Tuesday, April 22, 2014

SSDA 8: Calculating Darts

Readers, these are our final calculations. They may seam a little complicated now, but once we will be doing our real measuring session it will all become clear and easy. I will organize calculations on one sheet with measurements, so you will have only a few clear steps.

The purpose of this post is to explain the logic behind this patternmaking approach, as well as to close the darts chapter with final calculation.


  • we measured depth (intake) and length of front, side and back darts. 
  • we checked the sum of all dart intakes against the difference between the hip and waist circumference (in ideal world these two values should be equal)
  • we clarified some terminology: dart depth for the measured value, and dart intake for the final calculated value (we differentiate this terms only for our process, normally they mean the same thing)

so far, so simple, right?


  • calculate relative error (a concept from high school math that helps reduce measurement error)
  • calculate final dart intakes taking into consideration the relative error calculation. To simplify calculations, I use abbreviations which appear in brackets next to the corresponding term.

Do bear with me, it is still simple I promise.

RELATIVE ERROR (or how to reduce measurement error for darts)

Darts are little tricky to measure, that's why we introduce the relative error calculation (Er): (a) to make our final values more accurate and (b) to integrate the error margin (e) that we got by comparing the sum of measured dart depths (DD) with the difference between the hip and waist circumference (DI).

Er = DI / DD


All we need to do now is multiply the measured dart depth (FD, BD, SD) for each dart by the calculated relative error (Er). FI, BI, SI stand for the corresponding front, back and side intake.

FI = FD x Er
BI = BD x Er
SI = SD x Er

We are finished with all the measurements, readers! I hope I managed to write it in an easy and coherent manner, however, if you notice any errors or typos please do let me know in a comment. What's left before the final drafting is determine the side seam placement, or, in other words, the front and back width. I'll be happy to answer your questions about darts, and if you want, I can add a post about the logic behind splitting the darts into two or more. Just let me know

Previous Straight Skirt Draft Along (SSDA) posts:

Coming up:

SSDA 9: Side Seam Placement and the Front/Back Width Controversy
SSDA 10: The Measuring Session Summarized
SSDA 11: Drafting the Skirt
SSDA 12: Making a Toile and Analyzing the Fit


  1. Thanks a lot for the series on darts, it's eye-opening. I would very much like a post on the logic behind making one or more darts

  2. I am so glad you can take the time to help us understand the reasoning behind all of this...Thank you. I am able to print each of them to take to my sewing room to re read and "do the math".

  3. My DI is zero. Totally 0, so my Er=0 and BI=SI=FI=0. Can I make a good skirt? :(

    1. You do need some dart intake for your skirt to hang from the waist without shifting down, provided, of course, your measurements are accurate. Before you give up on a skirt, however, try to measure yourself several times, until you get consistent results in two-three measurement sessions in a row. Maybe there was a mistake and even a small dart can make a difference. It will also help you to develop the skill.

    2. You do need some dart intake to make a skirt that will hang from the waist without shifting down. But before you give up on it completely, try to measure yourself several times again, until you get consistent results after two or three sessions. There may have been a mistake, and even a small dart can make a difference. It can be on the front, or back, or side only. Send me your images if you want and I will have a look.

    3. My measuring was correct. :( I am a big big girl in a big big world.... :(

  4. We have calculated the DI but what about the length of the darts ? (forget my question if you have answered on the following SSDA articles)



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