15mm – the rule of thumb


15mm. I never knew this tiny measurement would become such a key part of my life. But it’s the standard seam allowance in most sewing patterns and now I can eyeball 15mm with impressive accuracy.

It’s not always clever or wise to eyeball, so sometimes I use measuring tools. Get me! If I don’t have a measuring tool, I use my thumb. From thumb tip to the base of my nail is about 15mm. (That’s how long my thumb is. You should measure first if you want to sew by your own rule of thumb.)


I’ve heard that the tip of a tape measure is the equivalent of 15mm, but not in this house. I checked. That’s the type of detail that could really impact on sizing.


I’d also be cautious around the accurate readings from retractable tape measures. They can stretch.


If you’re lucky, your sewing machine will have seam allowances marked on the throat plate. If yours doesn’t, you can always add a strip of masking tape. Gawd bless masking tape – so many functions. (It’s also great for when you’re pinching out or repairing paper patterns as it can be removed easily and doesn’t melt under an iron.)


Once upon a time, I cared about a 24-inch waist. (I mean, I never had one. I just cared about never having one.)

Now, all I care about is 15mm.

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22 Responses to 15mm – the rule of thumb

  1. Elizabeth H says:

    I love your blog and love all item you sew. You make me laugh and that is a good thing!

  2. Jen (NY) says:

    After largely abandoning a switch to metric in the mid 70s, we’re still mostly on standard measure here on the left side of the Atlantic. My non-American spouse is appalled by that, but 5/8 inch has part of my life as long as I can remember. (Both my mother and grandmother sewed my clothes). I just bought a serger (overlocker) and now I’m suddenly concerned with 3/8 of an inch. Non-sewers have no idea!

  3. sewbobbi says:

    The width of my thumb is about 5/8″ or 15mm. I use that rule of thumb ( pun intended) frequently.

  4. Kathy Lynch says:

    Ummm, have I been missing the obvious? Is that where the expression “as a general rule of thumb” comes from I wonder? Never actually thought about it before…

    Very glad to hear you aren’t worried anymore about a 24″ waist. They’re overrated I’m sure. 🙂

    • LinB says:

      No, the “rule of thumb” is that you could beat your wife and children with a stick the same diameter as your thumb, or smaller. Anything larger than that and you were committing a crime.

  5. Judith says:

    Masking tape is great, I agree, but I’ve had a couple of bad experiences using it long term to mark seam allowances on my sewing machine. Think…major residue. I now use blue painters tape, which sticks nicely, but comes off clean, days/weeks/months/years later. (Available on Amazon UK as ScotchBlue Painters Tape.)

    Love your makes. Keep ’em coming!

  6. 24″ waistlines? only on 6 year olds.
    ps: i added that tape that has the measurements on it to my throatplate, extending the line.
    the tape is sold for marking seamlines or topstitching, very useful for the [also legendary like unicorns] straight lines

  7. Jacqueline says:

    Somewhere I was told that ‘rule of thumb ‘ referrred to the width of a stick that men were allowed to chastise their wives with. I may be wrong and could have googled before posting but ……. Still loving your blog but the arthritis in my hands is making sewing very difficult. So easy patterns with little or no hand sewing is the only way for me and 5/8 inch seams. Thanks for the masking tape hint.

    • PsychicSewerKathleen says:

      You’re right Jacqueline – the rule of thumb is the width measure of the switch you’re legally allowed to BEAT your wife with” – every time I hear that expression I hear my teeth grinding!

  8. A totally unrelated note: do you know where the expression ‘rule of thumb’ comes from? Apparently it relates to a old pre victorian law that it was legal for a man to hit his wife with a stick, as long as the stick was not thicker than his thumb. I stopped using that expression since I learnt that…

  9. sarah says:

    Such a lovely little post. My biggest bug bear with indie patterns is the variation in seam allowance 1.5 comes so naturally to me after years sewing big 4

  10. Another little fact I like to throw at my maths students [who remain unimpressed] is that ‘inch’ comes from the Latin ‘uncia’ and is more likely to be close to the length of the top joint of your thumb [unless you have big thumbs!]. It’s quite useful for rough measuring small items, as it’s always …wait for it…to hand!

  11. Mary says:

    At five foot four inches tall, 27 inch waist…I was five pounds or so above the anorexia section of the height/weight chart. Just for perspective. (back when I worked a very physical job full time, and biked for miles, and swam for hours in free time.)

  12. PsychicSewerKathleen says:

    I’d always thought the width of a tape measure was 5/8″ so I guess that doesn’t line up too well in mm 🙂 I like your Stoff and Stils gauge! Where did you buy that?

  13. Kathy Zachry says:

    And what about measuring fabric from your nose to the tip of your outstretched fingertips to approximate a yard while you’re out buying fabric? It’s close enough for a guesstimate!

  14. Do you know your post made me get out a measuring tape and have a look? Mine is bang on 2cm, bah, though that I was onto a good thing there. That Stoff and Still gauge looks really useful. I have a plastic Prym one and it is the most useful sewing gadget that I own. Xx

  15. esewing says:

    My tape measure is 2mm wide too, there is a permanent tape on my machine, I use electrical tape but that might be because my husband is an electrician 😊
    Might have to get a gauge , tried measuring my thumb , but nothing measures 1.5 neatly,:(

  16. My machine does have the measurements on the throat plate, but I’m very guilty of eyeballing measurements – often leading to disasters. I totally take responsibility for those. My pet hate is where seam allowances differ within a single pattern, though. Strange how a few mm can mean the difference between something fitting. Or not.

  17. Lakaribane says:

    On the issue of the width of measuring tapes, on the package, they state the width. I purposefully bought various widths for quick seam allowance addition. I sew mostly Burdastyle patterns.

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