---
title: "SDI Calculator: Silt Density Index for RO Feed Water | Filtsap"
description: "Free SDI calculator: enter t0, t15 (or t5, t10) from your SDI test to get SDI, percent plugging and whether your RO feed water is acceptable."
source: https://www.filtsap.com/tools/sdi-calculator/
updated: 2026-10-23
---

Free calculator

# SDI (silt density index) calculator

Enter the time to collect 500 mL at the start of the test (t0) and after the test interval (usually 15 minutes). The calculator gives SDI, the percent plugging and what it means for your RO membranes.

## Result

Results appear here as you type.

## Formula

**SDIT = (1 − t0 ÷ tT) × 100 ÷ T** , where T is the test interval in minutes. Percent plugging = (1 − t0 ÷ tT) × 100. If plugging is above 75%, the result is not valid; repeat the test with a shorter interval.

Test at 30 psi (2.07 bar) through a 47 mm, 0.45 µm membrane disc. The full method is in our [SDI test procedure](https://www.filtsap.com/guides/sdi-test-procedure/) .

Related: [RO plant calculator](https://www.filtsap.com/tools/ro-plant-calculator/) , [ultrafiltration before RO](https://www.filtsap.com/build/membrane-systems/) , [RO membrane cleaning](https://www.filtsap.com/guides/ro-membrane-cleaning-procedure/) .

## FAQs

What SDI is acceptable for RO?

Below 3 is good; up to 5 is acceptable to most membrane makers. Above 5, improve pre-treatment.

What if the filter plugs in less than 15 minutes?

Run the test with a 5 or 10 minute interval and use that interval in the formula.

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