When drafting the Libeth pants, I made sure to add some extra width in the thigh, but it still may not be enough. I find that even if the pattern fits well, with some fabrics like Tencel twill, I need just a bit more room in the thigh. Do you find that? So strange.
The thigh is a little tight in this pair, 
but is perfectly fine in this one!
HOW MUCH TO ADD
Start by measuring your thigh, about 2” (10 cm) below the crotch. This should be the widest part of your thigh. Compare this measurement to the finished thigh measurement for your chosen size.
You should have about 3/4” (1.9 cm)* ease in the thigh. This means that your thigh measurement should be at least 3/4” (1.9 cm) smaller than the finished thigh circumference. If your thigh circumference is larger than the finished thigh measurement and your fabric does not have any stretch, I recommend making a thigh adjustment.
* 3/4" (1.9cm) is not a hard a fast number. As you can see in the two examples above, I was able to get away with just 1/2" (1.3 cm) ease in the thigh in the beige viscose-linen pair of Lisbeth pants, but it was a tad tight in the pink viscose pair of pants.
Next, let’s figure out how much to add to the pattern at the thigh:
(your thigh measurement + 0.75” (1.9 cm) ease* – finished thigh measurement for your chosen size) / 4
*We divide this by 4 because the amount will be added to the inseam and side seam on both front and back legs.
ADJUST THE PATTERN
You will need – front and back leg pattern pieces, scrap paper, a pen and a ruler. We'll start with the back leg.
1. At the crotch, measure 2” (5 cm) down and draw a horizontal line through the entire pant leg. A quick way to make sure the line is truly horizontal is to draw it perpendicular to the grainline
2. Place a mark on the line the required distance from the side seam and the inseam. The required distance is the number you calculated above.
3. Start drawing a slightly curved line down from the mark, blending the line into the side seam at the knee notch. Repeat for the inseam.
4. At the side seam, continue drawing the line upward, blending it into the side seam at the top.
5. At the crotch seam, continue drawing the line upward. At the crotch level, pivot and blend it into the crotch curve. This will slightly extend the crotch curve, which will also help accommodate the wider thigh.
6. Repeat the above steps for the front leg.
CHECK THE PATTERN
Next, we need to check the new side seams to make sure they match on both front and back legs.
1. Place the front leg on top of the front pocket bag, lining up the side seams. Use pins to pin the two pattern pieces together. Draw in the stitching line on the yoke and back leg at the yoke attachment seam. At the side seam, line up the yoke and the back leg along the yoke attachment seam. The yoke won’t lie flat since there is some shaping through that seam, and that is okay. It only needs to lie flat right at the side seam. Pin the two pattern pieces together.
2. Check the side seam on the front and back legs to make sure they are the same length. Since there were no changes below the knee notch, you only need to check the side seam above it. Adjust as needed.
3. Check the inseam on the front and back legs to make sure they are the same length. Since there were no changes below the knee notch, you only need to check the side seam above it. Adjust as needed.
Of course, there are a few methods on how to adjust the thigh, some are more involved, and some are considered more “proper”. I believe there are many ways to do the same thing, and some methods are better suited for specific situations. I find this fast and dirty method works well for the minor adjustments I need!
Let me know if you have any questions.
Xoxo,
Anya