Using smaller needles in a pattern than the pattern calls for

I have not been knitting for very long but have found that I really enjoy it. I made an afghan last year and loved the pattern and size of the afghan but did not like the size of the needles used as the afghan fell apart kind of easily. I just knitted a baby blanket as a gift and really liked the needle size and how the blanket turned out.

My question: when casting on stitches with a smaller needle size, how many additional stitches should be cast on to acquire the same length as in the pattern? I don't think a baby sized blanket for an adult is the right size.

My pattern requires size 17 needles and to cast on 143 stitches for a width of 47 inches.

I want to use size 11 needles and would like to know how many additional stitches to cast on for the 47 inch width or more?

The gauge is 9 stitches and 12 rows = 4 inches.

Here is the pattern for the first row:
K3, P2, *M1, K3, P2, slip 1, P2 tog, PSSO, P2, K3, M1, P2, repeat from * across to last 3 sts, K3.

I am confused. What do you recommend. Thank you for your help.

YOUR ANSWER

To work it out you use the recommended tension of the needles you wish to use.

Divide the total width required by the tension measurement (in your example above that would be 4) and multiply that number by the number of stitches in your tension (9).

Then you work out how many stitches are in your repeating pattern, in this case it seems to be multiples of 15 plus 8. So you adjust the number of stitches you have calculated up or down to be a multiple of 15 plus 8.

I have to admit though I am having trouble working out your number for you. If 143 stitches is 47 inches on size 17 needles the tension would be 12 stitches for every four inches and you seem to be saying that your tension with size 11 needles is only 9, which seems wrong to me.

I suggest that you recheck the tension using the yarn you intend to make the blanket out of using the size 11 needles and then do the calculations.

These are not the real numbers, but I will give you an example to help you through the process:

Lets assume that your new tension is say 16 stitches per 4 inches.

So you divide your width by the width of your gauge (in this case 4 inches)

47 / 4 = 11.75

Then multiply that number by the number of stitches

11.75 x 16 = 188

To work out the total number of stitches needed for your pattern (multiple of 15 plus 8 in this case), divide that number by the number of stitches in your repeating pattern:

188 / 15 = 12.53

Then multiply 15 * 12 = 180 and add 8 stitches = 188.