Seam allowance: the complete guide for sewing and quilting

Standard seam allowance by pattern company and region

The golden rule sits above every row of that table: read the pattern's own statement. It is printed on the envelope, the instruction sheet or the first page of a PDF, and it always wins over any chart, including this one. Even within one Big 4 pattern, a neckline or collar piece can carry a narrower allowance than the rest, and when it does the pattern says so on that piece.

Seam allowances in quilting

Quilters work to a smaller, stricter allowance, because a quilt is dozens or hundreds of seams side by side, and every fraction repeats.

Seam allowance conversion: inches to mm and cm

One inch is exactly 25.4 mm by international agreement, so every value in the "Exact mm" column is precise. The metric conventions are not conversions: they are their own round numbers, chosen because they are easy to sew. That difference matters more in quilting than in dressmaking, and the next sections show exactly why. To convert any allowance in one tap, use the free seam allowance converter.

What is a seam allowance?

Every pattern piece has two edges that matter: the cutting line, where your scissors or rotary cutter go, and the seam line, where the stitches go. The strip of fabric between them is the seam allowance. When two pieces are placed right sides together and stitched on the seam line, the allowances end up inside the garment or quilt, out of sight.

The allowance is not waste. It gives the seam strength so it does not pull out under stress, room to finish the raw edges, room to press, and, in garments, a little fabric you can let out for fit.

Why is the standard seam allowance 5/8 in?

Because 5/8 in does every job a garment seam needs. It is wide enough to press open flat, wide enough to zigzag or overlock each edge, and wide enough to let out a side seam if a fitting shows you need a little more room. It is also exactly the width that classic seam constructions use. A French seam is sewn at 1/4 in, trimmed, turned and sewn again at 3/8 in: 1/4 + 3/8 = 5/8 in. In metric the same seam is 6 mm + 9 mm = 15 mm. The numbers are not an accident; the allowance was built around the techniques.

Is 1.5 cm the same as 5/8 in?

Close enough for garments, and that is why metric pattern companies chose it. 5/8 in is 15.9 mm (15.875 mm exactly), so 1.5 cm is 0.875 mm narrower. If you sew a 5/8 in pattern at 1.5 cm, each seam adds 1.75 mm to the circumference of the garment (0.875 mm on each side of the seam). A skirt with four vertical seams grows by 7 mm, about 1/4 in, which disappears into the wearing ease. Your needle plate is almost certainly marked in both, so sew the one the pattern names.

Quilting is a different story, as you will see below.

Does the pattern include seam allowance?

Three checks settle it in under a minute.

1. Read the envelope or the instructions. Look for "seam allowance included" or "5/8 in (1.5 cm) seam allowance included" on the back of the envelope, the instruction sheet or the first page of the PDF.
2. Look at the edge of the pieces. Single size patterns often print a dashed stitching line inside the cutting line; if you see one, the allowance is included. Multi size patterns usually show only the nested cutting lines, with the allowance hidden inside them.
3. Check the source. Burda Style magazine, OTTOBRE and Japanese pattern books print the net pattern, the exact seam line, and ask you to add allowance as you trace. Burda envelope patterns vary, so read each one. Indie designers state their own value, often 1 cm or 3/8 in.

How do you add seam allowance to a pattern without it?

1. Trace your size onto tracing paper, using a ruler for straight lines and a curve for the rest.
2. Copy every marking: grainline, notches, dots, fold lines and the piece name.
3. Add the allowance outside the traced line: 1 to 1.5 cm (3/8 to 5/8 in) on seams and 2 to 4 cm (3/4 to 1 1/2 in) at hems, or exactly what the pattern asks for. Use a seam gauge or a double tracing wheel to keep it even all the way round.
4. Add nothing on a fold line, and nothing on edges the instructions say will be bound or finished at the net line.
5. Square off the corners: extend both new allowance lines until they meet, then fold the paper along the seam line and trim, so the allowance has the right shape when the seam is sewn.
6. Walk the seams: lay matching edges together and check that their lengths agree between the notches.
7. Write the allowance on every piece. Future you will be grateful.

Our tip: when you add your own allowance, choose 1.5 cm on side seams (room to fit) and 1 cm on necklines, armholes and collars (less to trim). Mixed allowances are fine, as long as each piece is labelled.

How to change a 5/8 in seam allowance to 3/8 in, and why you might

Plenty of sewists reduce the allowance on some or all seams. Good reasons:

• Curves and enclosed seams: necklines, armholes, collars, cuffs and facings are trimmed after sewing anyway. Sewing them at 3/8 in (1 cm) saves the trim and makes the curve easier to steer.
• Overlocker construction: a four thread overlocker sews and trims in one pass. Knits drafted for an overlocker already use 6 mm to 1 cm (1/4 to 3/8 in).
• Matching another system: if you mostly sew indie patterns at 1 cm, you may want your Big 4 patterns to match.

