# Nutrient mix, dilution and reservoir calculator

> Free nutrient calculator: how much water to dilute a strong mix to target EC, how much nutrient to raise a weak one, reservoir size for 3–5 days, elemental ppm from N-P-K labels (P₂O₅ and K₂O), US vs imperial gallons, and EC to ppm 500/700.

Interactive: https://genetixbay.com/knowledge/tools/nutrient-mix.html

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.

## How it works

### 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.

## Constants

| Constant | Value | Unit | 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

1. 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
2. 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
3. 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
4. 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
5. GenetixBay Knowledge Base. EC vs PPM: the 500 and 700 scales. https://genetixbay.com/knowledge/ph-ec/ec-ppm-scales.html