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.

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.