Strap yourself in for an epic post about altering pants pattern pieces! This post covers how to compare your body measurements to flat pattern measurements and how to adjust the pattern pieces.

You get a lot of information about whether a pattern will fit you by measuring the flat pattern pieces and comparing those dimensions to your body’s dimensions. That’s what I’m covering in Part 2 of the pants-sewing series: the ins and outs of altering pants pattern pieces.

If you haven’t read Part 1, please go back and check it out: Sewing Pants, Part 1: The Measurements You Need for Success. Part 2 will confuse the bejesus out of you if you don’t have context.

Also, for the record, this post on altering pants pattern pieces is my longest post ever! So, take this article slow. If you’re test driving this pants-fitting technique, you’ll probably have to read it more than once.

AND — I have to state, as I did in Part 1, that I’m NOT a pants-fitting expert. I learned this technique for altering a pants pattern from sewing educator Cynthia Guffey. If it’s a method that you’d like more information on, check out her website, where you can find fitting books and videos.

OK, pressing on! I hope I haven’t freaked you out, LOL. This technique isn’t difficult, but because there are so many measurements in the mix, it’s a bit fiddly. BUT — because there ARE so many measurements in the mix, I think you can supremely fine tune the fit. There’s potential here to sew the best-fitting pants of your life. Take your time and believe in yourself! Sewing pants is within your reach! I believe your butt can look cute! Woo, cute butts!

For your reference, these are the pants I made using this technique. I talk about these pants in this post:

From Measurements to Altering Pants Pattern Pieces

In Part 1, you took measurements. Like, a gazillion measurements. And you recorded them in a mega-sized table.

In Part 2, you can discard your body-measuring attire and put your regular clothes back on. Now that you’re comfortable again, here’s what you need:

  • a pants pattern
  • the mega-sized table filled with your measurements
  • tissue paper
  • clear plastic quilting ruler with a grid — 2 x 18 inch or 6 x 24 inch work great
  • scissors
  • Scotch tape
  • measuring tape
  • different colored pens — it can be nice to mark pattern changes in different colors, and pen doesn’t smear like pencil

Pick the size that you think will work best for you. Refer to finished measurements, if possible. Don’t get too hung up on choosing a size, because you’ll be making gobs of adjustments.

The following adjustments must be made in this order!

Marking the Sewing Pattern Pieces

Altering pants pattern pieces starts will marking seam allowances, the crotch line, the knee line, and the hem line.

Before you can compare pattern piece measurements to your body measurements, you have to prep the pattern.

1.) Cut out the (traced, if you please) front and back pattern pieces.

If the pattern has front slant pockets, cut out the pocket piece, too, and pin it to the front pattern piece, matching symbols. And if you’re using a tissue-paper pattern, give it a gentle, dry press to remove wrinkles.

2.) Mark ALL seam allowances.

Seam allowances are the backbone of the flat-pattern measurements!

3.) Mark grainline from top to bottom.

My fitting instructor, Guffey, showed us how to find grainline. Fold the pattern piece in half at the bottom edge, inseam and outseam touching. This is the center of the hem; mark it.

Make a fold, about three inches long, from the center of the hem up toward the top of the pants. This fold should be perpendicular to the bottom edge. Draw a straight line in the valley of the fold.

Using a gridded ruler to make sure the line is straight, continue the line to the top of the pattern piece. This is the grainline!

Guffey said it’s common for the grainline on the printed pattern to NOT be the true grainline.

4.) Mark crotch depth.

The crotch depth line is perpendicular to the grainline and (surprise!) passes through the crotch. (This is one reason it’s important the grainline extends from top to bottom.)

5.) Mark the hem and knee point.

Read your pattern instructions to determine hem depth. Draw a line at the hem depth, perpendicular to the grainline.

Using the grainline as a guide, fold up the hem depth line to meet the crotch depth line. Mark the valley of the fold with a straight line. This is the knee point, and the knee point should be perpendicular to the grainline.

Now you’re ready to compare measurements! Bust out your table. This is where your tape measure gets a serious workout.

For each element, I’m going to share how to measure the flat pattern piece and how to adjust it based on your body measurements. I will cover how to make adjustments for the front and back pattern pieces under each element.

Also: If your hips are uneven, keep reading to the end. The following directions are for even hips.

Waist Slope

Hip to floor measurements and center front and center back to floor measurements determine waist slope when altering pants pattern pieces.

Back

Draw a line perpendicular to the grainline from the side seam to the center back. This is zero slope (think X-axis marked 0).

