Silt Density Index (SDI) Calculator

Q: What is a good SDI value for reverse osmosis?

Most major RO and NF membrane manufacturers, Dow Filmtec, Hydranautics, Toray, LG: require an SDI₁₅ of 5 or less, and prefer values under 3. SDI under 3 is considered excellent feed water that needs minimal pretreatment; SDI between 3 and 5 is marginal; SDI above 5 means significant fouling risk and you need additional pretreatment before the membranes.

Q: What does SDI15 mean?

The subscript on SDI indicates the test duration in minutes. SDI₁₅ is the standard 15-minute test described in ASTM D4189. When the filter plugs too fast to wait 15 minutes (final fill time more than ~1 minute), operators run SDI₁₀ or SDI₅ instead. You cannot directly compare an SDI₅ result to an SDI₁₅ result, always report the duration.

Q: What is the difference between SDI and turbidity?

Turbidity (NTU) measures light scattering caused by suspended particles at one instant in time. SDI measures how quickly those particles plug a 0.45 µm membrane over a 5-, 10-, or 15-minute interval. Two waters can have the same turbidity but very different SDI values because turbidity does not capture colloidal or biological fouling agents that membrane systems care about most.

Q: What pretreatment reduces SDI?

In rough order of effectiveness: (1) coagulation and media filtration for raw surface water, (2) microfiltration or ultrafiltration upstream of the RO, (3) cartridge filters of 1–5 µm rating, and (4) chemical pretreatment with antiscalants. The choice depends on what kind of fouling is dominant, particulate, biological, or organic, which you identify with a fouling-factor analysis, not SDI alone.

Q: Can SDI be negative?

Not in a valid test. SDI assumes the membrane gets slower (more plugged) over the test interval. If your final fill time is shorter than the initial fill time, something is wrong: usually pressure was not regulated, air was trapped in the holder, or the membrane tore. Re-run the test.

WaterOperatorPracticeTest.com Updated May 21, 2026 Free · ASTM D4189

Enter your two 500 mL fill times and the run duration to get your SDI and a feed-water rating.

Inputs

Initial fill time (time to collect first 500 mL)

seconds

Final fill time (time to collect 500 mL after the run period)

seconds

Total test duration (typically 15 min; use 5 or 10 if 15 plugs the filter)

minutes

Result

Show formula with your numbers

How SDI is calculated

SDI_T = (1 − t_i / t_f) × 100 / T

where t_i is the initial 500 mL fill time, t_f is the final 500 mL fill time, and T is the elapsed test duration in minutes. The result is dimensionless; an SDI of 3 means the membrane lost about 3% of its flow capacity per minute over the test interval.

Interpreting your result

SDI < 3, Excellent feed water. Most RO membrane manufacturers accept this without additional pretreatment.
SDI 3–5, Marginal. Acceptable for some installations but may require additional pretreatment (cartridge filters, coagulation, or media filtration).
SDI > 5, Poor. Significant fouling risk. Additional pretreatment is required before RO/NF feed.

Most major membrane manufacturers (Dow/Filmtec, Hydranautics, Toray, LG) specify an SDI₁₅ of ≤ 5 with a preference for < 3. Check your specific membrane's warranty conditions.

Common test conditions

Sample feed pressure: 30 psi (207 kPa) regulated upstream of the filter
Sample temperature: 20–30 °C (note temperature on the data sheet)
Filter media: 0.45 µm mixed-cellulose-ester membrane, 47 mm diameter
If the second 500 mL takes more than 1 minute to collect, the membrane is essentially plugged. Re-run with a shorter test duration (5 or 10 min).

Typical SDI values by source water

These ranges are typical of what operators encounter in the field. Treat them as expectations, not guarantees: your actual SDI depends on season, source-water turbidity events, and upstream pretreatment.

