Project 003 Exercise 1: Finished Measurements & Fabric Requirements
In today's post, I'll show you how to calculate the finished circumference of the dress and how to estimate the amount of fabric you'll need to sew it.
Last week, I shared How to Draft a Shirred Babydoll Dress and while we did use numbers, it didn't get too mathy. The drafting relies more on diagrams than formulas. So, if you were dying to stretch your computation muscles a bit more and play around with trigonometry and algebra, then this post is for you.
In today's post, I'll show you how to calculate the finished bust measurement and how to estimate the amount of fabric you'll need to sew your dress.
How to Calculate the Finished Bust/Hip Measurement
The method of drafting this pattern uses measurements of the chest and shoulders and a gathers multiplier of 2 to draft the pattern pieces. But, if you're like me, you may want to know the finished bust/hip circumference BEFORE you draw your pattern piece or make a muslin.
Fortunately, we can use a little math to calculate this before we start drafting. We are going to use trigonometry again, just like in this lesson.
For this exercise we'll call the Finished Bust/Hip Circumference “X”. Because the front and back bodice are the same pattern piece and it's cut on the fold, we know that the width from the center fold to the seam allowance is X/4. And we know that the top of the bodice is 2A from our last lesson.
The missing piece of the puzzle is the width of the pattern underneath the 45 degree angle we drew to make the armhole. Fortunately, we can use the sine function to solve for that missing measurement.
We know that sin θ = opposite / hypotenuse. (Watch this video for an explanation on this formula.) Here, the hypotenuse is equal to C+G (the Armpit plus the Armpit Ease) and θ is equal to 45. We also know that the opposite side of the triangle equals (X/4)-2A.
Our angle is 45 degrees and lucky for us we know that sin 45=1/√2.
After, replacing the parts of our trig formula, we get to use algebra (my favorite!) to simplify the equation and solve for X. (Seriously, if I could solve algebra problems all day I think I'd be pretty happy.)
sin θ = opposite / hypotenuse Replace these symbols with our known numbers
sin(45)=(X/4-2*A)/(C+G) First, let’s replace sin(45) with 1/√2
1/√2=(X/4-2*A)/(C+G) Now, let’s simplify to solve for X. Start by multiplying both sides by (C+G).
(C+G)/√2 = (X/4-2*A) Next, add 2*A to both sides.
(C+G)/√2 + 2*A = X/4 Finally, we multiply each side by 4 and we have a formula for X.
4*((C+G)/√2 + 2*A) = X
In conclusion, the Finished Bust/Hip Measurement “X” equals 4*((C+G)/√2 + 2*A)
How to Estimate Fabric Requirements
If you are planning to order or buy fabric you probably want to know how much you need. With a little math we can cut the guess work. Since fabric comes in pretty set widths, we have to work under those parameters. But, since we only have two pattern pieces and they are nearly rectangles, this is a fairly easy cutting layout to determine.
As I see it, there are two likely cutting layouts. For narrower fabric, you can probably only fit one pattern piece per width of fabric, option 1 below. But for 60" wide fabric you might be able to get two pattern pieces per width.
If you want to save, or maximize, your fabric usage, you can adjust the Gathers Multiplier to change the width of the pattern pieces. For example, if your pattern piece is just slightly too big for the fabric, you can REDUCE the gathers multiplier to make it fit. (Just check that you’ll still have enough wearing ease.) Alternately, if you want to reduce the scraps leftover, you can INCREASE the Gathers Multiplier so that the pattern piece runs the full width of the fabric. (Just be aware that it doesn’t get too bulky.)
To estimate your fabric requirements, you’ll want to calculate the finished width and length of each pattern piece. (If you don’t remember what the different letters mean, refer back to the drafting lesson and look at the worksheet for a good diagram of the pattern pieces.)
Bodice Width = I + (C+G)/√2 +2*A
Sleeve Length = I + H + D + J
Sleeve Width = I + (C+G)/√2 +2*B
Sleeve Length = I + H + E + F + J
Compare the pattern piece widths to the fabric width to determine the best cutting layout (option 1 or 2). If your pattern pieces are too big to double stack, you’ll need to use Option 1.
Finally, to calculate the length of fabric required, and add up the lengths as follows.
For LAYOUT OPTION 1, you will need 2 x (BODICE LENGTH + SLEEVE LENGTH).
For LAYOUT OPTION 2, you will need BODICE LENGTH + SLEEVE LENGTH.
I hope that you found this post helpful and/or interesting. It’s the kind of stuff that clanks around in my head as I’m working on a drafting project. Please let me know if you have any questions. I will post soon on how to do a Full Bust Adjustment to this pattern.
Happy mathing!
Beth
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It would be helpful to see a photo of your pattern pieces. I wonder if my c + g is too long.