# Calculate it properly: nutrient dilution, EC targets and reservoir size

> Mix to an EC target, not a bottle dose: measure your water's EC, add nutrient in steps, and read the EC each time. Dilution follows C1 × V1 = C2 × V2, so to bring a 100 L reservoir from EC 2.4 down to 1.8 with EC 0.2 water you add about 38 L of water. Size a reservoir to hold 3–5 days of the plants' water use (about 30 L for a 4×4 ft tent in flower) and change it at least when you've topped up about one reservoir's worth.

- Canonical: https://genetixbay.com/knowledge/ph-ec/calc-nutrient-dilution-reservoir.html
- Category: pH and EC · Level: Intermediate
- Evidence: Supported · Status: Draft: awaiting expert review
- Stage: Veg, Flower · Systems: DWC, RDWC, NFT, Ebb and flow, Drip, Coco, Rockwool
- Updated: 2026-11-01 · Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay Knowledge Base (genetixbay.com/knowledge)"

## Questions and answers

**How do I dilute a nutrient solution that's too strong?** Add plain water using the dilution formula: water to add = reservoir volume × ([EC](https://genetixbay.com/knowledge/glossary/ec.md) now − EC target) ÷ (EC target − EC of your water). For a 100 L reservoir at EC 2.4 that should be 1.8, with [tap water](https://genetixbay.com/knowledge/glossary/tap-water.md) at EC 0.2: 100 × 0.6 ÷ 1.6 ≈ 38 L. Mix, wait a few minutes, then re-measure EC and [pH](https://genetixbay.com/knowledge/glossary/ph.md) before adjusting further.

**How big should my hydro reservoir be?** Make a hydro reservoir hold about 3–5 days of the plants' water use, so EC and [pH swing](https://genetixbay.com/knowledge/glossary/ph-drift.md) less between checks. Plants in flower often use 3–6 L per m² of canopy a day; at 4 L/m²/day a 4×4 ft tent uses about 6 L a day, so about 30 L (8 US gal) is a good minimum. A 10×10 ft room uses about 37 L a day, so roughly 190 L (50 gal).

**How often should I change my nutrient reservoir?** A common rule is to change a [recirculating](https://genetixbay.com/knowledge/glossary/recirculating-system.md) reservoir when you've topped up about one full reservoir volume, or every 1–2 weeks, whichever comes first. Topping up keeps [EC in](https://genetixbay.com/knowledge/glossary/input-ec.md) range, but ions the plants don't use, such as sodium and chloride from tap water, slowly build up. Change it sooner if EC climbs while plants drink, or pH drifts hard.

**How do I work out ppm of nitrogen from a fertiliser?** [ppm of an element](https://genetixbay.com/knowledge/glossary/elemental-ppm.md) = grams of fertiliser per litre × the element's percentage × 10. So 1 g/L of a 15-5-15 fertiliser gives 150 ppm N. Labels list phosphorus as P₂O₅ and potassium as K₂O: multiply by 0.436 for P and 0.830 for K. That 15-5-15 at 1 g/L gives about 22 ppm P and 125 ppm K.

**Is a gallon on a nutrient label US or imperial?** Most North American nutrient labels use US gallons, 3.785 L. An imperial (UK) gallon is 4.546 L, about 20% bigger. Mixing a US-gallon dose into an imperial gallon makes the solution about 17% weaker, and the reverse makes it about 20% stronger. Convert to millilitres per litre: ml per US gallon ÷ 3.785 = ml per litre.

**Short answer, in plain words.** Don't trust the bottle's dose alone. Measure your water first, add nutrient a bit at a time, and stop when the [EC meter](https://genetixbay.com/knowledge/glossary/ec-meter.md) hits your target. If you overshoot, add water using simple maths: the strength times the volume stays the same. Make your reservoir big enough for **3–5 days** of what the plants drink, so the numbers don't swing wildly, and change it after you've topped up about **one reservoir's worth**. For the 500 and 700 ppm scales, see [EC vs PPM](https://genetixbay.com/knowledge/ph-ec/ec-ppm-scales.md). *Supported (dilution is plain maths; reservoir timing is grower practice and extension advice).*

There's no calculator for this one yet; the [grow-room calculators](https://genetixbay.com/knowledge/tools/) cover the rest of the room.

