About 1/2 yard for little kids, 1 yard for older kids and up to 2 yards for an adult. First of all you need to find a pair of pants that currently fit the person you are making these pajamas for. (If you are not comfortable with this you can absolutely get a pattern for these.
Regarding this, How do you make Pyjamas without a pattern? 
What can I use for PJ drawstring? 
How do you make pajama bottoms? 
Beside above, How do you make a PJ collar?
How do you make homemade pajamas?
How do you make pajama bottoms without a pattern? How To Make Pajama Pants Without a Pattern
- Use a pair of pajama pants that you already have and fold them in half. …
- Lay the fold of the pants on the fold of the fabric.
- Pull the crotch area out a bit so that it is pointy.
- Cut 2 pieces (one for each leg) on the fold, using your old pair as a guide.
How do you make a PJ pants pattern?
How do you make fleece pajama bottoms?
How do you put Nada in pajama? 
How do you put Nada in pajamas?
What are pajama pants made out of? If you’re making kids’ pj pants, these are technically supposed to be called lounge pants because they are made from regular woven cotton or flannel and worn by children.
How do you make a sleepwear?
How do you make satin Pyjamas?
How do you make satin pjs? 
How do you draft a pajama pattern? 
How do you spell pyjamas in Canada?
Pajamas and pyjamas both refer to loose-fitting clothes worn for sleep. Pajamas is the preferred spelling in American English, while pyjamas is preferred in the main varieties of English from outside North America. Canadian usage in this century is inconsistent, though pyjamas appears to have the edge.
How do you make pajama trousers?
How do you make kids pajamas?
How do you make mens pajama pants? 
How do you make sleep shorts?
Can you make pajama pants out of fleece? DIY Pajama Bottom Tutorial
Making them out of fleece makes it even easier because, like knit, fleece doesn’t fray and you don’t have to hem it. I have put together a quick little tutorial as a guide if you want to try a pair of pajama bottoms on your own.
How do you make women’s pajama pants?
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