How do I sew a straight seam with an accurate seam allowance?

Accuracy comes from a fixed guide and from watching the edge rather than the needle — and from realising that small consistent errors are what make a garment come out the wrong size.

Difficulty: beginner
Time: 20 min
Read: 5 min

Short answer

Do not watch the needle. Watch the raw edge of the fabric against a fixed mark on the needle plate or a guide taped to it, and keep the edge exactly on that mark. Set the stitch length to about 2.5mm for ordinary sewing, guide the fabric with light pressure rather than steering it, and let the feed dogs do the moving.

A garment is assembled from pieces cut with an allowance around them, and every seam sewn at the wrong distance from the edge changes the finished measurement. A five-millimetre drift on eight seams is four centimetres off the finished garment, which is a whole size. This is why seam allowance accuracy matters far more than the stitching being beautiful.

What you'll need

A machine with a visible needle plate
Masking tape or a magnetic seam guide
Scrap fabric for testing
A ruler marked in millimetres

Step by step

Find out what allowance the pattern uses.Commercial paper patterns commonly use 15mm; many independent and quilting patterns use 10mm or 6mm; Japanese patterns often include none at all and expect you to add it. It is stated on the instruction sheet and it governs everything else.
Find the guide lines on your needle plate.Most machines have engraved lines to the right of the needle marking distances from the needle position. Measure them with a ruler rather than trusting the printed numbers, especially on an older or an imported machine.
Make a physical guide if the lines are unhelpful.Lower the needle onto a ruler at the exact distance you want, then run a strip of masking tape along the ruler's edge, building it up in several layers so it forms a small ridge. A magnetic guide does the same job on a steel bed. The ridge lets you feel the edge as well as see it.
Watch the fabric edge, not the needle.This is the whole technique. Your eye should be on the point where the raw edge crosses the guide, a couple of centimetres ahead of the needle. Watching the needle is why beginners wander — you cannot correct a deviation you can only see after it has happened.
Guide, do not steer.Rest your left hand flat on the fabric behind the foot and your right hand ahead of it, applying almost no force. Steering the cloth sideways fights the feed dogs and pushes the needle off line.
Set a sensible stitch length.About 2.5mm suits most woven fabric. Go shorter — 2mm — for fine fabric, curves and anything under strain. Go longer — 3 to 4mm — for topstitching, thick fabric and for tacking. Very long stitches on a structural seam pull out.
Sew at a steady moderate speed.Straight seams are more accurate slightly faster than most beginners sew, because a steady speed makes it easier to hold a line. Slow down for curves, corners and thick crossings rather than sewing everything slowly.
Check the first seam with a ruler.Measure from the raw edge to the stitch line at both ends and in the middle. Two minutes here catches a systematic error before you have sewn eight of them.
Handle the fabric's weight.A large piece hanging off the table drags the seam sideways and stretches it. Support the bulk on the table or your lap so nothing is pulling on the part being sewn.

Tips

Sew a scrap first at the start of every session. It confirms the tension, the stitch length and your guide position in fifteen seconds.
Seam allowances on a curve are measured along the curve, not straight across, so a curved seam needs a narrower allowance or clipping. Never simply cut a wide allowance on a tight curve.
If your machine has a needle-position adjustment, moving the needle rather than the guide is a quick way to fine-tune the allowance.
Consistency matters more than the exact number. A garment sewn consistently at 14mm when the pattern says 15mm will fit; one sewn at anything between 10 and 20 will not.

Common mistakes

Watching the needle — By the time you can see the stitching is off line, it already is. Watching the edge against a guide a little ahead of the needle lets you correct before the error is sewn.
Assuming the presser foot edge is the right distance — It is a convenient guide but only if you have measured it. Standard foot widths vary between machines and are rarely exactly a common seam allowance.
Trusting the plate markings without measuring — They are stamped to the machine's own reference and are not always accurate to the millimetre, particularly at the wider settings.

If it doesn't work

Stitching wanders in and out along a long seam

Cause: Watching the needle, or steering the fabric — Fix: Fix your eye on the edge against a raised tape guide and let your hands only support the cloth.

The seam is straight but the finished piece is too small

Cause: Consistently sewing wider than the stated allowance — Fix: Measure your actual allowance with a ruler and reset the guide. Consistency is worth more than a nominal number.

Stitching drifts at the end of every seam

Cause: The bulk of the fabric dragging off the table as the piece runs out — Fix: Support the whole piece level with the machine bed throughout.

Questions people ask

Does it matter if my allowance is slightly different from the pattern?

Only if it is inconsistent. A uniform difference shifts the finished size predictably and can be allowed for; a varying one distorts the shape of each piece and cannot.

What stitch length should I use?

2.5mm covers most ordinary sewing on woven fabric. Shorten to 2mm for fine fabric and curves, lengthen to 3 to 4mm for topstitching and heavy cloth, and use the longest setting for tacking that you intend to remove.

Should I pin or tack?

Pin at right angles to the seam so the pins can be removed as the foot reaches them, and tack as well on slippery fabric, curved seams and anything where the two edges have to match precisely.