How Sewly drafts a pattern: blocks, ease and your measurements

Sewly drafts a pattern in two stages. First, the AI looks at your photo, link or description and decides what the garment is: its type among Sewly's 20 garments, and options such as length, neckline, sleeves, fullness, closure and pockets. Then a drafting engine, with no AI involved, draws every piece from flat-pattern blocks using your measurements and the right ease for the fit. Same inputs, same pattern, every time.

Why separate the AI from the drafting?

Pattern drafting is geometry with tight tolerances. A set-in sleeve cap has to be slightly longer than the armhole it is sewn into, by about 1.8 cm (3/4 in) for a regular fit in a woven fabric, so that it eases in without puckers. Front and back side seams have to be the same length. A waistband has to match the skirt's waist plus an overlap. Language and image models are good at recognizing things and poor at guaranteeing exact lengths. So Sewly lets the AI do the recognition and gives the drawing to code that calculates.

Two practical consequences follow. When you change a measurement or an option by hand, the pattern redraws instantly, without an AI call, and that is free and unlimited on every plan. And if you enter the same measurements and options twice, you get the same pieces twice.

What does the AI decide?

From a photo, a screenshot, a link to a public Pinterest pin, Instagram post or shop page, or a description in words, the AI returns a structured choice:

• The garment type, such as fit-and-flare dress, wrap skirt or sweatshirt.
• Its options: length, neckline, sleeve length and shape, fit, skirt style, gathering ratio, closure, pockets, ties.
• The fabric's colors and print, for the 3D preview.

If the picture shows something Sewly does not draft yet (a tailored blazer with lapels, a coat, fly-front jeans, a shirt with a collar stand, lingerie or swimwear), it picks the closest garment and says so. You can correct any choice afterwards.

What is a pattern block?

A pattern block is a basic, close-fitting pattern drafted from measurements, with no style: a bodice, a skirt, a sleeve, a pair of trousers. Designers create styles by changing blocks. Sewly's engine has a block for each, following the proportions of the usual flat-pattern systems, simplified for clean, sewable results. For the bodice, for example, the armhole depth comes from about a quarter of the bust plus a constant, the back and chest widths come from the bust, and the side bust dart is sized from your cup, the difference between your bust and underbust.

Then each garment applies its style to the blocks: lowering the neckline into a scoop, adding flare at the side seams, cutting the skirt as a half circle, adding a cut-on facing for buttons.

How much ease does Sewly add?

Ease is the difference between your body and the garment. Sewly adds it according to the fit you choose and whether the fabric is woven or knit. At the bust, at the time of writing:

Waist and hip have their own tables along the same lines. Knits get less ease because the fabric stretches; a fitted knit can be slightly smaller than your body, which is called negative ease. Our guide to ease in sewing patterns explains wearing ease and design ease in more depth.

How does Sewly decide lengths?

Lengths come from your body, not from a fixed number of centimeters. Skirt and dress lengths are measured from your waist using your waist-to-knee and waist-to-floor measurements: a knee length ends about 2 cm below the knee, a midi halfway between knee and floor plus 4 cm, a maxi 4 cm above the floor. Top lengths are measured from the waist too; hip length, for instance, ends about 21 cm (8 1/4 in) below it. The length adjustment then adds or removes up to 25 cm (about 10 in) if you want the hem somewhere else.

That is why the same "midi" lands at mid-calf on someone 155 cm tall and on someone 180 cm tall.

What happens when you change an option?

Every option is a parameter of the draft. Switching a sleeve from straight to puff widens the sleeve and lengthens its cap so there is fullness to gather, adds a gathering step to the sewing guide and adds narrow elastic to the notions, with its length. Switching a skirt waist from a waistband to elastic removes the waistband and the zip and calculates the elastic from your waist, a little shorter than the waist itself so that it holds. Raising the gathering ratio of a gathered skirt from 1.8 to 2.5 widens the panels and updates the fabric quantities.

How does the draft become a printable pattern?

The engine then finishes the pieces: seam allowances (1.5 cm or 5/8 in on woven fabric, 1 cm or 3/8 in on knits by default, adjustable), notches where seams meet, grainlines, fold lines, labels and cut counts. Large plain rectangles such as waistbands, ties and gathered skirt panels are given as cutting dimensions rather than printed. The pieces are laid out for your paper (A4, US Letter, A3, Tabloid, A0, 36 × 48 in or a projector file) on lettered and numbered pages with a test square, and the sewing guide lists the steps of that garment with its options, the notions, and fabric quantities for 110, 140 and 150 cm (45, 55 and 60 in) wide fabric. Printing needs a paid plan; everything else described here is on the free plan too. See how it works for the whole journey from photo to print.

What if you only enter a few measurements?

Height, bust, waist and hip are enough to start. Sewly fills in the rest (shoulder width, back waist length, arm length, waist to knee and so on) from the nearest standard size. Refining the lengths that matter for your garment, such as waist to knee for a skirt or arm length for long sleeves, makes the most difference. The body measurements page lists each one and how to take it.