Written and maintained by CASRAI Editorial Board
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Short answer
Molecular weight cut-off (MWCO, also written “molecular weight cut off”) is the nominal rating of a dialysis or ultrafiltration membrane, given in daltons (Da) or kilodaltons (kDa). By the common convention it is the molecular weight at which a solute is about 90% retained by the membrane. It is a rating, not a sharp dividing line, and it is not a pore diameter.
Definition and scope
Thermo Scientific’s dialysis guide defines MWCO as the average molecular weight of a molecule that can no longer diffuse across the membrane, so that it is retained at more than 90% because it no longer fits through the pores. Sterlitech’s technical note states the same convention for filtration membranes: the lowest molecular weight at which the membrane retains more than 90% of a solute of known molecular weight. A French standard (NF X 45-103), as quoted in a 2020 Membranes paper, uses the same 90% retention wording and notes that the threshold could be set at another value.
MWCO values appear on dialysis tubing, dialysis cassettes, centrifugal ultrafiltration devices and tangential-flow cassettes. Thermo’s dialysis range, for example, is offered at 2K, 3.5K, 7K, 10K and 20K ratings.
Why a rating is not a sharp line
Thermo states directly that molecular weight cut-off is not a “defined” value, because diffusion of molecules near, but below, the MWCO is also significantly slowed. In the same guide, molecules with a molecular weight below one tenth of the MWCO rating diffuse most rapidly and reliably across the membrane, and dialysis is described as suited to separating molecules with a roughly 10- to more than 50-fold difference in size.
Thermo lists what governs diffusion through a membrane: pore size, number of pores and membrane thickness, plus temperature, the geometry, concentration, interactions and hydrophobicity of the molecules, buffer volume, agitation, exchange frequency, and the ratio of membrane surface area to sample volume.
How ratings are tested, and why they differ between vendors
Sterlitech notes that there is currently no industry standard for determining MWCO, and that manufacturers may use dextran, polyethylene glycol or proteins as test solutes. The 2020 Membranes paper (Fadel, Wyart and Moulin) reaches a similar conclusion: an acceptable, fast, cost-effective universal standard method is still unavailable for ultrafiltration and microfiltration membranes. It reports that retention curves shifted with filtration time (fouling), that a pH change distorted quantification, and that membrane charge influenced repulsion and adsorption of the test particles. Earlier work it cites used PEGs, oligostyrenes, alkanes and dextrans.
Cross-source comparison
| Question | What the source says | Source |
|---|---|---|
| Retention threshold | More than 90% retained of a solute of known molecular weight | Thermo Scientific; Sterlitech |
| Is it a fixed value? | No; diffusion near but below the MWCO is also slowed | Thermo Scientific |
| Is there a standard test? | No industry standard; test solutes vary (dextran, PEG, proteins) | Sterlitech; Fadel et al. 2020 |
| Passing rule of thumb | Below 1/10 of the MWCO diffuses most rapidly and reliably | Thermo Scientific |
| Retaining rule of thumb | Choose an MWCO at least 3 times smaller than the target; for proteins, 3 to 6 times smaller (6 times for maximum retention, 3 times if flow rate matters) | Sterlitech |
The two rules of thumb answer different questions and come from different vendors: one describes which small molecules cross a dialysis membrane quickly, the other describes how far below a protein’s molecular weight to set the rating to keep it. Neither is a substitute for the manufacturer’s own retention data for the device in hand.
Where it matters in the lab
Dialysis and buffer exchange, centrifugal and stirred-cell ultrafiltration, and tangential-flow filtration all select membranes by MWCO. See the CASRAI guide on tangential flow filtration for the process-scale context and the guide on microdialysis for a sampling technique that also relies on a membrane cut-off. Protein workflows that use buffer exchange are covered in the guide on protein refolding.
Record-keeping checklist (editorial, derived from the sources above)
- MWCO value and unit (Da or kDa) as stated by the manufacturer.
- The test solute and retention threshold behind the rating, where the manufacturer publishes them, since solute type changes the result.
- Conditions that the sources say affect transport: temperature, buffer volume, agitation, exchange frequency, and surface area relative to sample volume.
- For ultrafiltration, the filtration time and buffer pH, since fouling and pH shifted measured retention in the cited study.
Common misunderstandings
- MWCO is a pore size. It is a molecular-weight rating derived from retention of test solutes; the sources describe the pores only as the reason larger molecules are retained.
- Everything above the MWCO is held back and everything below passes. Thermo describes slowed diffusion near the rating and recommends a wide size difference for clean separation.
- Two membranes with the same MWCO behave identically. Without a common test method, ratings from different vendors rest on different test solutes and conditions.
FAQ
What does MWCO mean?
Molecular weight cut-off: the nominal molecular weight, in Da or kDa, at which a membrane retains more than 90% of a solute of known molecular weight.
Is “molecular weight cut off” the same as “molecular weight cut-off”?
Yes. The hyphenated and unhyphenated forms, and the abbreviation MWCO, refer to the same rating.
Is there a standard way to measure MWCO?
Sterlitech states there is no industry standard, and a 2020 peer-reviewed paper describes a universal standard method as still unavailable for ultrafiltration and microfiltration membranes.
Sources
- Thermo Scientific, High-Performance Dialysis Guide (manufacturer PDF), read 2026-11-08. PDF.
- Fadel, Wyart, Moulin, Membranes (Basel) 2020;10(10):271, read 2026-11-08. PMC7600232.
- Sterlitech, “What is Molecular Weight Cut-Off (MWCO)?”, read 2026-11-08. sterlitech.com.
