The washed gauge differs significantly from the knitted gauge

I need some advice.
My knitted gauge swatch measures 25 stitches and 29 rows for a 10 cm x 10 cm square.
I washed it, and now it measures 20 stitches and 36 rows.
This means I can’t try it on while I’m knitting.
If I base my calculations on the washed swatch now, the sweater will be too narrow at first, but then too long until I wash it. Is that right?
But: what if I wash the yarn (54% cotton + 46% viscose) before knitting?? How does the washed yarn behave once it’s knitted??
What should I do now?
Please help me!
Bea

I find it a bit strange that a piece would get longer in one direction after washing. It doesn’t really make any sense. Nothing can be added through washing and drying. Could it be that you hung your piece up and stretched it lengthwise that way? Normally, you should dry it flat.

I hand-washed it. I partially dried the gauge swatch in a towel and then let it dry flat.
My daughter will surely just throw the sweater in the washing machine and probably put it in the dryer, too. It’s going to be a children’s sweater.
Or rather, it’s supposed to become one.

I’m a little frustrated. I finally figured out the raglan, and now this. How am I supposed to work out how much yarn I’ll need? Now I’ll definitely have to buy more … an expensive business with the vendors.

Dear Mupfelbea,
it’s unusual for the gauge to vary so much after washing—and in the wrong proportions, at that. However, viscose is a tricky material because it’s inelastic and tends to warp. Cotton is also rather inelastic. As much as I hate to say it, I get the impression that the yarn you bought is problematic. Maybe you should get in touch with the yarn manufacturer—they can certainly weigh in on this...
Best regards,
Steffi

I didn’t explain myself very clearly. What I mean is, my washed gauge is 20 stitches and 36 rows for a 10 cm x 10 cm swatch.
It confuses me that the finished piece expands in width after washing but loses a significant amount of length.
If the length and width were both to become either larger or smaller at the same time, that would seem logical to me.
I think I’m just going to throw it in the trash and give up on knitting.

Thanks, Steffi.
As a beginner knitter, I guess I really didn’t buy the best yarn.
Unfortunately, my grandson can only wear cotton because of skin problems. Cotton/viscose works too (in store-bought sweaters), and I actually find it cozier than pure cotton. Too bad.