--- title: "RO Plant Calculator: Recovery, Reject, Membranes and Pump Size | Filtsap" description: "Free RO plant calculator: enter permeate flow, feed TDS and recovery to get feed and reject flow, number of 4\" or 8\" membranes, pressure vessels, permeate and reject TDS." source: https://www.filtsap.com/tools/ro-plant-calculator/ updated: 2026-10-23 --- Free calculator # RO plant calculator Enter the treated water you need, your feed water TDS and the recovery. The calculator works out feed and reject flow, how many RO membranes and pressure vessels you need, the expected permeate and reject TDS, and the typical operating pressure. ## Result Results appear here as you type. ## How the calculator works - **Feed flow** = permeate ÷ recovery. **Reject flow** = feed − permeate. - **Membranes:** permeate ÷ (design flux × membrane area). Design flux depends on the feed: about 25 LMH for borewell water, 20 LMH for surface water, 15 LMH for treated wastewater and 14 LMH for seawater, in line with membrane makers' design guidelines. An 8" × 40" element has about 37 m² of membrane; a 4" × 40" element about 7.9 m². - **Permeate TDS** assumes about 98% system salt rejection for brackish RO and 99.4% for seawater RO. **Reject TDS** comes from a salt balance. - **Pressure** is the typical range for that feed TDS. The exact figure depends on temperature, membrane type and plant design. Keep recovery at or below about 75% for brackish water and 45% for seawater unless a water analysis shows it is safe: higher recovery concentrates salts and causes scaling. Use the result as a first estimate; we confirm the design with membrane projection software and your water analysis. Related: [industrial RO plants](https://www.filtsap.com/build/membrane-systems/) , [RO membranes and pressure vessels](https://www.filtsap.com/supply/ro-components/) , [SDI test procedure](https://www.filtsap.com/guides/sdi-test-procedure/) , [RO membrane cleaning](https://www.filtsap.com/guides/ro-membrane-cleaning-procedure/) . ## FAQs What recovery should an RO plant run at? Typically 60 to 75 percent for brackish borewell water and 35 to 45 percent for seawater. The safe limit depends on the hardness, silica and other scaling salts in your water analysis. How many membranes does a 1000 LPH RO plant need? About two 8-inch brackish water elements on borewell water, or five to six 4-inch elements, depending on feed water quality and design flux. What is the difference between LPH and GPD? LPH is litres per hour. GPD is US gallons per day. 1 GPD is about 0.158 LPH, so a 100 GPD domestic membrane makes about 16 litres per hour. More calculators - [Water softener sizing calculator](https://www.filtsap.com/tools/water-softener-calculator/) - [Pressure sand and carbon filter sizing](https://www.filtsap.com/tools/pressure-filter-sizing-calculator/) - [Cartridge filter housing sizing](https://www.filtsap.com/tools/cartridge-filter-sizing-calculator/) - [SDI calculator](https://www.filtsap.com/tools/sdi-calculator/) - [Water treatment unit converter](https://www.filtsap.com/tools/water-unit-converter/) ## Need an exact design? Send your water analysis and flow. An engineer checks it and replies on WhatsApp. [Request a quote](https://www.filtsap.com/get-quote/) ## Need filters, spares or a new plant? Send the details on WhatsApp or through the form. An engineer reads every enquiry before we quote. [Get a quote](https://www.filtsap.com/get-quote/) [WhatsApp](https://wa.me/918160852169?text=Hello%20Filtsap%2C%20I%20need%20a%20quote.)