Free Engineering Tools

Reverse Osmosis Calculators

Five engineering-grade RO calculators: recovery & concentration factor, salt rejection, membrane flux, concentrate LSI scaling risk, and normalized permeate flow — all client-side, no login required.

Units:
GPM · °F · psi · ft²
Calculator 1 of 5

Recovery & Concentrate Factor

Calculate system recovery, concentrate flow, and concentration factor (CF) from feed and permeate flows — or from a target recovery percentage.

If entered, concentrate TDS is estimated (100% rejection assumed)

Results

  • R (%) = (Q_p / Q_f) × 100
  • Q_c = Q_f − Q_p = Q_f × (1 − R/100)
  • CF = 1 / (1 − R/100)
  • TDS_concentrate = TDS_feed × CF (assumes 100% rejection)

CF interpretation: A CF of 4× means all dissolved solids in the concentrate stream are 4 times more concentrated than the feed.

Calculator 2 of 5

Salt Rejection & Passage

Evaluate membrane performance by calculating observed salt rejection and salt passage from feed and permeate TDS. Temperature-corrected normalized rejection is also shown.

Default 77°F (25°C)

Results

  • SR (%) = (1 − TDS_p / TDS_f) × 100
  • SP (%) = TDS_p / TDS_f × 100
  • Temperature correction factor (TCF):
  • T ≥ 25°C: TCF = exp(2640 × (1/298 − 1/T_K))
  • T < 25°C: TCF = exp(3020 × (1/298 − 1/T_K))
  • T_K = T_°C + 273.15; T_°C = (T_°F − 32) / 1.8
  • Normalized SR ≈ 1 − (1 − observed_SR/100) / TCF (expressed as %)

Temperature normalization accounts for increased water flux at higher temperatures, which dilutes the permeate and artificially lowers observed rejection.

Calculator 3 of 5

Membrane Flux

Calculate permeate flux (GFD and LMH) from permeate flow and membrane element count. Compare against typical operating ranges for your application type.

Results

  • Total Area = n_elements × A_element
  • Q_p (GPD) = Q_p (GPM) × 1440
  • Flux (GFD) = Q_p (GPD) / Total Area (ft²)
  • Flux (LMH) = GFD × 1.699

GFD = gallons per square foot per day. LMH = liters per square meter per hour. Typical design flux ranges vary by feed water quality and application.

Calculator 4 of 5

LSI at Concentrate

Assess CaCO₃ scaling risk in the RO concentrate stream using the Langelier Saturation Index. Optionally check CaSO₄ saturation when sulfate is known.

Feed Water Parameters

Optional Inputs

Applied to feed to reduce alkalinity before concentration
Enables CaSO₄ saturation check

Results

  • CF = 1 / (1 − R/100)
  • Ca_c = Ca_f × CF
  • Alk_adjusted = Alk_f − acid_meq × 50.04 (if acid dosed)
  • Alk_c = Alk_adjusted × CF
  • TDS_c = TDS_f × CF
  • pH_c ≈ pH_f − log₁₀(CF) (simplified; actual pH depends on CO₂ equilibrium)
  • T_K = (°F − 32)/1.8 + 273.15
  • A = log₁₀(TDS_c/1000 + 1) / 10
  • B = −13.12 × log₁₀(T_K) + 34.55
  • C = log₁₀(Ca_c) − 0.4
  • D = log₁₀(Alk_c)
  • pHs = (9.3 + A + B) − (C + D)
  • LSI = pH_c − pHs
  • CaSO₄: IP = [Ca_c/40080] × [SO4_c/96060]; K_sp ≈ 4.93×10⁻⁵

pH_c formula is a simplification assuming closed system with no CO₂ degassing. For precise pH modeling, use process simulation software.

Calculator 5 of 5

Normalized Permeate Flow

Temperature- and pressure-correct observed permeate flow to baseline conditions to track fouling or scaling trends over time, per ASTM D4516 methodology.

Baseline Conditions (from clean system or initial commissioning)

Net driving pressure at baseline

Current Operating Conditions

≈ TDS/100 for dilute brackish

Results

  • NDP_current = (P_f + P_c)/2 − P_p − (π_f + π_c)/2
  • T_°C = (T_°F − 32) / 1.8
  • T_K = T_°C + 273.15
  • T ≥ 25°C: TCF = exp(2640 × (1/298 − 1/T_K))
  • T < 25°C: TCF = exp(3020 × (1/298 − 1/T_K))
  • NPF = Q_current × (NDP_baseline / NDP_current) × TCF
  • NPF % = (NPF / Q_baseline) × 100

Per ASTM D4516, a >10% decline in normalized permeate flow indicates that cleaning is warranted. A >15% decline requires urgent action.