Before the first real seam of any project, sew a 15 cm (6 in) test seam on two layers of scrap from the same fabric, with the same needle, thread, stitch and interfacing you intend to use, then look at both sides, press it, and fix whatever is wrong before the garment goes under the foot. It takes five minutes and catches the problems that otherwise appear on the center front: wrong tension, puckering, skipped stitches, a needle that leaves holes, a stitch length too short for the fabric, an iron too hot for the fiber. Make it a habit every time the fabric, thread, needle or stitch changes.
Why test on a scrap?
A sewing machine does not know what fabric you have put under it. The settings that gave a perfect seam on last week's cotton give a puckered seam on this week's viscose and a wavy one on jersey. Finding that out on the garment means unpicking a seam, with pinholes along the stitching line that may or may not close, on a piece you cannot recut. Finding it out on a scrap means turning a dial.
The same is true of the iron, of interfacing, of marking pens, of the buttonhole foot and of every technique you have not done on this fabric before. Everything that can go wrong on the garment can go wrong on a scrap first, at no cost. Many of the errors in beginner sewing mistakes are caught by this one habit.
What do you need for a test seam?
- Offcuts of the actual fabric, pre-washed with the garment fabric. Keep every scrap larger than a hand until the project is finished.
- The thread, needle and bobbin you will use, with the bobbin already loaded.
- A scrap of the interfacing if the garment has any, fused to one of the test pieces.
- A ruler or a seam gauge, and the iron set to the temperature you plan to use.
- A notebook, or the back of the pattern envelope, to write the settings down.
How do you run a five-minute test seam, step by step?
- Cut two pieces about 15 × 10 cm (6 × 4 in), on the same grain as the seam you will sew. Most seams are on the straight grain; if the garment has a bias seam or a crosswise seam, test that direction too.
- Set the machine as you intend to sew: the needle size and type for the fabric, the stitch, the stitch length (2.5 mm for most wovens, 3 mm for thick fabric, a narrow zigzag or stretch stitch for knits), and the tension at its normal setting.
- Sew the seam 1.5 cm (5/8 in) from the edge at your normal speed, with the same amount of guiding you would use on the garment. Do not hold the fabric taut unless you plan to on the real seam.
- Look at the top side: are the stitches even, is the seam flat, are there puckers or a wavy edge?
- Look at the underside: does the bobbin thread lie flat in a line, or are there loops, or is the top thread visible as dots? Then pull the seam open gently and look at the stitching line from the right side: does the top thread show as dots on the underside or the bobbin thread on the top? The thread tension glossary entry shows what balanced stitching looks like.
- Press the seam open with the iron at the temperature you plan to use, with a pressing cloth if you intend to use one on the garment. Check for shine, scorching, a melted edge or a seam allowance that shows through on the right side.
- Measure and adjust. If anything is off, change one thing (needle, tension, length, foot pressure, iron) and sew a second 15 cm seam. Repeat until it is right, then write the settings down.
What should you look for on a test seam?
| What you see | What it means | Fix |
|---|---|---|
| Loops of top thread on the underside | Top tension too loose, or the top thread is not seated in the tension discs | Rethread with the presser foot up; raise the top tension one step |
| Bobbin thread showing as dots on top | Top tension too tight, or bobbin thread not in its tension spring | Rethread the bobbin; lower the top tension one step |
| Seam puckers along its length | Stitch too short, tension too high, or the needle is blunt | Lengthen the stitch, lower the tension, change the needle |
| Wavy seam on a knit | Fabric stretched as it fed, or foot pressure too high | Lower the foot pressure, use a walking foot, do not pull the fabric |
| Skipped stitches | Wrong needle type or size, or an old needle | Ball-point for knits, microtex for fine wovens, a new needle every project |
| Holes along the stitching line | Needle too big | Go down a size, 70/10 on lawn and silk |
| Thread shreds or breaks | Needle too small for the thread, or burr on the needle | Go up a size or change the needle |
| Fabric does not feed | Foot pressure too low, feed dogs lowered, or the fabric is sticking to the bed | Raise the pressure, check the feed dog lever, try a walking foot or tissue paper |
| Shine or a melted edge after pressing | Iron too hot for the fiber | Lower the temperature, use a pressing cloth |
Tension problems and puckers have their own posts, sewing machine tension problems and fabric puckering when sewing, if the fix in the table does not work.
What else should you test on a scrap?
The seam is the minimum. Anything that happens once on the garment and cannot be undone is worth doing once on a scrap first.
- Buttonholes, every time. Sew one on the same layers as the garment (fabric, interfacing, facing), cut it open, and check that the button passes. The buttonhole guide explains why the first one is never on the garment.
- Fusible interfacing: fuse a 10 cm square and check that it bonds, that it does not show through, that the fabric does not shrink or shine. A bubbled interfacing on a front band is permanent.
- Hems: a twin-needle hem on a knit, a blind hem, a rolled hem, each on the exact depth you plan to use. Twin needles in particular tunnel on some fabrics until the tension comes down.
- Topstitching: thread, needle, length (3–3.5 mm) and whether the color works.
- Seam finishes: zigzag width and length on this fabric, or the serger's settings, which a new fabric thickness can throw out.
- Marking tools: draw a line with your chalk, pen or wheel, and check that it comes out with water, heat or time, before you mark a dart on a pale silk.
- Steam: some viscose and polyester fabrics water-spot, and some wool felts under too much of it.
If you are following a pattern with its own sewing guide, keep the test next to the relevant step. Sewly's sewing guide answers questions about the step you are on, such as what to do when a fabric is slippery, but the scrap is still where you find out what your own machine does with it.
Common mistakes with test seams
- Testing on a different fabric, such as a cotton scrap for a viscose dress. The point is to test this fabric.
- Testing one layer when the seam has two, or two when the collar has four plus interfacing.
- Testing on the wrong grain. A bias seam behaves nothing like a straight one.
- Changing three settings at once and not knowing which one fixed it.
- Not pressing the test. Half of what can go wrong, from shine to show-through, only appears under the iron.
- Throwing the scraps away at cutting time. Keep a bag of offcuts for each project until the last hem is done.
When can you skip the scrap test?
When nothing has changed: the same fabric from the same bolt, the same thread, a needle that has sewn under three hours, the same stitch, the same machine. A second pair of pull-on trousers in the cotton you used last week does not need a new test. Change any one of those and the five minutes are back, and they are never wasted: the test seam of a new fabric is also where you learn how it handles. The sewing machine basics guide walks through the first test seam from threading onward if you are starting from zero.