## Which numbers do I work in?

Work in **EC (mS/cm)**. Every "ppm" meter measures EC and multiplies by a scale factor: 500 or 700 (sometimes 640). EC 1.0 is 500 ppm on one meter and 700 on another, so a ppm number without its scale is meaningless. See [EC vs PPM](https://genetixbay.com/knowledge/ph-ec/ec-ppm-scales.md) for the conversion table.

Units that trip people up [2]:

- **1 US gallon = 3.785 L; 1 imperial gallon = 4.546 L.** Most North American labels mean US gallons.
- **ml per litre = ml per US gallon ÷ 3.785.** Example: 5 ml/gal = 1.32 ml/L.
- **1 US teaspoon ≈ 4.93 ml; 1 US tablespoon ≈ 14.8 ml.** Use a syringe, not a kitchen spoon.

## How do I mix to a target EC?

Mix to the meter, not to the label. *Grower practice; EC monitoring is standard in hydroponics* [1].

1. **Measure your water's EC** (tap water is often 0.1–0.6; [RO](https://genetixbay.com/knowledge/glossary/ro-water.md) is near 0).
2. **Work out how much EC the nutrient must add:** target EC − water EC. Example: target 1.8, water 0.3, so add **1.5**.
3. **Add part of the dose,** mix, wait a few minutes, read the EC.
4. **Scale up:** if 1 ml/L raised EC by 0.4, you need about 1.5 ÷ 0.4 ≈ **3.75 ml/L** in total. Write that "EC per ml/L" number down for your brand and water; it saves guessing next time.
5. **Check pH last,** after the nutrients are in.

Multi-part nutrients: add each part separately into the water and mix between parts; never mix concentrates together. EC is close to proportional to concentration at feeding strengths, but not exactly, so always finish by measuring.

**Mixing to the meter.** Measure the water's EC → Add nutrient in steps: Each part separately → Read EC each time: Note your EC per ml/L → Adjust pH last. Source: [1]

## How do I dilute a solution that's too strong (or weak)?

Use **C₁ × V₁ = C₂ × V₂** (strength × volume is conserved), adjusted for the EC your water already has.

**Water to add (L or gal)** = V × (EC now − EC target) ÷ (EC target − EC of water).

Worked example: 100 L (26.4 US gal) at EC 2.4, target 1.8, tap water EC 0.2: 100 × 0.6 ÷ 1.6 ≈ **37.5 L (9.9 gal)**. If the reservoir can't hold that much, [drain](https://genetixbay.com/knowledge/glossary/runoff.md) first: to fix a full reservoir, drain the same volume you're about to add.

**Too weak?** Add nutrient at the "EC per ml/L" rate from your notes, working on the whole volume: to raise 100 L from 1.4 to 1.8 at 0.4 EC per ml/L, add 1.0 ml/L × 100 L = **100 ml** of each part in the brand's ratio.

## How big should the reservoir be?

**About 3–5 days of the plants' water use.** A small reservoir that the plants drink down by half in a day swings in EC and pH fast. *Grower practice.*

Water use: indoor cannabis in flower often uses about **3–6 L per m² of canopy per day** (1 mm per day = 1 L/m² [4]). This is a planning range; measure your own (see [dehumidifier sizing](https://genetixbay.com/knowledge/equipment/calc-dehumidifier-sizing.md) for the same numbers).

| Space | Canopy | Water use at 4 L/m²/day | Reservoir for 3 days | Reservoir for 5 days |
|---|---|---|---|---|
| 2 × 4 ft tent | 0.74 m² | ~3 L (0.8 gal)/day | ~9 L (2.4 gal) | ~15 L (4 gal) |
| 4 × 4 ft tent | 1.49 m² | ~6 L (1.6 gal)/day | ~18 L (4.7 gal) | ~30 L (7.9 gal) |
| 5 × 5 ft tent | 2.32 m² | ~9 L (2.5 gal)/day | ~28 L (7.4 gal) | ~46 L (12 gal) |
| 4 × 8 ft tent | 2.97 m² | ~12 L (3.1 gal)/day | ~36 L (9.4 gal) | ~59 L (16 gal) |
| 10 × 10 ft room | 9.29 m² | ~37 L (9.8 gal)/day | ~111 L (29 gal) | ~186 L (49 gal) |

For recirculating systems ([DWC](https://genetixbay.com/knowledge/glossary/dwc.md), [RDWC](https://genetixbay.com/knowledge/glossary/rdwc.md), NFT), count the water in the buckets or channels too; see [RDWC](https://genetixbay.com/knowledge/systems/rdwc.md).

