Why? How? Wherefore?
Why would you use decimals in a pattern? First of all, it’s more scientific. I think of patterns as techinical writing and decimals fit better than fractions with a slash. 1.5 is much easier to read than 1 1/2. The only time that fractions would be preferable over decimals is for those pesky thirds and eighths of inches. But for that, read ahead.
How do you use decimals? Only use decimals that are rounded to the nearest quarter or half. Zero is not written after the decimal but always written before the decimal if the value is less than 1. Whole values are written as the number without a decimal point. 1 skein. 1 inch. Not 1.0 skeins. Use quarter values of 0.25, 0.5, 0.75 where applicable (read ahead). Do not use other values in between like 1.1, 1.2, 1.3, 1.4, 1.6, 1.7, 1.8, 1.9.
Wherefore? The biggest question to ask yourself is why you are using decimals or partial values at all! If you’re writing gauge and using them for stitch counts STOP! Don’t ever use fractions of a stitch in gauge for crochet, it’s virtually impossible to measure a partial stitch in crochet. For more information on measuring gauge, check my How to measure Gauge and How to Write Gauge posts.
Are you giving measurements for bust values? Think about what you’re making. If you’re writing for a garment that has 2-4 inches of positive ease, does a quarter of an inch really matter? Base your values on the whole inches. When you convert to centimeters, is someone really going to be disappointed with a half of a centimeter difference? That’s assuming they’ve measured accurately to get that half centimeter. Does accuracy matter for your item?
To talk about accuracy, let’s talk about centimeters. Centimeters are REALLY SMALL. Have you looked at a ruler lately? When you’re measuring a garment or a blanket, is a half centimeter difference really going to matter? Almost always, your centimeter values can be rounded to the nearest whole value.
When do decimals matter? Small items and snug fitting items are when the fraction of the whole might have an effect. If you’re recommending negative ease, maybe a half of an inch will have an effect on the final fit. Anything fitting the hands, wrists, feet, or head might need to be measured to the half inch. If you’re precision only matters to the half-inch then your centimeter values can still be rounded to the whole cm value. I emphasize might in this instance. If you’re using super-bulky yarn making a hat, is it really possible to measure something to a half-inch value? When making something for the feet, you might think about half-inches for a close-fitting sock made with sock weight yarn compared to bulky slippers.
If you’re going to have a measurement that is smaller than a half-inch then you had better be working with small stitches. If you can’t add or subtract 1 stitch to reach that quarter-inch value, then it’s not possible to achieve. Why frustrate your maker?
So when are fractions allowed? I said for only the third and eighths of inches. The ONLY times that those values would be in your pattern are for measurements of added items: buttons, zippers, ribbons, eyes, etc. Only items that are traditionally measured in values of thirds or eighths of inches. These values will also be listed only once on your pattern thus less annoying for the reader.
Want more information on how to write crochet? Check out my ebooks or other blog posts. I have posts on Repeat punctuation, how to write repeats on squares, how to write to the stop, etc. My number one recommended post to new designers is my How to write a Yarn description post.