A broad back adjustment adds width across the back bodice between the armholes, so you can reach forward, drive or hug someone without the fabric pulling tight across your shoulder blades and the sleeves dragging at the back. You slash the back piece from the middle of the armhole toward the center back and spread it 1 to 2.5 cm (3/8 to 1 in), keeping the armhole curve and side seam exactly as drafted, then absorb the extra length on the shoulder seam with a small dart or by easing. The front, the sleeve and the armhole size do not change.

How do you know you need a broad back adjustment?

Put on a toile or a fitted woven top and cross your arms in front of you as if reaching for a steering wheel. The signs are:

  • Horizontal pulling across the shoulder blades when you reach forward, and a feeling that the top is holding your arms back.
  • Drag lines from the back armhole toward the center back, angled slightly downward, even with your arms at rest.
  • The back armhole seam pulls toward the center, so the sleeve sits inward of your shoulder blade and the sleeve seam at the back is strained.
  • The center back seam or zip strains across the shoulder blades, while the waist fits.

If the top is tight across the whole back, including the side seams and the waist, the problem is overall width and the fix is a larger size or wider side seams. A broad back is specifically the upper back, between the armholes, with the rest of the garment fitting. Many sewists with a broad back also have a front that is drafted too wide for them, which is why going up a size never solves it: the front gapes at the armholes while the back is still tight. The post on reading drag lines helps separate the two.

How do you measure your back width?

The measurement is the cross back (also called back width): across the back, between the creases where the arms join the body, at the level of the shoulder blades. That is about 10 to 12 cm (4 to 4 3/4 in) below the prominent bone at the base of the neck. Have someone measure it with your arms relaxed at your sides; measuring it alone pulls the shoulders forward and gives a number that is too small.

Then measure the pattern. On the back bodice, measure horizontally between the armhole stitching lines at the same level, about halfway down the armhole. Double it if the piece is a half back cut on the fold. The pattern needs your cross back plus wearing ease: 1.5 to 2.5 cm (5/8 to 1 in) in total for a blouse or dress, more for a jacket. The difference between what the pattern gives and what you need is the adjustment, and it is divided between the two halves of the back.

For example, a cross back of 38 cm plus 2 cm of ease is 40 cm. If the pattern measures 37 cm, you need 3 cm in total, or 1.5 cm (5/8 in) on each half of the back.

The arms-forward test on a toile is a good check: pin the center back closed and reach forward. If the fabric strains, let the center back seam open at the shoulder-blade level and measure the gap. Half of that gap is your amount per side.

How do you do a broad back adjustment, step by step?

Work on a traced copy of the back bodice with the seam allowances drawn in or removed.

  1. Draw a vertical line from the middle of the shoulder seam straight down to the waist, parallel to the grainline. It should pass through the point of the shoulder dart if there is one.
  2. Draw a horizontal line from the middle of the armhole, at cross-back level, across to meet the vertical line. The two lines make an L shape that boxes off the armhole and side section of the pattern.
  3. Cut along the vertical line from the shoulder down to the horizontal line, and along the horizontal line out to the armhole, leaving a hinge of paper at the armhole edge. The armhole-and-side section is now a flap attached by a hinge at the armhole.
  4. Spread the cut at the shoulder by your amount, say 1.5 cm (5/8 in), so the gap is widest at the shoulder and at the cross-back line and tapers to nothing at the armhole hinge. Tape paper underneath.
  5. Check the armhole. The curve has not been cut, so it is the same length and shape as before. The sleeve still fits. The side seam has not moved either.
  6. Deal with the longer shoulder seam. The back shoulder is now 1.5 cm longer than the front shoulder. Either draw a small shoulder dart of that width at the vertical line, 7 to 8 cm (3 in) long and pointing toward the shoulder blade, or ease the extra length onto the front shoulder between the neck and the armhole if it is 1 cm or less. The back shoulder dart is the classic solution and it also shapes the fabric over the blades. The guide to sewing darts shows how to press it so it disappears.
  7. True the center back. If the spread has pushed the lower part of the back out of line, redraw the center back straight from the neck to the waist. The extra width at the waist level is small and can be taken out at the waist dart or ignored.
  8. Add the seam allowances and cut a new toile.
Note: A back with a center back seam can be widened more simply, by adding 1 to 1.5 cm (3/8 to 5/8 in) at the center back from the neck down to the cross-back line and tapering to nothing at the waist. It is quick and works for small amounts, but the extra width lands at the center rather than across the blades, and the neckline grows by the same amount.

What if the top has a yoke or is a knit?

On a shirt with a yoke, the width is added to the lower back piece in a straight strip below the yoke, usually as a center back pleat or two small pleats at the yoke seam. This is why so many shirts have a box pleat at the back yoke: it is a broad back adjustment built into the design.

Knits forgive a broad back up to a point. A T-shirt with 25 % stretch or more stretches over the blades when you reach. If a fitted tee still binds, add 1 cm (3/8 in) per side with the L-slash method and ease the extra back shoulder onto the front; a dart is rarely wanted in jersey.

How does made-to-measure drafting handle a broad back?

A pattern drafted from your measurements starts from your bust, shoulder width and back waist length, which puts the shoulder seam and the armhole in the right place for your frame. The cross back is a separate number, and a made-to-measure draft that does not take it uses a proportion of the bust and shoulder width instead. Sewly measures your shoulder width across the back from bone to bone, but not the cross back at shoulder-blade level, so a broad back relative to your shoulders is still something to look for on a toile with the arms-forward test. If you find it, apply the slash-and-spread above to the back bodice and keep the amount noted for future garments.

The guide to making a toile explains how to cut a quick test from calico and what to pin, and the woven top pattern page shows the shell, boxy and button-front options; the boxy version has enough ease across the back to skip the adjustment entirely.

Common mistakes with the broad back adjustment

  • Cutting through the armhole. Slashing straight across to the armhole and spreading there changes the armhole length and the sleeve no longer fits. Keep the paper hinge at the armhole.
  • Adding the width at the side seams. It widens the body at the underarm and the waist, where you do not need it, and does nothing across the blades.
  • Widening the front too. A broad back usually comes with a normal or narrow front. Alter the back only and check the front armhole for gaping on the toile.
  • Ignoring the extra shoulder length. Sewing a longer back shoulder to a shorter front shoulder without a dart or easing leaves a pleat at the armhole end.
  • Skipping the reach test. The back can look fine with the arms at rest and fail the moment you reach forward. Test the movement, not only the mirror.