Add width through the bicep with a slash-and-spread that keeps the sleeve cap length close to what the armhole needs.
Measure before adjusting. Take your bicep at the fullest point with the arm slightly bent, and compare it against the pattern’s sleeve width at that level minus the seam allowances. The difference plus at least 30 to 50 mm of movement ease is what you need to add.
The simple version. Draw a vertical line down the centre of the sleeve from the cap point to the hem, and a horizontal line across at the bicep. Cut both. Spread the vertical cut by the amount required, which also raises the cap, then lower the cap by overlapping along the horizontal line until the cap height returns to roughly its original measurement.
Then walk the cap against the armhole. Adding width increases the cap seam length. A little extra is absorbed as ease; a lot has to be removed by lowering the cap further or by taking a small amount from the underarm seams.
Keep the underarm points level and redraw the cap curve smoothly through the spread, without a corner at the centre.
If the whole sleeve is tight, consider going up a size on the sleeve alone and easing it into the existing armhole, which is often less work than a full slash and spread.
Check the cuff or hem width afterwards, since the spread widens the whole sleeve unless you taper the added width away below the elbow.
Questions people actually ask
How much ease should a sleeve have?
Enough to move in — commonly 30 to 50 mm over the bicep on a fitted sleeve, more on a casual one. Measure a garment you find comfortable.
My sleeve now will not fit the armhole.
Lower the cap slightly and re-walk it. Cap height and sleeve width trade against each other, which is why the horizontal cut exists.
Last reviewed 2026-08-11