Seam Allowances Explained: Standard, French, and Flat-Felled

A pattern's finished measurements describe the garment after it's sewn together. Getting from that finished size to the size you actually cut requires adding a seam allowance to every edge that gets stitched to another piece — and how much to add depends on which seam construction you're planning to use, not just on habit.

What a seam allowance is for

When two pieces of fabric are stitched together, the seam line sits some distance in from the cut edge. That margin — the seam allowance — is what gets folded to the inside (or pressed open) once the seam is sewn. An edge that isn't seamed to anything, like one cut on a fold or bound with tape, needs no allowance at all.

Standard and overlocked seams: no surprises

A plain seam, stitched once with the raw edges left as-is or finished with a zigzag, uses exactly the allowance you cut — nothing more. In US home-sewing patterns, 5/8in (1.5cm) is the long-standing standard for wovens; quilting cottons and many knit patterns instead default to 1/4in (0.6cm). An overlocked or serged seam typically gets trimmed down to somewhere between 3/8in and 1/2in as the machine finishes the edge, but it's still a single-pass seam, so it doesn't need any allowance beyond what you specify.

French seams: sewn twice, so cut a bit more

A French seam fully encloses the raw edges inside a second row of stitching — useful on sheer or unlined fabrics where a visible raw edge or a serged finish would show through. The construction is: sew a narrow first seam with wrong sides together, trim it close, then flip the fabric right sides together and sew a second seam that encloses the first entirely. Because two passes and a trim step eat into the fabric, a French seam typically needs about one and a half times a plain seam's allowance to leave enough margin for both passes to land cleanly — a 5/8in plain allowance becomes roughly 15/16in for a French seam on the same fabric.

Flat-felled seams: only one side needs to grow

A flat-felled seam is the durable, topstitched construction you'll recognize from the outseam of a pair of jeans: one seam allowance is trimmed narrow, the other is folded over it and pressed flat, and the folded edge is topstitched down on the surface, fully enclosing the raw edges in a flat band. Because only one side of the seam does the folding-over, only that side needs the extra allowance — commonly worked out to about double the plain allowance — while the other side of the same seam stays at the standard width.

Don't forget the hem

A hem allowance is a different kind of addition — it's added to the LENGTH of a piece, not to a seamed edge, and it scales with the finish you want. A narrow, rolled hem needs as little as 1/4in. A typical garment hem runs around 1in. A deep hem — the kind used on formalwear, coats, or curtains, where extra weight helps the fabric hang properly — can run 2in or more, and has the side benefit of leaving room to let a garment down later if needed.

Seam allowances by market and pattern tradition

Everything above uses US-style numbers because that's the most common convention this site's readers start from, but "the" seam allowance isn't one universal figure — it's a convention that varies by market and by pattern publisher, and treating any single number as standard everywhere is a fast way to cut a piece wrong. Many European pattern lines, and a good number of independent patterns published outside North America, use a flat 1cm seam allowance instead of 5/8in. It's not simply a metric relabeling of the American figure — 1cm is meaningfully narrower than 5/8in (about 1.6cm) — so a seam sewn at "the standard allowance" means a genuinely different cut size depending on which tradition the pattern came from.

Japanese pattern drafting takes a third approach entirely: many Japanese sewing books and pattern magazines print pattern pieces at the finished stitching line only, with no seam allowance included at all, and explicitly expect the sewer to add their own before cutting. Tracing a Japanese pattern piece straight onto fabric and cutting exactly on the line — treating it like a US pattern that already includes 5/8in — produces a garment with zero seam allowance anywhere, which is unsewable as drafted. There's a full walkthrough of exactly how to add an allowance correctly to a zero-allowance pattern, piece by piece, in a separate guide; the short version is that cutDimension() with a seam allowance of 0 simply returns the finished size unchanged, which is precisely the trap — the tool will happily confirm "no change" if you leave the allowance at zero, so the fix has to happen by deliberately entering a real allowance, not by trusting a default.

Two worked examples, start to finish

A bodice side seam with a finished width of 18in, sewn as a French seam (5/8in base allowance, so 15/16in per side) on both the front and back pieces, needs a cut width of 18 + 0.9375 × 2 = 19.875in — not 19.25in, which is what the same piece would need with a plain 5/8in seam on both sides instead. The extra 5/8in across the two edges is exactly what the second, enclosing pass of a French seam consumes.

A jeans outseam is the classic flat-felled case, and it only adds allowance on one side of the seam, not both. With a finished outseam length of 42in and a flat-felled allowance of 1 1/4in (double the 5/8in base) on the single side that gets folded over and topstitched, the cut length for that side is 42 + 1.25 = 43.25in, while the other side of the same seam stays at the standard 5/8in allowance. Mixing up which side gets the doubled allowance is one of the more common flat-felled mistakes — only the folded-over side needs it.

Combining a seam allowance and a hem on the same piece

A skirt panel with a finished length of 40in, a standard 1in hem, and a 5/8in seam allowance at the top waist edge needs a total cut length of 40 + 1 + 0.625 = 41.625in — the hem and the seam allowance stack because they're both additions to the same dimension (length), even though one comes from a fold-under and the other from a stitched seam. Swap the hem for a deep, 2in coat-weight hem with no seam allowance needed at that edge (say it's cut on a fold instead), and the same 40in finished length instead needs 42in cut — the two additions are independent, and skipping either one produces a piece that's short exactly where that allowance was supposed to go.

Common questions about seam allowances

Do curved seams need a different allowance than straight ones? The allowance width itself doesn't need to change for a curve, but curved seams usually need to be clipped (on an outward curve) or notched (on an inward curve) within that allowance so it can spread or overlap smoothly when pressed — a plain 5/8in allowance left uncut on a tight curve will pucker no matter how accurately it was sewn.

Do knit fabrics need less allowance than wovens? Many knit patterns are drafted with a narrower default — often 1/4in or 3/8in — because knit seams are frequently sewn with a stretch stitch or a serger that doesn't need much margin to work with, and a knit garment's fit usually depends less on a precisely pressed seam allowance than a tailored woven garment's does. It's still a convention set by the specific pattern, not a universal rule for "knits" as a category.

What if I want to switch a pattern's plain seam to French or flat-felled after the pieces are already cut? If the pieces were cut with only the pattern's original plain allowance, there usually isn't enough fabric in the seam to convert to French or flat-felled without trimming into the garment itself. Decide on seam finish before cutting whenever possible; converting afterward generally means re-cutting the piece with the correct allowance added, not just resewing the existing seam differently.

Getting the cut size right the first time

Put together, the cut size of any piece is: finished size + (seam allowance × number of seamed edges) + any hem allowance, where the seam allowance itself is adjusted up for a French or flat-felled finish. Working this out before cutting — rather than adding a generic allowance and hoping — is what keeps a French seam from coming up short of fabric to enclose properly, and keeps a flat-felled seam's folded edge from being too narrow to topstitch confidently. The seam allowance calculator runs both halves of this math: the cut dimension from a seam type and edge count, and the total cut length once a hem is added.

When your pattern already tells you

All of this matters most when you're adapting a pattern, drafting your own pieces, or switching a plain seam over to a French or flat-felled finish partway through a project. If a commercial pattern already specifies its own seam allowance and construction — and most do, right on the pattern piece or in the instructions — follow that first. Use the general conventions here as a fallback, and as a way to sanity-check a number that looks unusually large or small for the seam you're planning, and remember that "standard" itself means something different depending on which pattern tradition you're holding.