Calculate the difference between the right side to floor measurement and the center back to floor measurement. Record the difference. (There’s not a great place on the table for this, but it’s important to WRITE DOWN all adjustments you make!)

Use the difference to draw a new cutting line. For example, if center back to floor is 1 inch lower than right side to floor, mark 1 inch below the zero slope.

The new cutting line now gently curves from the side seam to the mark 1 inch below zero slope.

Front

Follow waist slope for back directions, substituting center front for center back.

If center front to floor is higher than right hip to floor (e.g., 1/4 inch higher), mark 1/4 inch above zero slope. Draw the new cutting line.

Crotch Depth

Measure from the waist seam to the crotch line on the outseam to determine crotch depth adjustments.

Back

Turn your tape measure to the side (skinny edge). Measure from the waist seam to the crotch depth line along the outseam seam allowance. Record the measurement on the table.

Calculate the difference between your body and the pattern and record on the table.

If your body crotch depth is longer, slash and spread the pattern piece above the crotch line to make the crotch depths the same. (This is where you’ll need tissue paper, tape, and scissors.)

If your body crotch depth is shorter, take a tuck in the pattern.

Front

Make the same length adjustment to the front pattern piece as you did to the back.

Crotch Curve

Fine tuning the crotch curve is critical when altering pants pattern pieces.

Back

Here’s where you transfer the crotch curve captured with a flexible ruler to the back pattern piece.

Trace your curve shape. A flat(ter) crotch curve will be toward the inside of the pattern’s original crotch curve. A round(er) crotch curve will be toward the outside of the pattern’s original crotch curve.

This traced line is a stitching line (seam allowance). Now adjust your pattern piece’s cutting line (and seam allowance).

Front

Guffey didn’t have us capture the front crotch curve with a flexible ruler, so there’s nothing here for your to do!

Crotch Length

When altering pants pattern pieces, add or subtract crotch length from the inseam.

Back

Turn the tape measure on its side. Measure from the center back waist seam along the new crotch seam allowance to its intersection with the inseam seam allowance. Record in the table.

Ready for some math-a-magic? Great!

Multiply the body crotch length by 0.55. Fifty-five percent of the total crotch length should be in the back crotch.

Calculate the difference between the pattern length and the 55 percent number. Record in the table.

Make crotch length adjustments by adding or subtracting from the inseam as necessary.

Front

So, if 55 percent of the total crotch length is in the back, that means 45 percent is in the front.

Measure the pattern’s front crotch length, from the center front waist seam along the front crotch seam allowance to its intersection with the inseam seam allowance.

Like you did with the back crotch length, compare the 45 percent number to the pattern measurement. Add or subtract length as needed at the inseam.

Hip Width

When altering a pants pattern across the front and back hip, start at the largest hip point and gently curve to the waist.

Back

Mark 2-inch increments — 2, 4, 6, 8 — and the largest hip point along the outseam seam allowance. Measure across the back hip at each increment and record the measurement in the table.

The next step is calculating the difference between the pattern and body measurements. But before you can do this, you must divide your body measurements in half. For example, let’s say your body back hip width at 2 inches measures 17.25 inches. The pattern back hip at 2 inches measures 9.5 inches.

9.5 – (17.25 / 2)
9.5 – 8.625
= 0.875 [this is 7/8 inches]

In the example, the pattern is 7/8 inches wider than the body measurement.

Record the differences for all the increments in the table. Now, add or subtract these differences from the pattern side seam. Going back to the example, at the 2-inch mark, subtract 7/8 inches from the side seam. (In the illustration, the differences are red dots, and the new side seam is light blue.)

I have a couple of notes about hip width:

1.) When measuring across the hip, don’t include the lower crotch curve/upper inseam wedge. This part of the pattern curves inside the leg and isn’t captured when you measure across the hip.

2.) For a lot of us with female human bodies, there’s an indentation about 8 inches below the waistline. (For me, it’s the high point of the leg opening at the side of my undies. You can see it in this silhouette of my body.) You don’t want to fit this indentation. In her “Pants Fitting Workbook,” Guffey writes, “Start at the largest part of the hip maintaining a smooth line, but do not abruptly angle in at the waist.”

3.) If the pants have back darts, don’t fold the darts when measuring the flat pattern.

Front

Measure the front hip width like the back hip width. If the pattern has a front fly, don’t include the fly extension in the flat pattern measurements.

If the pattern has pleats, fold the pleats and carefully measure over the bubbled-up area. This excess is design ease and shouldn’t be included in the measurements.