How to do it without changing the fit:

1. Change the paper, not the stitching. Trim exactly 1/4 in (6 mm) off the cutting line of both pieces that meet at a seam, so the seam line stays where the designer put it.
2. Change both sides of every seam you alter. A sleeve at 3/8 in sewn into an armhole at 5/8 in will not fit.
3. Move the notches in with the edge.
4. Keep 5/8 in (1.5 cm) on side seams and centre back seams of fitted garments until you have made the pattern once. That extra width is your fitting insurance.

And if you only want a seam a little looser or tighter, there is a gentler route: Simplicity's own guidance is that on most fabrics you can stitch as much as 1/4 in (6 mm) closer to the cut edge of a 5/8 in allowance, which lets you fine tune fit on the go.

Grading, clipping and notching: what to do with the allowance after sewing

A 5/8 in allowance is perfect while you sew and too bulky once a seam is turned to the inside. Three small cuts fix that.

• Grading layers the allowances so they do not form a ridge. On an enclosed curve such as a collar or a faced neckline, trim one allowance to 1/4 in (6 mm) and the other to 1/8 in (3 mm). Keep the wider one against the outside of the garment.
• Clipping lets an inward (concave) curve spread. Clip straight into the allowance every 3/8 to 1/2 in (1 to 1.3 cm), stopping a thread or two short of the stitches.
• Notching removes bulk from an outward (convex) curve. Cut small V shapes so the allowance folds flat without lumps.
• Corners are trimmed diagonally across the point, about 1/8 in (3 mm) from the stitches, so they turn out sharp.

One more cut people overdo: pattern notches. When you mark notches by snipping, cut no more than 3 mm (1/8 in) into the seam allowance. A deeper snip can show on the outside once the seam is sewn, and on a reduced 3/8 in allowance it can reach the seam line itself.

The quilter's 1/4 inch seam allowance

In quilting the allowance is 1/4 in, and it is built into every cut size. That is why every quilting square is cut finished size + 1/2 in: each of the two opposite edges loses 1/4 in to a seam. A 4 in finished square is cut 4 1/2 in. A 12 in finished block measures 12 1/2 in before it is sewn into the quilt. In metric the convention is 0.75 cm, so a square is cut finished + 1.5 cm: a 10 cm finished square is cut 11.5 cm.

Why quilters never mix 1/4 in and 0.75 cm

Here is the worked example that shows why. 1/4 in is 6.35 mm, and 0.75 cm is 7.5 mm. Cut a square 4 1/2 in (114.3 mm) from an imperial pattern but sew it with a 0.75 cm seam, and it finishes at 99.3 mm instead of 4 in (101.6 mm). Ten of those across a quilt come out 23 mm short, just under 1 in. The border is now too long, and the quilt will never lie flat.

So a metric quilt is planned in metric from the start, with finished sizes in round centimetres and a 0.75 cm seam, and an imperial quilt is planned in inches with 1/4 in seams. Pick one system per project and stay in it, cut list to binding.

The quarter inch accuracy test

A quarter inch foot gets you close. This test tells you whether you are exactly right.

1. Cut three strips 1 1/2 x 3 1/2 in (4 x 9 cm in metric).
2. Sew them together along the long edges with your quarter inch seam (0.75 cm in metric).
3. Press both seams away from the centre strip.
4. Measure. The unit must be exactly 3 1/2 in (9 cm) wide, and the centre strip must show exactly 1 in (2.5 cm).

If the unit is narrower, your seam is too wide. Move your guide one or two thread widths toward the needle and sew a scant quarter inch. The scant seam makes up for the thread and the fold that a pressed seam takes up, so the unit finishes at its true size. Mark the perfect line with a stack of masking tape or a magnetic seam guide, and test again.

How much does a tiny seam error cost?

More than you would think, because every seam takes the error twice, once from each piece. Across a row of eight squares there are seven seams, so the row loses 14 times the error.

A seam 1/16 in (1.6 mm) too wide, which is hard to see by eye, makes that row 7/8 in (22 mm) short. That is the whole reason the quarter inch test exists.

Seam guides: how to sew a straight, even allowance

Your needle plate is engraved with lines for the common allowances, measured from the needle in its centre position. If the line is hard to follow, stick a strip of masking tape exactly along it, or use a magnetic seam guide. Then watch the fabric edge against the guide, never the needle, and let the feed dogs do the pulling.

Check yourself with a seam gauge: measure from the edge to the stitches in three places along a test seam. If the three numbers agree, your guide is doing its job.

Seam allowance mistakes and how to fix them