A construction spec that spells out stitch class, stitches per inch (SPI), seam allowance and thread tension gives a factory sewing room everything it needs to set machines on the first day of cutting — no back-and-forth emails, no surprise samples. This tutorial shows you exactly how to build that section of your tech pack, field by field, so nothing gets lost in translation between your design studio and the production floor.
Key takeaways
- ISO stitch class notation (e.g. 301, 401, 504) is the universal language between technical designers and factory mechanics — use it alongside a plain-English description.
- SPI is fabric-dependent: a heavier woven needs fewer stitches per inch than a lightweight knit, and your spec should state the tolerance band, not just a single number.
- Seam allowance, seam type and stitch type must be called out together on the same row of your construction table — separating them creates ambiguity at the machine.
- Thread tension and needle size belong in the spec even if they feel like factory territory; stating them sets a baseline for dispute resolution if seams fail QC.
- A well-structured construction section reduces first-sample correction cycles and is one of the most time-saving investments in a tech pack.
What do you need before you start?
Before you open your spec template, gather the following:
- A completed flat sketch or tech sketch with all seam lines labelled
- Your fabric composition and weight (GSM or oz/yd) for every main material
- The seam type reference you use (ISO 4916 is the international standard; ASTM D6193 is common in North America)
- A stitch type reference chart — ISO 4915 covers the full classification
- Your brand's approved thread count and fibre type (if you have a preferred supplier, note the thread ticket number)
- A blank construction section in your tech pack template (rows for each seam location, columns for seam type, stitch class, SPI, seam allowance, thread, and notes)
If you are building a tech pack from scratch, our guide to what a tech pack contains covers the full document structure before you reach the construction section.
Why does the construction spec matter so much?
A tech pack is the primary document a brand hands to a manufacturer for production. According to the guide published by White Label Manufacturing, a well-prepared tech pack reduces cost and minimises risk by giving factories a single source of truth — and the construction section is where the most machine-level decisions live. When stitch type or SPI is missing, a factory mechanic makes a judgement call. That call may be perfectly competent, but it may not match your quality standard, and proving the gap at QC is far harder than preventing it in the spec.
Step 1 — Map every seam location on your flat sketch
Go through your flat sketch and list every seam in the garment: centre back, side seams, shoulder seams, armhole, neckline, hem, cuff, pocket attachment, bartacks — everything. Give each seam a reference code (CB-01, SS-01, SS-02, and so on). This code becomes the row identifier in your construction table.
Expected result: a numbered seam map that you can cross-reference between the sketch and the spec table. Anyone reading the tech pack can find the seam on the drawing and its spec in the table without guessing.
Step 2 — Assign an ISO stitch class to each seam
ISO 4915 organises stitches into six classes. The ones you will use most often in apparel are:
- Class 300 (lock stitch) — 301 is the standard single-needle lock stitch, used for most woven seams, topstitching and flat-fell seams.
- Class 400 (multi-thread chain stitch) — 401 is a two-thread chain stitch common in denim and workwear; 406 and 407 are cover stitches used on knit hems.
- Class 500 (over-edge / overlock) — 504 is a three-thread overlock; 514 is a four-thread safety stitch (overlock plus chain stitch in one pass).
- Class 600 (covering chain stitch) — 602 and 605 are flat seam stitches for activewear and underwear where bulk and chafe are a concern.
For each seam on your map, choose the class and sub-type. Write both the ISO number and a plain-English label in your table (for example: '301 — single-needle lock stitch'). The number is for the mechanic setting the machine; the plain label is for anyone who does not have the ISO chart in front of them.
Note: If a seam uses two stitch operations — for example, a 504 overlock followed by a 301 topstitch — list both on the same row, in order of operation. Use a slash or a numbered sub-row to keep them together.
Step 3 — Set SPI for each stitch type and fabric weight
SPI (stitches per inch) controls seam strength, stretch and appearance. There is no single correct number; it depends on the stitch class and the fabric.
