Your swatch does not match the pattern. This works out what that actually costs you in inches, what stitch count fixes it, and whether a hook change is the better answer.
Gauge is the only number in a crochet pattern that is about you rather than about the design. Everything else — the stitch count, the row count, the finished dimensions — is fixed by the designer. Gauge is the conversion rate between them, and yours is almost never identical to the designer's. Hand tension, hook brand and yarn brand all move it.
The maths is a single conversion done twice. The pattern's intended finished width is its stitch count divided by its stitches per inch. Your finished width, if you work that same stitch count, is the stitch count divided by your stitches per inch. The gap between those two numbers is the problem. To close it, multiply the pattern's intended width by your stitches per inch and you have the stitch count you should actually work. Rows behave identically, using rows per inch.
In formulas, with gauge given over 4 in:
pattern stitches per inch = pattern sts ÷ 4
pattern finished width = pattern stitch count ÷ pattern stitches per inch
your width at that count = pattern stitch count ÷ your stitches per inch
stitch count you should work = pattern finished width × your stitches per inch
If the pattern has a stitch repeat — “a multiple of 6 plus 2” — the adjusted count is rounded to the nearest number that still fits the repeat, because a shell pattern that is three stitches short will not close at the end of the row. The tool shows you the rounded count and the fractional one it came from, so you can decide which way to go.
A worked example, with real numbers
A blanket pattern states a gauge of 14 stitches and 16 rows to 4 in in worsted with a 5 mm hook, and tells you to work 120 stitches across, 96 rows.
Your swatch, in the yarn you actually own, comes out at 16 stitches and 18 rows to 4 in. Tighter than the designer.
You: 16 ÷ 4 = 4 stitches per inch. Working 120 stitches gives 120 ÷ 4 = 30.0 in. Working 96 rows gives 96 ÷ 4.5 = 21.3 in.
That is 4.3 in narrower — about 12.5 % small. A lap blanket has quietly become a large baby blanket.
To hit 34.3 in you need 34.3 × 4 = 137 stitches. To hit 24 in you need 24 × 4.5 = 108 rows.
Or leave the counts alone and change the hook: you are getting 2 stitches per 4 in more than the pattern, so go up roughly 2 × 0.5 = 1.0 mm, from 5.0 mm to 6.0 mm, and swatch again.
Both answers are correct. Which one you want depends on whether you care more about the fabric or about the stitch count — see below.
How to measure a gauge swatch so the number is real
Make it bigger than you think. At least 6 in square. Edge stitches are always a different tension, and a 4 in swatch is all edge.
Use the stitch the pattern names. Single crochet and double crochet have completely different gauges in the same yarn and hook. If the pattern's gauge is “in pattern”, swatch the pattern stitch.
Block it the way you will block the finished thing. Cotton and acrylic both move when washed. If the blanket will be machine-washed, wash the swatch.
Lay it flat and do not stretch it. Pinning it taut gives you a lovely number and a garment that does not fit.
Measure across the middle, in two places. Count stitches over 4 in, including part stitches as fractions — 14.5 is a legitimate answer and this tool accepts quarter stitches.
Do the rows too. Row gauge is the one people skip, and it is the reason so many finished blankets are the right width and the wrong length.
Hook change or stitch-count change?
Changing the hook changes the fabric: drape, density, stiffness, and how much yarn the project eats. Changing the stitch count leaves the fabric exactly as it is and changes only the size. So:
Change the hook when the fabric matters or the shape is fitted — hats, bags, garments, amigurumi (where a loose fabric shows stuffing), and anything worked to a stitch repeat that you would rather not re-map.
Change the stitch count when the fabric is already what you want and only the dimensions are off — most blankets, scarves, wash cloths, rectangles.
The hook suggestion this tool gives is a rule of thumb, not a law: roughly 0.5 mm of hook per one stitch per 4 in of gauge difference. It is a starting point for your next swatch, not a substitute for making one. Hook brands vary by up to 0.25 mm from their stamped size, and some yarns barely respond to hook changes at all.
Substituting a different yarn
A yarn substitution is a gauge problem wearing a disguise. If you swatch the substitute and put both gauges in above, the tool answers it directly. If you have not swatched yet, the weight categories give a first estimate — these are the Craft Yarn Council's standard single-crochet gauge ranges, so they tell you roughly what to expect, not what you will get.
Two things the stitch count will not tell you. First, yardage changes: finer yarn covers less area per yard, so a DK substitution for worsted can need 20–40 % more yards even at the same finished size. Second, fibre changes drape: swapping acrylic for cotton at identical gauge still gives you a heavier, less bouncy fabric that hangs differently. Gauge is necessary, not sufficient.
Common questions
How do I adjust a crochet pattern for a different gauge?
Work out the pattern's intended finished size first: divide its stitch count by its stitches per inch. Then multiply that finished size by your own stitches per inch to get the count you should work. A pattern with 120 stitches at 3.5 sts/in is 34.3 in wide; if you get 4 sts/in you need 137 stitches for the same width. Do the same with rows, then round to the pattern's stitch repeat if it has one.
How do I measure a crochet gauge swatch?
Make it at least 6 in square in the stitch the pattern names, block it the way you will block the finished piece, lay it flat without stretching, and count stitches across 4 in and rows up 4 in through the middle — counting part stitches as fractions. Measure in two places and average. Half a stitch per inch changes a blanket by several inches.
What if my gauge is off by just one stitch?
One stitch per 4 in is about a 7 % size change on a 14-stitch gauge — roughly 2.5 in on a 36 in blanket. On a rectangle you can absorb it by adding or removing stitches. On anything fitted, change hook size instead: about 0.5 mm per stitch per 4 in, then swatch again.
Can I substitute a different yarn weight?
Yes, as long as you re-do the counts. One weight finer typically gives 2–4 more stitches per 4 in, so the same stitch count comes out roughly 15–25 % smaller. Swatch the substitute, enter both gauges above, and use the adjusted counts. Budget extra yardage: finer yarn needs more yards for the same area.
Should I change hook size or change the stitch count?
Change the hook when the fabric matters — hats, bags, garments, amigurumi, or anything with a stitch repeat you would rather not re-map. Change the stitch count when the fabric is already right and only the size is wrong, which covers most blankets and rectangles. Changing the hook changes drape and yardage; changing the count does not.
Do I really need to swatch for a blanket?
Not a formal one. Work the first 20 stitches and 10 rows of the blanket itself, measure that, put the numbers in here, then rip back and adjust the starting chain. What you cannot skip is measuring something before you commit — a 10 % gauge difference on a 50 in blanket is 5 in and about a skein of yarn.
Does this work for knitting too?
The arithmetic is identical — stitches per inch and rows per inch behave the same way in knitting. The only thing that does not transfer is the hook-size suggestion; for knitting, the equivalent rule of thumb is about 0.5 mm of needle per one stitch per 4 in, in the same direction.