Seam Allowance Converter
Quick imperial ↔ metric seam-allowance conversions.
Type the measure you have.
0.5 in = 1.27 cm
- Millimetres
- 12.7 mm
- Nearest standard
- 1/2 in
- That standard is
- 13 mm
Patterns written in metric often round a hair differently, so a 15 mm allowance is really 5/8 in even though 5/8 in is closer to 16 mm. Use the nearest-standard row to match a paper pattern to your machine’s taped throat plate without chasing a fraction of a millimetre.
About This Tool
The Seam Allowance Converter restates a seam allowance between inches and centimetres in a single keystroke, so a metric paper pattern and an imperial machine throat plate finally speak the same language. Type the measure you have, say whether it was written in inches or centimetres, and the tool shows the other unit, the millimetre value, and the nearest common sewing standard.
It snaps odd figures to familiar named allowances.
One field flips cleanly between the two systems.
Millimetres print for machines marked only in metric.
The nearest-standard row bridges rounded pattern notes.
No more mental arithmetic over a 5/8 inch line.
It keeps patchwork seams honest across unit changes.
Beginners learn which fractions map to which numbers.
Why Seam Allowance Converter Math Matters
Patterns from different countries quietly assume very different default seam allowances.
A metric paper draft and an inch-marked machine rarely ever line up exactly.
Guessing at a stray fraction of an inch shifts every single seam line.
Patchwork depends on a true quarter inch or the finished blocks will not match.
Reading a millimetre plate as inches sews a whole garment far too narrow.
Named standards let you match the drafter's real intent, not just raw decimals.
Consistent conversion keeps every seam even and honest across a long project.
Understanding the small gap between rounded numbers prevents slow, silent measuring drift.
A misread allowance compounds seam by seam until a piece no longer fits.
Quick conversion frees you to focus on sewing rather than mental arithmetic.
The Formula
One inch equals two point five four centimetres, so an inch figure multiplies by that for centimetres and by twenty five point four for millimetres, while a centimetre figure divides by two point five four to recover inches; the tool then compares the inch value against the named standards and reports whichever is closest, showing a metric allowance as its true imperial intent.
- inch —
- the allowance expressed on an imperial scale
- cm —
- the same allowance on the metric centimetre scale
- standard —
- the nearest common named seam allowance
Because patterns round a millimetre or two for convenience, the exact decimal and the named standard can disagree slightly, and it is the standard that usually reflects what the drafter truly meant you to sew.
Worked Example: Step by Step
- 1A pattern instructs a one and a half centimetre seam.
- 2Enter one point five in the value field, in centimetres.
- 3The tool converts that one point five centimetres to inches.
- 4It prints about zero point five nine inches on screen.
- 5The millimetre line confirms the same fifteen millimetres.
- 6The nearest standard reads five eighths of an inch.
- 7Five eighths is really sixteen millimetres, a hair wider.
- 8You now know the metric draft rounds a touch under it.
- 9Sew fifteen or sixteen and the seam still matches intent.
- 10Flip the unit back to inches to check your own machine plate.
How to Use This Calculator
- Type the seam allowance exactly as your pattern writes it.
- Choose inches or centimetres to match that written unit.
- Read the opposite unit printed in the large result line.
- Note the millimetre value for a metric-only throat plate.
- Use the nearest-standard row to interpret a rounded number.
- Set your guide or tape the plate to the converted figure.
- Re-check long seams so unit drift never creeps in.
Key Terms
- Seam allowance
- the strip of fabric between the stitched seam line and the raw edge
- Throat plate
- the metal plate around the needle often marked with guide lines
- Quarter inch
- the standard patchwork allowance, roughly six millimetres wide
- Five eighths
- the common garment allowance on US patterns, near sixteen millimetres
- Grade
- a set of fixed measurement steps between standard sizes
- Press cloth
- not a length but the cloth used when pressing a sewn seam open
- Notching
- trimming allowance curves in small wedges so a seam lies flat
- Conversion factor
- the fixed number, two point five four, linking one inch to centimetres
Seam Allowance Converter Reference Table
| Imperial | Metric intent | Used for |
|---|---|---|
| 1/4 in | 6 mm | patchwork |
| 1/2 in | 13 mm | craft seams |
| 5/8 in | 16 mm | apparel |
| 3/4 in | 19 mm | heavy curtains |
Machine-sewn garments most often call a five eighths inch seam while patchwork squares trust a quarter inch, and metric drafts round these to tidy whole millimetres, so treat the named standard as the goal and the raw decimal as a close guide rather than a law.
Common Mistakes & Pro Tips
- Reading a fifteen millimetre note as a plain half inch and losing a seam.
- Confusing centimetres with millimetres and sewing a wildly wrong allowance.
- Chasing an exact decimal instead of the standard the drafter intended.
- Assuming every country's pattern shares the same default seam width.
- Setting the guide once and ignoring it drift across a long seam.
- Mixing an imperial paper pattern with a metric-only marked machine.
- Trimming a curve allowance too narrow so the seam later splits open.
- Forgetting patchwork needs a scant quarter inch, not a full six millimetres.
- Rounding up hem allowances on a garment already running short.
Real-World Use Cases
- Converting a European dress pattern to an inch sewing machine.
- Matching a metric quilt block to a quarter inch patchwork foot.
- Checking a garment seam before committing an expensive cut.
- Teaching a class how inch and metric allowances correspond.
- Interpreting a rounded millimetre note on a commercial pattern.
- Taping a throat plate guide at the converted width.
- Confirming two patterns really want the same seam line.
Seam Allowance Converter: Frequently Asked Questions
Is five eighths inch the same as fifteen millimetres?
Almost, but not exactly. A true five eighths inch is about sixteen millimetres, so a pattern writing fifteen rounds just shy of it. Sew either and the garment still matches on most fabric.
Why does a patchwork quarter inch seem too small?
Patchwork often calls a scant quarter inch, a touch under the full six millimetre figure, so blocks finish square once several seams stack up. The millimetre row helps you catch a wide seam early.
Which unit should I enter?
Enter the number exactly as your pattern prints it and select that same unit. The tool handles conversion, so you never have to recall the two point five four factor or risk flipping it around.
What is the nearest-standard row for?
It snaps an odd decimal to the named allowance you actually want, such as a quarter or five eighths inch. That bridges a rounded metric draft and the imperial line your machine guides you to sew.
Does this convert hem and seam widths alike?
Yes, it converts any length you type, so use it for hems, allowances, or a pattern's grading step. The named-standard hints are tuned for seams, though, so ignore them when converting a plain hem.
This page explains how the seam allowance converter works, what each input means and where the numbers come from, so you can plan your next make with confidence.
Written by Bobbin & Form Editorial Team
Home sewists & machine-embroidery practitioners
Reviewed by Marie Delacroix
Custom sewist & machine-embroidery technician, 15 years at the machine
Formulas follow standard sewing and needlework references. Results are estimates — always confirm against your pattern and a pre-washed swatch.
Sources & further reading
- Threads Magazine — fabric cutting & yardage guides
- ISO 13934 — textile tensile & fabric property standards
- Your local fabric store's bolt-width charts