If the pattern has slant pockets, you’ll have to make adjustments to those side seams, too.

Waist Width

Back and Front

Guffey taught the class to adjust waist width, back and front, with darts and pleats, respectively. She cautioned against angling the side seam too aggressively at the waist.

I think the best way to make dart and pleat adjustments is to baste up your pants (after you’ve made all the other flat pattern adjustments) and see what’s going on. If you need things tighter/looser, play with the back darts and front pleats. I think waist adjustments are more art than science.

Per Guffey’s advice, I added pleats to my pants to take in my front waist even though the pattern didn’t call for them.

I know that pleats can be controversial. I wasn’t hot on them, either, but I figured that if I’d put this much effort into a pants-fitting class, I should listen to the instructor.

If you are dead-set against pleats, I think you could fiddle with the side seams of your muslin and get to where you want to be. But I definitely think back darts and the center back seam need to be part of the fiddling, too. Again, art not a science. Be patient with yourself!

Waist to Knee

To adjust waist to knee length, slash or tuck above the knee line.

Back

Turn your tape measure to the side and measure from the waist seam to the knee point. Compare this measurement to your body waist to knee measurement and calculate the difference. Record on the table.

Add (slash) or subtract (tuck) accordingly at a line perpendicular to the grainline about 5-6 inches above the knee point.

Front

Add or subtract the same amount from the same location.

Finished Length

When altering pants pattern pieces for finished length, slash or tuck between the knee line and hem line.

Back

Turn your tape measure to the side and measure from the waist seam to the hem (not the bottom of the pattern piece, but the line that represents where the bottom of the finished pants will be).

Compare this measurement to your body finished pants length measurement and calculate the difference. Record on the table.

Add (slash) or subtract (tuck) accordingly at a line perpendicular to the grainline about 6-7 inches below the knee point.

Front

Add or subtract the same amount from the same location.

Waistband

Guffey told us to throw out any waistband pieces that came with a pattern and draft our own. And, if you’ve made a bunch of adjustments, it makes sense why she’d say that.

Once you have your pants fitting great in the waist, transfer those adjustments to your front and back pattern pieces.

Mark (or remark) the waist seam on the front and back. Fold the pleats and darts.

Now turn your tape measure to the side and measure the waist seam. You’ll use this measurement to draft your own waistband! Don’t forget about seam allowances, and mark sewing landmarks — side seams, center back, center front, etc. — to make attaching it to your pants easy peasy.

When I drafted the waistband for my pants, I left extra length at the front to make sure everything around the fly went smoothly. Food for thought.

What If Your Hips are Uneven?

If your hips are uneven, you have a bit more work to do. Trace left and right leg pattern pieces for back and front (for four total pieces).

Waist Slope

Altering pants pattern pieces for uneven hips calls for a bit more work.

Use the smaller difference to slope at the center and the larger difference to slope at the waist. Above is an example for the back.

I struggled to write directions for the uneven hip waist slope directions. Here, pictures do the work for me!

Crotch Depth

Use the lower side measurement and make the same adjustment to all pattern pieces.

Hip Width

Apply all those left and right hip measurements to the left and right pattern pieces. (I know this is a lot of work, but think about how AMAZING your pants will be! SUPER customized! Keep going!)

Waist to Knee

Use the lower side measurement and make that adjustment to all pattern pieces.

Finished Length

Use the lower side measurement and make that adjustment to all pattern pieces.

Whew, I AM EXHAUSTED! I bet you’re tired, too! Altering pants pattern pieces is a lot of work, guys. But in the end, aren’t amazing-fitting pants worth it? Of course.

FYI: This is Part 2 of three parts. In Part 3, I’ll talk about the making a pair of pants using this technique and give some final thoughts.

OK, I’m ready for questions about adjustments! Please lay them on me in the comments, and I’ll do my best to answer them. Remember, I’m not a pants-fitting expert, but I’m reporting out on my experience taking this class. Guffey goes into more detail in her pants-fitting book, which I recommend.

P.S. ICYMI, here’s the previous post: Sewing Plans for November 2017 and the TNT Blues.

P.P.S. If you like this post, I’ve got gobs of other sewing resources for you! They live under “Sewing Resources” tab in the menu at the top of the page. Here’s your shortcut to my hard-earned sewing kernels of wisdom.

P.P.P.S. Here are the links to the other two parts of this three-part series (in case you forgot, what you’re looking at right now is Part 2):

Sewing Pants, Part 1: The Measurements You Need for Success

Sewing Pants, Part 3: A Completed Pair of Hampshire Trousers