Knitting is fairly simple maths. Four sums cover most of what you need, and here you can do them without digging out a calculator. It all builds on the same number: your gauge.
1. What is my gauge?
This is the number everything else hangs on. Knit a swatch of at least 15 × 15 cm, let it rest, and count in the middle of the swatch — never out at the edge.
Stitches ÷ cm × 10 = stitches per 10 cm. Rule of thumb: a difference of one stitch per 10 cm is roughly half a millimetre on the needle.
Change the needle and you change the whole garment. Two stitches too many per 10 cm on a jumper meant to measure 100 cm around leaves you with something more than ten centimetres too tight. That is the difference between a jumper you wear and a jumper that stays in the wardrobe.
2. How many stitches should I cast on?
Ease is how much wider the garment is than you are. Zero means it sits close to the body. If you want an oversized jumper, add twenty centimetres or more.
(Bust + ease) × gauge ÷ 10 = stitches around. If you are between two sizes in a pattern, this is the sum that decides which one you pick.
3. How often should I increase in the raglan?
A raglan adds eight stitches per increase round — two in each of the four raglan lines. The only question is how often.
Stitches missing ÷ 8 = number of increase rounds. Rounds in the yoke ÷ increase rounds = how often.
4. How much yarn do I need?
That calculator has a page of its own, together with the table of yarn weights and what you need to know when you substitute yarn halfway through a pattern.
5. Needles — metric, US and UK
English-language patterns rarely give millimetres. Here is the conversion:
| mm | US | UK |
|---|---|---|
| 2.5 | 1½ | 13 |
| 3 | 2½ | 11 |
| 3.5 | 4 | 10 |
| 4 | 6 | 8 |
| 4.5 | 7 | 7 |
| 5 | 8 | 6 |
| 5.5 | 9 | 5 |
| 6 | 10 | 4 |
| 6.5 | 10½ | 3 |
| 7 | — | 2 |
| 8 | 11 | 0 |
| 9 | 13 | 00 |
| 10 | 15 | 000 |
A 7 mm needle does not exist in the US system — it sits between US 10½ and US 11. Worth knowing, since 7 mm is the size I use in nearly all my patterns.
The sums, all in one place
| What you want to know | The sum |
|---|---|
| Gauge | stitches ÷ cm × 10 |
| Stitches around | (bust + ease) × gauge ÷ 10 |
| Rounds in a given length | cm × round gauge ÷ 10 |
| Yarn amount when substituting | balls × yardage ÷ new yardage |
| Raglan increases | (stitches missing) ÷ 8 |
| Even distribution | total stitches ÷ number of increases |
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