Weaving Sett (EPI) Calculator
Sett is the heart of a weaving draft: it sets how many warp ends sit in each inch, which decides whether your cloth is open and drapey or dense and firm. The classic approach measures the yarn's wraps per inch (WPI), then applies a structure factor for the weave you plan, because a plain weave with frequent interlacements needs a more open sett than a twill. This calculator takes your measured WPI and a structure factor (0.5 for balanced plain weave, higher for twills), returns the ends per inch and a reed-friendly rounded figure, and reminds you that WPI is a measurement, not a constant, so always weave and finish a sample before committing a full warp.
Sett formula
sett (EPI) = WPI * structure factor
rounded sett = round(EPI) to nearest whole end
balanced picks per inch = EPI (for balanced weave)
total warp ends = rounded sett * finished warp width
The structure factor reflects interlacement frequency. Use 0.5 for balanced plain weave (the Ashenhurst half-of-WPI rule), and a higher value (commonly around 0.66 to 0.75) for twills and other structures with fewer interlacements.
Using the result
- Wrap firmly but without crowding when measuring WPI; over-wrapping inflates the sett.
- Plain weave uses a factor near 0.5; 2/2 twill often uses about 0.66 to 0.75.
- A balanced weave uses the same picks per inch as ends per inch, so weft and warp show equally.
- Total warp ends sets how many threads to wind and thread through the reed and heddles.
- Always sample and wet-finish; fulling and shrinkage can shift the ideal sett substantially.
Weaving sett: frequently asked questions
What is sett in weaving?
Sett is the spacing of warp threads, measured in ends per inch (EPI). It controls how dense or open the woven cloth is. The right sett balances the warp and weft so the fabric is stable but not stiff. Sett is derived from how many wraps of the yarn fit in one inch and the weave structure you plan to use.
How is sett calculated from wraps per inch?
A widely taught starting point is the Ashenhurst method: take wraps per inch (WPI), and for a balanced plain weave use about half the WPI as the ends per inch. More generally, EPI equals WPI times a structure factor, where plain weave uses about 0.5 and twills and denser structures use a higher fraction because they have fewer interlacements.
Why does weave structure change the sett?
Plain weave interlaces every thread, so threads need room and sett is more open. Twill and satin interlace less often, so threads can pack closer and the sett rises. This calculator multiplies your WPI by a structure factor (0.5 for plain weave, higher for twill) so you can match your draft.
Is WPI an exact figure?
No. Wraps per inch depends on how tightly you wrap and the yarn's loft, so it is a measured input, not a fixed constant. Wrap your yarn around a ruler without crowding or gapping, count the wraps in one inch, and enter that number. The sett result is only as accurate as your WPI measurement.
Should I sample before warping?
Yes. Sett formulas give a starting point, not a final answer. Always weave a sample at the calculated sett, wet-finish it, and adjust up or down based on how the cloth behaves. Fiber, twist, and intended use all shift the ideal sett.
Official sources
- U.S. National Institute of Standards and Technology: Office of Weights and Measures (inch definition).
- NIST: National Institute of Standards and Technology (unit conventions).
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.