--- title: "SDI Test Procedure: How to Measure Silt Density Index for RO | Filtsap" description: "How to run an SDI test on RO feed water: equipment, 30 psi, 0.45 micron filter, timing and the SDI15 formula. What SDI value RO membranes need." source: https://www.filtsap.com/guides/sdi-test-procedure/ updated: 2026-10-23 --- Buyer guide · Updated 23 Oct 2026 · By the Filtsap technical team # SDI test procedure: measuring silt density index for RO feed water **Short answer** SDI (silt density index) shows how quickly RO feed water will foul a membrane. Pass the water through a 47 mm, 0.45 µm membrane filter at 30 psi (2.07 bar). Time how long it takes to collect 500 mL at the start (t0) and again after 15 minutes (t15). SDI15 = (1 − t0/t15) × 100 / 15. Most RO membrane makers want SDI below 5, and below 3 for long membrane life. ## Why SDI matters Turbidity tells you how cloudy water looks. SDI tells you how fast fine colloids will plug an RO membrane. Water can be clear to the eye and still have an SDI of 6 or 7, which will foul membranes in weeks. ## Equipment - SDI test kit with a 47 mm filter holder - 0.45 µm membrane filter discs - Pressure regulator and [pressure gauge](https://www.filtsap.com/monitor/flow-pressure-monitoring/) set to 30 psi (2.07 bar) - 500 mL graduated cylinder and a stopwatch ## Procedure - Connect the kit to the RO feed line, after the [cartridge filter](https://www.filtsap.com/products/pp-spun-filter-cartridge/) and before the high-pressure pump. - Flush the line for a few minutes. Fit a new 0.45 µm disc, wetted, without trapping air. - Set the pressure to 30 psi and keep it constant throughout. - Open the valve and start the stopwatch. Record the time to collect 500 mL: this is **t0** . - Leave the water flowing through the filter for 15 minutes. - Collect another 500 mL and record the time: this is **t15** . - Calculate: **SDI15 = (1 − t0 ÷ t15) × 100 ÷ 15** . **Example:** t0 = 30 seconds, t15 = 50 seconds. SDI15 = (1 − 30/50) × 100 / 15 = 2.7. Enter your readings in the [SDI calculator](https://www.filtsap.com/tools/sdi-calculator/) to get the result instantly. ## What the result means | SDI15 | Meaning | | --- | --- | | Below 3 | Good RO feed. Normal membrane cleaning interval. | | 3 to 5 | Acceptable for most membranes, but expect more frequent cleaning. | | Above 5 | Improve pre-treatment before the RO. | If more than about 75 percent of the filter plugs within 15 minutes, the test is invalid; repeat it with a 5 or 10 minute interval and use the matching formula. ## How to bring SDI down - Coagulant dosing ahead of a [multimedia filter](https://www.filtsap.com/build/water-treatment-systems/) - [Ultrafiltration](https://www.filtsap.com/build/membrane-systems/) ahead of the RO, which usually gives SDI below 3 - Correct micron and regular change of the RO [cartridge filter](https://www.filtsap.com/guides/filter-micron-rating-guide/) ## FAQs What is a good SDI value for RO? Below 3 is good. Most membrane makers allow up to 5, but fouling and cleaning frequency rise as SDI goes up. What is the SDI formula? SDI15 = (1 − t0/t15) × 100 / 15, where t0 and t15 are the times to collect 500 mL at the start and after 15 minutes at 30 psi. Where should I take an SDI sample? On the RO feed after the cartridge filter and before the high-pressure pump. ## Not sure what to order? Send your application and current filter. We reply on WhatsApp. [Ask an engineer](https://wa.me/918160852169?text=Hello%20Filtsap%2C%20I%20read%20your%20guide%20%22SDI%20test%20procedure%3A%20measuring%20silt%20density%20index%20for%20RO%20feed%20water%22%20and%20need%20advice.) [Request a quote](https://www.filtsap.com/get-quote/) More guides - [Filter micron rating guide](https://www.filtsap.com/guides/filter-micron-rating-guide/) - [Spun vs wound vs pleated filter cartridge](https://www.filtsap.com/guides/spun-vs-wound-vs-pleated-filter-cartridge/) - [How to choose a bag filter](https://www.filtsap.com/guides/how-to-choose-bag-filter/) - [When to replace a filter cartridge](https://www.filtsap.com/guides/when-to-replace-filter-cartridge/) ## Need help choosing? Send your application details and our team will recommend the right filter, at no cost. [Get a quote](https://www.filtsap.com/get-quote/) [WhatsApp](https://wa.me/918160852169?text=Hello%20Filtsap%2C%20I%20need%20a%20quote.)