**Chart: Reservoir size for 3–5 days of water use.** At 4 L per m² of canopy a day, a reservoir holding 3–5 days is about 9–15 L for a 2 × 4, 18–30 L for a 4 × 4, 28–46 L for a 5 × 5, 36–59 L for a 4 × 8 and 111–186 L for a 10 × 10 room.

| Space | 3–5 days of use (L) |
|---|---|
| 2 × 4 tent | 9–15 |
| 4 × 4 tent | 18–30 |
| 5 × 5 tent | 28–46 |
| 4 × 8 tent | 36–59 |
| 10 × 10 room | 111–186 |

4 L/m²/day is a planning assumption; measure your own.

Source: Table above; 1 mm = 1 L/m² [4].

**Too-strong reservoir, worked example.** 100 L at EC 2.4: Start; EC 1.8: Target; ~38 L: Water to add (EC 0.2). Source: Dilution formula above.

## When do I top up, and when do I change it?

**Top up daily** to the fill line. Which top-up depends on which way EC is moving. *Grower practice* [1]:

- **EC rising as volume falls:** plants are drinking more water than nutrient. Top up with plain (pH-adjusted) water or a weak mix.
- **EC falling:** plants are taking nutrient faster than water. Top up with feed at or a little above target.
- **EC steady:** top up at target strength.

**Change the reservoir** when you've topped up about **one full reservoir volume**, or every **1–2 weeks**, whichever comes first. *Grower practice.* Topping up keeps EC in range, but EC doesn't tell you *which* ions are left. Ions the plants barely use, such as sodium and chloride from tap water, build up while others get used up, so the solution drifts out of balance [1]. See [pH drift in hydro](https://genetixbay.com/knowledge/ph-ec/ph-drift-hydro.md) and [water quality](https://genetixbay.com/knowledge/ph-ec/water-quality.md).

**[Drain-to-waste](https://genetixbay.com/knowledge/glossary/drain-to-waste.md) (coco, rockwool):** you don't change a reservoir, but you mix fresh each time and check runoff EC against input; see [runoff testing](https://genetixbay.com/knowledge/ph-ec/runoff-testing.md).

## How do I work out ppm of an element from a fertiliser?

**ppm of an element = grams per litre × the element's percentage × 10.** Example: 1 g/L of a 15-5-15 fertiliser gives 1 × 15 × 10 = **150 ppm N**.

Labels give phosphorus as P₂O₅ and potassium as K₂O. Convert with atomic weights [3]:

- **P = P₂O₅ × 0.436** (2 × 30.974 ÷ 141.94).
- **K = K₂O × 0.830** (2 × 39.098 ÷ 94.20).

So 15-5-15 at 1 g/L gives about **22 ppm P** (5 × 10 × 0.436) and **125 ppm K** (15 × 10 × 0.830). These are elemental ppm (mg/L), not meter "ppm".

## Common mistakes

- **Comparing [ppm readings](https://genetixbay.com/knowledge/glossary/tds.md) from 500-scale and 700-scale meters** (see [EC vs PPM](https://genetixbay.com/knowledge/ph-ec/ec-ppm-scales.md)).
- **Forgetting your water's starting EC,** then underfeeding with high-EC tap water or [overfeeding](https://genetixbay.com/knowledge/glossary/nutrient-toxicity.md) with RO.
- **Mixing a US-gallon dose into imperial gallons** (or the reverse) [2].
- **Mixing [nutrient concentrates](https://genetixbay.com/knowledge/glossary/stock-solution.md) together** before they go into water.
- **Adjusting pH before the nutrients are in.**
- **A reservoir too small for the plants,** so EC and pH swing daily.
- **Topping up forever** without a full change.

## Sources

1. 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, solution replenishment and replacement). https://doi.org/10.1016/B978-0-444-63696-6.00012-8
2. Thompson A, Taylor BN. 2008. NIST Special Publication 811, Appendix B (US gallon = 3.785 412 L; imperial gallon = 4.546 09 L). https://www.nist.gov/pml/special-publication-811
3. NIST. Atomic weights and isotopic compositions (P 30.974, K 39.098, O 15.999), used for the P₂O₅ and K₂O conversion factors. https://www.nist.gov/pml/atomic-weights-and-isotopic-compositions-relative-atomic-masses
4. Allen RG, Pereira LS, Raes D, Smith M. 1998. Crop evapotranspiration (FAO Irrigation and Drainage Paper 56), Chapter 1 (1 mm over 1 m² = 1 litre). https://www.fao.org/4/x0490e/x0490e04.htm