Technique - Waist Shaping

Take girth and length measurements and use your knitting gauge (which matches the pattern) to determine the amount and location of increases and decreases. Note that thicker fabric will stand stiffer with less drape so should have less waist shaping while drapier garments eg in fingering weight yarn can tolerate greater waist shaping.

For example: For the schematic below, the gauge is 20 stitches and 20 rows to 10cm. Therefore 1cm = 2 stitches and 2 rows.

Point 1. Bust girth. Measure 1/2 (bust girth + desired ease) to calculate the number of stitches required at front bust eg if 1/2  (bust girth + ease) = 50cm then from the gauge you know you need 100 stitches.

Point 2. Waist girth. Measure 1/2 (waist girth + desired ease) to calculate the number of stiches required at front waist eg if 1/2 (waist girth + ease) = 40cm then from the gauge you need 80 stitches.

Therefore, between the bust and the waist you need to lose 10 cm which is 20 stitches. But how does this relate to where to do the decreases? Need to look at length measurements to find the location of your waist. 

Point 3. Total length. Measure where the garment will come to on yourself and place an elastic at this position eg 60cm. This is needed to find the location of the waist.

Point 4. Finding your waist on the pattern. Get the armhole depth from the pattern eg 20cm. Measure this on yourself as the location of the base of armhole. Measure the distance between an elastic at your waist to this base of armhole position in cm and convert this to rows. eg if bust to waist is 20cm your waist is 40 rows below the base of armhole.

Point 5. On your body, measure the distance from your waist to the hem elastic eg if waist to hem is 20cm your waist is 40 rows above the hem.

So for this example, the waist shaping ends up being a decrease of 20 stitches over 40 rows from the base of armhole to the waist, and an increase of 20 stitches over 40 rows from the waist to the hem.

But you never decrease then immediately increase, there is always some plain knitting in between ie a band of plain at the waist. The same at the bottom of a longer garment, there is a band of plain knitting at the hem. So it may end up that of that 40 rows between the hem and the waist there are only 20 rows where you want to do the decreasing. eg a total of 10cm waist shaping (5cm each side) =  20 stitches over eg 20 rows. ie 2 stitches per row, every other row.

Instead of doing increases and decreases, eg in Fair Isle which would be difficult, you could just use a smaller and larger needle size. Note that this changes the fabric.