How to calculate how much yarn you need for an afghan

baby knit blanket white 1

Because I mostly knit baby stuff , I have the  patterns written in baby sizes. To transform a baby blanket into an afgan or a big blanket, it is easy to calculate how many stitches to cast on to make a certain size. But then comes the question: how much yarn to buy???

Most of the skeins have the weight on them, so my calculations are based on this.

Well, my dears, this is when we need a bit of math … oh, no! don’t run away! I promise that is quite simple.

I am using the metric measurements, but even if you use the imperial, the logic is the same.

What do we need? A scale. An electronic one, is more accurate, so, it is better.

Don’t forget that the calculations are more for orientation, but this is what I use.



First knit your swatch. Mine was 15 x 8 cm and was weighing 8 grams.

The area of the swatch is: 15*8cm  = 120 square cm

If 120 sq cm weighs 8 grams

Then 100 sq cm weigh…”z” grams


“z” = 100*8 / 120 = 6.66 grams per 100 square centimeters

This means that an area of 100sq cm will weigh about 6.7 grams. (Aproximatelly)


Let’s say I want to knit an 100cm x 150 cm afghan.

It will have an area of: 100*150 = 15000 sq cm.

 Quantity of yarn: aprox  6.7 g * 150 = 1005 grams  (We multiply with 150 because 6.7 grams are for 100 sq cm)

That will be 10 skeins of 100 grams each. OR 5 skeins of 200 grams each. or what ever yarn you decide to use. 


Let’s say I want to knit a 75*110 cm crib blanket

Area: 110*75 = 8250 sq cm

Quantity of yarn: 6.7 g * 82.5 = 552.75 grams

That will be almost 6 skeins of 100 g each

Or 11 skeins of 50 grams each


I am so curious of what you think of this. Do you fond it helpful? What did you used it on (if you did)