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 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. 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 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 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].
- Measure your water's EC (tap water is often 0.1–0.6; RO is near 0).
- Work out how much EC the nutrient must add: target EC − water EC. Example: target 1.8, water 0.3, so add 1.5.
- Add part of the dose, mix, wait a few minutes, read the EC.
- 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.
- 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.
- Measure the water's EC
- Add nutrient in stepsEach part separately
- Read EC each timeNote 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 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 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, RDWC, NFT), count the water in the buckets or channels too; see RDWC.
4 L/m²/day is a planning assumption; measure your own.
Source: Table above; 1 mm = 1 L/m² [4].
Show the numbers
| 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 |
- 100 L at EC 2.4Start
- EC 1.8Target
- ~38 LWater 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 and water quality.
Drain-to-waste (coco, rockwool): you don't change a reservoir, but you mix fresh each time and check runoff EC against input; see runoff testing.
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 from 500-scale and 700-scale meters (see EC vs PPM).
- Forgetting your water's starting EC, then underfeeding with high-EC tap water or overfeeding with RO.
- Mixing a US-gallon dose into imperial gallons (or the reverse) [2].
- Mixing nutrient concentrates 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.
