Practical calculation tools for water treatment system sizing, performance, and process requirements. Results update as you type, in US or metric units.
Last reviewed 27 October 2026
Recovery, salt rejection, concentration factor and concentrate quality from your feed and permeate readings.
Recovery = permeate ÷ feed. Concentrate TDS comes from a mass balance: (feed flow × feed TDS − permeate flow × permeate TDS) ÷ concentrate flow.
How many RO elements and vessels you need for a target permeate flow at a chosen design flux.
| Feed water | gfd | LMH |
|---|---|---|
| RO permeate (second pass) | 20 to 30 | 34 to 51 |
| Well water, low fouling | 16 to 20 | 27 to 34 |
| Surface water, membrane pretreated | 13 to 17 | 22 to 29 |
| Surface water, conventional pretreatment | 10 to 14 | 17 to 24 |
| Tertiary wastewater, membrane pretreated | 8 to 12 | 14 to 20 |
| Seawater | 7 to 10 | 12 to 17 |
A first estimate only. Final element count, staging and pressures should come from the membrane manufacturer's projection software for your feed water analysis.
Resin volume and salt per regeneration for a softener, from flow, hardness and the run length you want.
Uses typical operating capacities for strong acid cation resin in the sodium form. Add a design margin (often 10 to 20%) and check service flow rate and bed depth before finalising. 1 gpg = 17.1 mg/L as CaCO₃.
Estimate total dissolved solids from a conductivity reading, or the other way round.
TDS (mg/L) ≈ factor × conductivity (µS/cm). The factor depends on which ions are present: commonly 0.55 to 0.7 for natural waters, and closer to 0.5 for sodium chloride. Calibrate it against a lab result for your water when you can.
Flow, pressure and hardness units used across North American and international projects.
The formulas behind our calculators, with worked examples.
Ask our teamRO recovery is the share of feed water that becomes permeate: recovery (%) = permeate flow ÷ feed flow × 100. For example, 75 gpm of permeate from 100 gpm of feed is 75% recovery, leaving 25 gpm of concentrate.
Salt rejection (%) = (1 − permeate TDS ÷ feed TDS) × 100. A feed of 800 mg/L and a permeate of 12 mg/L gives 98.5% rejection. Salt passage is the remainder: 1.5%.
Divide the permeate flow by the design flux multiplied by the area of one element. 100 gpm (144,000 gpd) at 16 gfd with 400 ft² elements needs 22.5 elements, so 23, which is four 6-element pressure vessels (24 elements). Confirm the final design with the membrane maker's projection software.
Multiply conductivity in µS/cm by a factor, usually 0.55 to 0.7 for natural waters: TDS (mg/L) ≈ 0.65 × conductivity. 1,200 µS/cm is about 780 mg/L. Check the factor against a lab result for your water.
Resin volume (ft³) = gallons treated between regenerations × hardness in grains per gallon ÷ resin capacity in grains per ft³. 50 gpm for 12 hours at 250 mg/L (14.6 gpg) is about 526,000 grains, or 21 ft³ of resin at 25,000 grains/ft³ (10 lb/ft³ of salt).
One grain per gallon (gpg) of hardness is 17.1 mg/L (ppm) as CaCO₃. To convert mg/L to gpg, divide by 17.1.
Send us your results along with your feed water analysis, and we'll help you select the membranes, pressure vessels or resin to match.
