Four inches and ten centimetres are the same thing
They are not, arithmetically — four inches is 10.16 cm. But every pattern in the world prints “10 cm (4 in)” as one measurement, and this calculator treats them as identical. If you convert between them the 1.6% error compounds across a garment and you end up a full stitch out over a sweater front. So: type your gauge as stitches per 10 cm, or per 4 in, and don’t convert. It is the same number.
The swatch is the whole tool
Every figure here is downstream of one measurement, so it is worth doing properly.
Knit it bigger than the area you measure. A 10 cm square tells you nothing because the edges curl and pull. Brooklyn Tweed suggests 6 to 8 inches square; 15 × 15 cm is the practical minimum. Cast on about 1.5 times the pattern’s stitch count so there is fabric to spare on both sides.
Wash and block it exactly as you will treat the finished thing. Then leave it alone. Pull the pins, let it dry, and wait another 12 to 24 hours before you measure — knitted fabric keeps relaxing after it looks dry, and a gauge taken straight off the blocking board reads tighter than the garment will be.
Measure dead centre. Lay a ruler across the middle, count the stitches in 10 cm, and ignore everything within three or four stitches of an edge.
A weight category is not a substitution chart
The Craft Yarn Council is explicit about this: two yarns can both be labelled Medium (4) and still knit at noticeably different gauges. The category tells you roughly which needle to reach for first. It does not tell you that one worsted can replace another. Swatch anyway — that is the official position, not a cautious one.
US and UK crochet are two languages
If you are following a British pattern, note that the terms are shifted by one whole step. US single crochet is UK double crochet; US double is UK treble. A UK pattern’s “gauge” is also usually called tension, and the gauges in this tool are stated in US terms. Getting this wrong does not produce a slightly wrong fabric — it produces the wrong stitch entirely.
Read the millimetres, not the number on the needle
US 8, UK 6 and 5.0 mm are the same needle, but manufacturers stamp them inconsistently, and the Japanese 号 system lands between metric sizes almost everywhere. The conversion table here is built on millimetres for that reason. When you buy, check the mm.
Yarn quantities are ranges
Tension, stitch pattern and yarn type together move real consumption by more than 30%, and crochet uses about 30% more yarn than knitting for the same piece. Buy the extra ball, and buy it in the same dye lot — a mismatched lot is visible, and you cannot hide it in the last few rows.