As a starting framework:
- Woven shirting (light, 80–120 GSM): 12–14 SPI for a 301 lock stitch
- Woven bottom-weight (200–300 GSM, denim, canvas): 8–10 SPI for a 401 chain stitch
- Single-jersey knit (150–200 GSM): 10–12 SPI for a 504 overlock
- Activewear / compression knit: 14–16 SPI for a 605 flat seam
- Topstitching (decorative): match the SPI of the structural seam it runs alongside, or go 1–2 SPI lower for a bolder visual
In your spec table, write SPI as a range with a target, not a single number: '12 SPI ± 1'. This gives the factory a tolerance band to work within and a clear pass/fail criterion for QC. A single number with no tolerance is unenforceable — every machine has natural variation.
Warning: Never copy SPI from a previous season's spec without checking the new fabric weight. A 12 SPI that worked on last season's poplin will pucker on a heavier twill.
Step 4 — Specify seam allowance and seam type together
Seam allowance and seam type are inseparable. A 1 cm seam allowance on a flat-fell seam is a different construction to a 1 cm allowance on a plain open seam — the factory needs both pieces of information to cut and sew correctly.
In your construction table, add a 'Seam type' column alongside 'Seam allowance (cm/in)'. Use the ISO 4916 seam type code (SSa, SSn, LSc, and so on) plus a plain description. Common combinations:
- Plain seam, pressed open — SSa — 1.5 cm SA — used on woven side seams
- Plain seam, overlocked and pressed to one side — SSa + 504 finish — 1.5 cm SA
- Flat-fell seam — LSc — 2 cm SA (the extra width is consumed in the fell)
- French seam — SSaw — 1.2 cm SA first pass, trimmed to 0.6 cm before second pass
- Bound seam — BSa — 1 cm SA under the binding
Note the pressed direction (towards CB, towards front, open) where it affects the silhouette or the topstitch placement.
Step 5 — Document thread and needle specifications
Thread and needle are often left to the factory, but including them in your spec gives you a documented baseline. If a seam fails a wash test or a burst strength test, you can check whether the factory used the specified thread ticket.
For each seam, record:
- Thread type: polyester core-spun, 100% polyester, cotton-wrapped polyester, or wool (for tailoring)
- Thread ticket (count): common tickets are 40 (general seaming), 60 (fine shirting), 20 (topstitching on denim)
- Thread colour: reference a Pantone thread number or your brand's internal colour code
- Needle size: in Nm (metric) or the equivalent Singer/Schmetz size — for example, Nm 80 for light wovens, Nm 100–110 for denim
- Needle type: universal, ballpoint (for knits), jeans/denim, or microtex
You do not need to specify thread tension as a machine dial number — that varies by machine brand and age. Instead, specify the outcome: 'thread tension balanced; no bobbin thread visible on face; no puckering on seam.'
Note: If your garment uses contrast topstitching, list the topstitch thread separately from the seam thread. They often have different ticket numbers and the factory needs to order both.
Step 6 — Build the construction table in your tech pack
Bring all of the above into a single table. A clean construction table has these columns:
| Seam ref | Location | Seam type (ISO 4916) | Stitch class (ISO 4915) | SPI (target ± tol.) | Seam allowance | Thread ticket | Needle | Notes |
|---|---|---|---|---|---|---|---|---|
| CB-01 | Centre back | SSa — plain open | 301 — SNLS | 12 ± 1 | 1.5 cm | Poly 40 | Nm 80 universal | Press open; no topstitch |
| SS-01 | Side seam | SSa + 504 finish | 301 + 504 | 12 ± 1 / 10 ± 1 | 1.5 cm | Poly 40 | Nm 80 / Nm 75 ballpoint | Overlock before joining; press toward CB |
| HEM-01 | Hem | LSc — flat-fell | 401 — chain | 9 ± 1 | 2 cm | Poly 40 (seam) / Poly 20 (topstitch) | Nm 100 jeans | Twin-needle topstitch 6 mm from fold |
Keep the table on the same page as the flat sketch it references, or add a clear cross-reference. A factory should never have to flip between unrelated pages to match a seam code to its spec.
