Water Treatment12 Nov 20252 min read

RO Plant Sizing Guide: How to Calculate the Right LPH (with Examples)

Correctly sizing your RO plant isn’t guesswork—it’s a calculated decision based on your daily water needs, usage pattern, operating hours, and system recovery. Get it right and you'll ensure consistent supply, better membrane life, and reduced operating costs.

In this guide, you’ll learn:

The exact formulas used in commercial RO sizing.
Real-world examples for offices, hotels, hospitals, and factories.
Tips to avoid common sizing mistakes.

Key Terms – Refresher (30 Seconds)

Term	Definition
LPH (Litres Per Hour)	Continuous product water capacity of your RO system.
Daily Demand (L/day)	Total purified water required in 24 hours.
Operating Hours (h/day)	Actual plant run time (e.g., 8–16 hours).
Peak/Diversity Factor (PF)	Accounts for short bursts or overlapping usage (typical 1.2–1.5).
Safety Factor (SF)	Buffer for future expansion or membrane aging (1.1–1.25).
Recovery (%)	% of feedwater converted to product water.
Recovery Factor (RF)	= 1 / Recovery (as a decimal). E.g., 70% → RF = 1 / 0.70 = 1.43

The RO Sizing Formula (Step-by-Step)

Step A: Calculate Base LPH (Product Output) Base LPH=(Daily Demand (L/day)Operating Hours (h/day))×PF×SF\text{Base LPH} = \left( \frac{\text{Daily Demand (L/day)}}{\text{Operating Hours (h/day)}} \right) \times \text{PF} \times \text{SF}Base LPH=(Operating Hours (h/day)Daily Demand (L/day))×PF×SF

Step B: Adjust for Recovery Feed LPH=Base LPH×RF\text{Feed LPH} = \text{Base LPH} \times \text{RF}Feed LPH=Base LPH×RF

Pro tip: RO plants are selected based on Product LPH. Feed pumps and pretreatment must match Feed LPH.

Quick Estimation Chart

Application	Typical Water Use	Notes
Office (drinking + pantry)	3–5 L/person/day	Add 10–20% for visitors
School/College	2–3 L/person/day	Canteen water calculated separately
Hotel Guests	3 L/room/day	Add laundry, kitchen separately
Hospital Beds	4 L/bed/day	Add lab, CSSD, OPD loads
Manufacturing	50–200 L/operator/day	Varies by process

Worked Examples (with Calculations)

A) Office: 120 Staff

People demand: 120 × 4 L = 480 L/day
Pantry use: 120 × 1 L = 120 L/day
Total demand = 600 L/day
Hours: 10/day, PF = 1.3, SF = 1.15
Recovery = 70% → RF = 1.43

Base LPH=(60010)×1.3×1.15=90 LPH\text{Base LPH} = \left( \frac{600}{10} \right) \times 1.3 \times 1.15 = 90 \text{ LPH}Base LPH=(10600)×1.3×1.15=90 LPH Feed LPH=90×1.43=129 LPH\text{Feed LPH} = 90 \times 1.43 = 129 \text{ LPH}Feed LPH=90×1.43=129 LPH

Select: 100 LPH RO with 130 LPH feed pump + 150–200 L RO tank.

B) Business Hotel: 80 Rooms @ 65% Occupancy

Guests: 80 × 0.65 × 3 = 156 L/day
Kitchen & Ice: 600 L/day
Laundry: 800 L/day
Total = 1,556 L/day

Hours = 14/day, PF = 1.35, SF = 1.2, RF = 1.43 Base LPH=(155614)×1.35×1.2=180 LPH\text{Base LPH} = \left( \frac{1556}{14} \right) \times 1.35 \times 1.2 = 180 \text{ LPH}Base LPH=(141556)×1.35×1.2=180 LPH Feed LPH=180×1.43=257 LPH\text{Feed LPH} = 180 \times 1.43 = 257 \text{ LPH}Feed LPH=180×1.43=257 LPH

Select: 200 LPH RO + 300–500 L storage.

Storage Tank Sizing (Don’t Skip!)

Tank Size (L)≈Peak Demand (L/h)×Autonomy (h)\text{Tank Size (L)} ≈ \text{Peak Demand (L/h)} \times \text{Autonomy (h)}Tank Size (L)≈Peak Demand (L/h)×Autonomy (h)

Example: Pantry draws 120 L in 30 mins → 240 L/h.
Recommended storage = 240–400 L for smooth breaks.

Recovery Rate & Pretreatment Selection

Recovery	Application	Pretreatment Needed
35–50%	Domestic	Cartridge, carbon
60–80%	Commercial	Softener, antiscalant, UV, multimedia

Higher recovery = better OPEX, but needs better feedwater conditioning.

RO Sizing Cheat Sheet

Use Case	RO Product LPH
Small office (<60 staff)	50–100 LPH
Mid office (60–150)	100–200 LPH
Café/Cloud Kitchen	100–250 LPH
100-bed hospital	150–300 LPH
Light Industry Cell	200–500 LPH