Source	Typical raw SDI₁₅	After pretreatment
Deep groundwater (well)	1–4	< 1
Shallow / GUDI groundwater	5–15+	< 3 (with MF/UF)
Lake / reservoir surface water	5–10	< 3 (with conventional or MF/UF)
River water (variable)	5–>15	< 5 (with coag + filtration + MF)
Seawater open intake	10–>20	< 3 (with UF or DAF + UF)
Beach-well seawater	2–6	< 3 (often direct to RO)
Municipal tap water	1–5	< 3 (with cartridge filters)
Conventional filter effluent	3–6	< 3 (with MF / UF polishing)

Common SDI testing mistakes

Letting feed pressure drift from 30 psi. Pressure must be regulated upstream of the filter holder. Even a 5 psi change makes SDI results non-comparable.
Trapped air in the filter holder. Air bubbles change the effective filtration area and skew the initial fill time. Vent before starting.
Using a wetted or used membrane. ASTM D4189 calls for a fresh, dry 0.45 µm membrane each test. Pre-wetted membranes give artificially low initial times.
Test duration too long. If the second 500 mL takes more than ~1 minute, the membrane is essentially plugged. Re-run as SDI₁₀ or SDI₅, and clearly label the result with the actual duration.
Comparing SDI₅ to SDI₁₅. You can't. Always specify the test duration when reporting the value, and only compare SDI results that share the same duration.
Temperature-induced viscosity changes. SDI is sensitive to temperature at the extremes. Run tests at typical RO feed temperature (20–30 °C) and note the temperature on the data sheet.

When to use this calculator

Use it any time you've run an SDI test bench-side and need to convert the two fill times into an index value, when you're studying for the Level 3 or 4 water-operator certification exam (membrane fouling questions are common at the advanced levels), or when commissioning a new RO/NF plant and verifying that pretreatment is meeting your membrane manufacturer's warranty conditions. The math is the same regardless of feed-water source, what changes is the interpretation.

Frequently asked questions

What is a good SDI value for reverse osmosis?

Most major RO and NF membrane manufacturers, Dow Filmtec, Hydranautics, Toray, LG, require SDI₁₅ ≤ 5 and prefer values under 3. SDI under 3 is considered excellent feed water that needs minimal pretreatment; 3–5 is marginal; above 5 means significant fouling risk and you need additional pretreatment before the membranes.

What does SDI15 mean?

The subscript on SDI indicates the test duration in minutes. SDI₁₅ is the standard 15-minute test in ASTM D4189. When the filter plugs too fast to wait 15 minutes (final fill time more than ~1 minute), operators run SDI₁₀ or SDI₅. SDI values from different test durations cannot be directly compared, always specify the duration.

What is the difference between SDI and turbidity?

Turbidity (NTU) measures light scattering by suspended particles at one instant. SDI measures how quickly those particles plug a 0.45 µm membrane over a 5-, 10-, or 15-minute interval. Two waters can have the same turbidity but very different SDI values because turbidity doesn't capture colloidal or biological fouling agents that membrane systems care about most.

What pretreatment reduces SDI?

In rough order of effectiveness: coagulation and media filtration for raw surface water, microfiltration or ultrafiltration upstream of the RO, cartridge filters of 1–5 µm rating, and chemical pretreatment with antiscalants. The right choice depends on what kind of fouling dominates, particulate, biological, or organic, which you identify with bench testing, not SDI alone.

Can SDI be negative?

Not in a valid test. SDI assumes the membrane gets slower (more plugged) over the test interval. If your final fill time is shorter than the initial fill time, something is wrong: usually pressure wasn't regulated, air was trapped in the holder, or the membrane tore. Re-run the test.

What is the MFI (Modified Fouling Index)?

MFI is a refinement of SDI developed at Delft to better predict actual fouling rates on RO membranes. It measures the slope of cumulative volume over time, which correlates more directly with colloidal fouling. MFI requires more elaborate apparatus but is gaining acceptance for seawater RO commissioning. SDI remains the industry-standard quick check.

How to run the test

This calculator handles the arithmetic once you have your fill times. If you need the procedure itself, our guide walks through the silt density index test procedure step by step: the 0.45 µm membrane, the 30 psi requirement, how to time the initial and final 500 mL fills, and the mistakes that quietly ruin a result.

It also covers what an SDI reading actually tells you about membrane fouling, and where SDI testing fits alongside turbidity and MFI when you are commissioning or troubleshooting an RO system.

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Disclaimer: This calculator is provided as a free study aid. Always follow your membrane manufacturer's published pretreatment requirements and the most current ASTM D4189 procedure for compliance testing.