.999), for the P₂O₅ and K₂O conversion factors. https://www.nist.gov/pml/atomic-weights-and-isotopic-compositions-relative-atomic-masses
- NIST. Guide for the Use of the International System of Units (SP 811), Appendix B: conversion factors (1 W = 3.412 142 BTU/h; 1 ft³ = 0.028 316 8 m³; 1 US gal = 3.785 412 L). https://www.nist.gov/pml/special-publication-811
- GenetixBay Knowledge Base. EC vs PPM: the 500 and 700 scales. https://genetixbay.com/knowledge/ph-ec/ec-ppm-scales.html