Step 7 — Add a bartack and reinforcement callout
Bartacks, rivets and reinforcement stitching are construction details that often get missed in the main seam table. Add a short supplementary table or a bulleted list that covers:
- Bartack locations (pocket corners, belt loop base and top, fly opening)
- Bartack length and stitch count (for example: 8 mm bartack, 42 stitches)
- Rivet placement (if applicable)
- Any chain-stitch lockdown at the start and end of seams
These details are quick to write and prevent the factory from making placement decisions that affect the finished look.
Troubleshooting common spec problems
The factory used a different stitch class than specified. Check whether you listed both the ISO number and the plain-English label. If only the number was present and the mechanic misread it, add the label for future specs. If the factory substituted intentionally, ask for written confirmation of the reason — this is a quality record.
SPI is within spec but the seam is puckering. Puckering is usually a thread tension or needle size issue, not an SPI issue. Check whether the factory used the specified needle type (ballpoint vs universal on a knit, for example) and whether the thread ticket matches. Add a 'no puckering' outcome statement to your thread tension field if it is not already there.
The seam allowance is correct but the seam is twisting. This is often a pressing direction problem. Go back to your spec and confirm you noted the pressed direction. If it was missing, add it.
The construction table is correct but the factory is still asking questions. This usually means the seam reference codes on the flat sketch do not match the codes in the table. Audit the sketch and the table together and make sure every code appears in both places.
Topstitch thread is the wrong weight. If you listed only one thread ticket for a seam that has both a structural stitch and a topstitch, the factory will use the same thread for both. Add a separate row or a sub-row for the topstitch thread.
What success looks like
When your construction spec is complete, a factory sample room supervisor should be able to sit down with the tech pack, set every machine on the floor for your garment, and produce a first sample without a single clarification email. The seam reference codes appear on the sketch and in the table. Every seam has an ISO stitch class, an SPI range, a seam type, a seam allowance, a thread ticket and a needle spec. Bartacks are called out. Thread tension is defined by outcome, not dial setting.
The first sample may still need fit corrections — that is what sampling is for. But it should not need construction corrections. Those belong in the spec.
Platforms that handle the product definition and workflow layer of tech pack creation — such as Vibe IQ, which covers product specs, visual line planning and PLM integrations — can help teams keep construction tables connected to the broader product record so nothing drifts between seasons. On the consumer-facing side, tools like Aiuta show how precise garment construction data (silhouette, fit, fabric) translates directly into better virtual try-on accuracy for shoppers — a reminder that what you write in a construction spec eventually shapes the end customer experience.
For a broader look at how construction documentation fits into the full product development cycle, our piece on fashion's AI ambitions versus production reality is worth reading alongside this tutorial.
FAQ
What is the difference between SPI and stitch density? SPI (stitches per inch) measures how many stitches fit in one inch of sewn seam. Stitch density is a broader term that can refer to stitches per centimetre or per unit area. In apparel specs, SPI is the standard unit in North America; stitches per centimetre (SPC) is common in metric markets. Always state which unit you are using.
Do I need to specify SPI for every seam or just the critical ones? Specify it for every seam. Leaving any seam unspecified hands a decision to the factory, and inconsistent SPI across a garment shows up in QC. It takes less than a minute per row to fill in.
Which ISO stitch class should I use for a knit hem? For a standard knit hem with a visible topstitch, a 406 or 407 cover stitch (Class 400) is typical. For a flat, chafe-free finish on activewear, a 602 or 605 covering chain stitch (Class 600) is more appropriate. The choice depends on the fabric weight and the required stretch recovery.
Can I use ASTM seam types instead of ISO? Yes. ASTM D6193 and ISO 4916 cover similar territory with different notation. Choose one system and use it consistently throughout your tech pack. If you work with factories in multiple regions, note which standard you are referencing at the top of the construction section.
How do I handle a seam that uses two different stitch operations? List both operations in the same row, in order of execution, separated clearly (for example: '504 overlock, then 301 topstitch'). Include the SPI and needle spec for each operation. This is more common than it sounds — overlocked-and-topstitched seams appear in almost every casual woven garment.