Hydroponic PPM Calculator
Work out how much water (to dilute) or stronger solution (to boost) you need to add to bring your reservoir to the PPM you want.
How the calculation works
Mixing a volume x of liquid at concentration Cadd into a reservoir of volume V at concentration Ccur gives a final concentration Ctgt:
(Ccur × V + Cadd × x) ÷ (V + x) = Ctgt
Solving for x: x = V × (Ctgt − Ccur) ÷ (Cadd − Ctgt)
Example: a 20 L reservoir reads 1,200 ppm and you want 800 ppm. Adding plain water (0 ppm): x = 20 × (800 − 1200) ÷ (0 − 800) = 10 L. Check: (1,200 × 20) ÷ 30 = 800 ppm.
This is the same idea as the dilution formula C1V1 = C2V2. It assumes the final volume is the sum of the two volumes, which holds for dilute nutrient water. Not sure of your ppm figure? Convert it with the EC to PPM converter.
Make sure your reservoir has room for the added volume, and re-measure after mixing.
Frequently asked questions
How do I lower the PPM of my reservoir?
Add plain water. Enter your current ppm, reservoir volume and target ppm, and leave the added liquid at 0 ppm. The calculator tells you how much water to add.
How do I raise the PPM?
Add a stronger solution. Enter the ppm of the nutrient solution you will add (for example your stock tank). The calculator gives the volume to add. If your target is higher than the added liquid, it cannot be reached and you will see a message.
Does this work with EC?
Yes. Dilution is linear, so you can enter EC values instead of ppm as long as you use the same unit for current, target and added liquid.
Why is my reading higher than expected after topping off?
Plants take up water faster than nutrients, so ppm rises as the reservoir level drops. Mix the new water in fully and re-measure after a few minutes before adjusting again.