The three numbers
Your gauge, in stitches per inch, from a swatch you knit in the yarn and stitch pattern of the sweater, washed and dried the way the sweater will be, and measured across the middle without stretching. The gauge swatch guide is the method; the gauge calculator turns a count across 4" into the number. Count the half stitches. A gauge of 5 and a gauge of 5½ are two different sweaters.
The body, measured around the fullest part of the chest, over whatever will be worn under the sweater.
Ease, which is how much bigger than the body the finished sweater is: 0 to 2" for a fitted pullover, 2 to 4" for a classic fit, 4 to 8" for a relaxed or oversized one. A pattern states its ease somewhere in the first page; pick the size by finished chest, not by the size name.
Finished chest = body + ease. A 38" chest and 2" of ease is a 40" sweater.
38"
Chest, measured
2"
Ease, classic
40"
Finished chest
From inches to stitches
Stitches = finished width × stitches per inch. A 40" sweater at 5 stitches to the inch is 200 stitches around. Knit flat, that is 100 for the front and 100 for the back, plus a selvedge stitch at each side of each piece.
Round to the stitch pattern. A 2×2 rib wants a multiple of 4; a 6-stitch cable panel with 2 purls between wants a multiple of 8; stockinette does not care. Rounding 198 up to 200 is under half an inch on a worsted sweater and nobody will notice.
Rows work the same way with row gauge, but most patterns give lengths in inches, so you rarely need to convert them. The exception is a yoke or a raglan shaped every so many rows, where the row count is the design.
When your gauge is not the pattern's
Say the pattern is written at 20 stitches to 4" — 5 to the inch — and its 40" size casts on 200. Your washed swatch comes out at 22 stitches to 4", 5½ to the inch. Knit the 40" size as written and you get:
200 ÷ 5½ = 36.4". A fitted sweater instead of a classic one, and the sleeves and yoke narrower to match.
The same 200 stitches
You have two honest choices.
Change the needle and swatch again. More stitches than the pattern means your fabric is tighter and the needle goes up; fewer means down. One needle size moves the count by about one stitch per 4" at worsted and about two at fingering, so from 22 you would try two sizes up. Within half a stitch over 4" you are on gauge; a wash and a block absorb that. This is the right choice whenever the fabric at the pattern's gauge is a fabric you like.
Knit a different size. If you like your fabric at 5½ and do not want to change it, find the pattern size whose stitch count, divided by your gauge, lands on the chest you want:
| Pattern size | Stitches | At 5 sts/in (pattern) | At 5½ sts/in (you) |
|---|---|---|---|
| 36" | 180 | 36" | 32.7" |
| 40" | 200 | 40" | 36.4" |
| 44" | 220 | 44" | 40" |
| 48" | 240 | 48" | 43.6" |
Finished chest at 5½ stitches to the inch
You knit the 44" instructions for every stitch count and get a 40" sweater around. Lengths are the other half: the 44" size's body length, armhole depth and sleeve length were drawn for a bigger body, so take those from the 40" size's schematic, or from the person, and knit to inches rather than to row counts. What does not scale is anything shaped by rows — a raglan that decreases every other row for 30 rows, a yoke chart — which needs the next section. This trick works for a size or two in either direction; beyond that, the proportions drift.
Shaping by rows
A raglan, a yoke, a sleeve taper: the pattern says decrease every 4th row 12 times and that was worked out at the designer's row gauge. Rebuild it from yours.
A top-down sleeve, for instance. Upper arm 48 stitches, cuff 36, so 12 stitches to lose, which is 6 decrease rounds of 2. Sleeve length from underarm to cuff 17", your row gauge 7 rows to the inch, so 119 rows. Leave the first 2" below the underarm plain, which is 14 rows, and the last inch above the cuff plain, 7 rows: 98 rows for 6 decreases, one every 16th row. Round down to every 16th and the decreases finish a little early, so the sleeve is a hair slimmer above the cuff; round up to every 17th and the last decrease lands almost at the cuff. Either is fine.
One sleeve, top down, 119 rows
The same arithmetic does a raglan: stitches to lose at each seam, divided by 2 per decrease round, spread over the raglan depth in rows at your row gauge.
Yardage when gauge changes
Yarn goes by fabric area, and by how thick the yarn is. The same yarn at a slightly different gauge takes nearly the same yardage for the same finished sweater — a tighter fabric uses a little more, a looser one a little less, within about 10%. A different weight of yarn is a different table: the sweater yardage guide has the totals for fingering through chunky by chest size, and the yardage estimator does the division for the yarn in your cart.
If you knit a 44" size to get a 40" sweater, buy for the 40" sweater: yardage follows the finished size, not the size name. Then add a skein. The skein is for the swatch you kept, the body you lengthened, and the gauge that drifted a quarter stitch after the first sleeve.
A worked example, start to finish
Body 38", classic fit, so a 40" sweater. Pattern at 5 stitches to the inch; your washed swatch of Malabrigo Rios on a US 7 says 5½. You like the fabric. The 44" size casts on 220: 220 ÷ 5½ = 40". Knit the 44" instructions, follow every length in inches, redo the raglan at your row gauge, buy for a 40" worsted sweater — 1,000 to 1,250 yards, six skeins of Rios plus one — and the sweater fits.