How Many Stitches Per Inch for Leather

When I look at the guides for stitches per inch, there are several ‘rules of thumb’ out there, and upon further digging, it seems like most of them are made up or there are many exceptions to the rules (…psst, which means they aren’t really rules of thumb in the first place). Today, I want to dig into the question, ‘What is the best stitch length for leather?” and help you decide the best lengths for your project. We’ll touch on strength, looks, and other factors that go into making a sturdy and elegant stitch.

What is SPI?

‘Stitches per inch’ is the number of stitches per inch on a seam. This differs from teeth per inch, which is the number of teeth per inch on a pricking iron. They sound similar, but they’re a little different. In the same span, you’ll have one less stitch than teeth because you need two teeth to make one stitch.

What is the Best Stitch Length for Leather?

I also want to clarify something I don’t see many other leatherworkers discussing: stitch length or millimeters per stitch. Many pricking irons and sewing machines measure their stitches in the individual lengths of each stitch and further in millimeters, not inches. When you say ‘3mm SPI’, you just sound ridiculous; as a leatherworking community, we should get with the program. Modern sewing machines follow suit and measure their stitch length in millimeters, while older ones can be found to measure in inches. SPI is Stitches Per Inch and stitch length is usually, though not always, in mm.

An Old Stitch Per Inch Formula For Seam Strength

A while back, a thread manufacturing company, A&E, developed a formula for the relation between seam strength and stitches per inch. It is as follows:

Seam Strength = Stitch Per Inch * Thread Strength (lbs.) * 1.5 (sewing threads’ average loop strength ratio).

This formula is meant for woven fabrics for a lockstitch sewing machine. It’s a good starting point; however, we can’t use it as-is for leather because:

1. Leather’s strength is different than woven fabric. Leather is strong per square inch, but its strength decreases rapidly as you put more holes in it. Unlike fabric, where you can sometimes push the sewing thread between the fabric weave, leather weakens more with each hole.
2. Not all leather seams use a lock stitch. Many readers and students will be using a saddle stitch. A saddle stitch will have a different strength ratio.

So, is the formula entirely useless? No, because it gives us a base to determine seam strength. Let’s do two common thread and SPI combinations, and I’ll add some details about them. I have the data for MBT’s polyester thread strength but not linen, so we’ll figure out linen vs polyester strength in a future post. I suggested linen equivalents below based on their relative diameters.

Seven Stitches Per Inch / 3.8 Millimeters Stitch Length
#5 MBT polyester (or 432 linen). Thread strength: 102 newtons / 23 lbs. Diameter: 0.6mm, 168 TEX

Nine Stitches Per Inch / Three Millimeters Stitch Length
#8 MBT polyester  (or 632 linen). Thread strength: 63 newtons / 14 lbs. Diameter: 0.45mm, 100 TEX

Let’s plug these values into our seam strength formula:
7 (SPI) x 23 (lbs) x 1.5 = 241 lbs
9 (SPI) x 14(lbs) x 1.5 = 189 lbs

Wow, that’s a lot.

I don’t make heavy-duty items like saddlery or bridles, so either of these is plenty strong for my work in handbags, wallets, and watches. Remember, this is for one seam. If I were to make something that gets a lot of stress, like a bag strap or a belt, it would have two rows of stitches closing on the buckles. That may not double the seam strength, but that should account for some very full bags or fat, ahem… large, waists. 

Additional factors to consider:

The strength ratio for a saddle stitch will be different than a lockstitch’s. The latter is 1.5; can we assume the saddle stitch will be higher?

Leather is not always stronger than woven fabric (see Kevlar). I think the failure here will be at the seam, not the material being held together, so let’s ignore material strength except for the next point.

Assume less strength at higher SPIs for leather. As mentioned earlier, I think more holes will weaken leather more than fabric. You can pass a thread between the fabric’s weave, whereas leather will simply get more holes.  Take the following common high SPI and thread pairing:

12 Stitches Per Inch / 2.1 Millimeters Stitch Length
#20 MBT polyester (or 832 linen). Thread strength: 48 newtons / 10.8 lbs. Diameter: 0.3mm, 77.5 TEX

Seam Strength: 12 x 10.8 x 1.5 = 198 lbs

While I want to believe that a 12 SPI seam will be stronger than a nine SPI in leather, I just can’t bring myself to believe it. There are so many holes in that small of a seam. The numbers don’t lie, though, so I’m open to being wrong.

Test the Seam Strength Formula

The above calculations are well and nice, but does this really play out? I wanted to know, so I set up an experiment.

I chose bridle because thinner leather would certainly fail before the thread would. I also didn’t want the thread to bite so hard into the leather that it tears through. I chose a 3/4” wide strap because it was handy. A wider strap would be more robust; however, I think the seam will fail before the strap because of the thickness of the leather. Here are the individual components of the setup:

To apply the force with consistency, I used a 3-ton jack that I use for my car. This should provide as much force as I need. I used our large package scale to measure the force, which tops out at 440 lbs. This scale and jack setup won’t be super precise to measure the force, but I think we’ll get a pretty close measurement of how much weight this seam can hold.

This is the completed setup. I wrapped the strap around the jack. As it rises, it puts pressure on the scale oriented upside down, and you can see the digital readout of the lbs measuring the approximate pressure. I put a wood board between the jack to prevent the scale from snapping in half. So, let’s take a look at the results.

Amazing!

The leather broke before the stitches did. If you pause the video, you can see that the scale was up to 200 lbs before the strap broke. There are only nine stitches on this strap, and they still look pristine. They fared much better than the leather. I want to try this again with some additional variations, but my initial conclusion is that the leather will fail long before the thread breaks.

Some additional thoughts:

In a future iteration, I want to widen the strap to 1 or 1.5 inches to see if it holds up better.

I noticed the leather elongating. At higher pressure, the weight readout would lower until I raised the jack again. This was the leather stretching under the increased force. Thread also elongates under stress. See the next point.

Stress over time. This experiment does not consider stress over time, like tugging on a handle over months or years. I suspect how the stitch is sewn will factor higher than the thread strength or SPI. More about this later.

Material degradation. Both the leather and the thread will deteriorate over time. How long depends on exposure to light and moisture. I’ve heard from leather repair people that sometimes synthetic thread lasts longer than the leather, causing it to cut through the worn-out leather. They said organic threads like linen and cotton will age more closely with the leather. I haven’t owned pieces this long to notice except for really cheap leather goods. They fail before the thread, which is probably more about the cheap leather.

So where does that leave us? There are three other considerations: tradition, aesthetics, and effort.

Traditional Leatherworking Stitches Per Inch

I was taught to do nine SPI on small accessories like wallets and pouches and seven spi on medium to large items like bags and cases. You can increase the stitches per inch to twelve for very small items like watch straps and bracelets. Over the years, I’ve talked to other leatherworkers using different traditional methods in many countries, and they responded similarly. Some traditions went a little higher or lower for each item size, but these were the majority.

For a long time, the primary mode of transportation in the world was horses. Leatherworking went hand-in-hand with this tradition, and if there were a category of leather goods where the strength of your SPI mattered, it would be in saddlery and bridlework. I corresponded with a few saddlemakers, Cary Schwarz, Pedro Pedrini, and Chuck Stormes. All three are well-regarded saddlemakers and have seen many saddles through their work, adjudicating contests, and viewing collections worldwide.

They all indicated that 6 SPI is most commonly used today. Cary observed, “I’ve known really good saddlers who sew their cantle bindings at 4 spi and everything else 6.”

He and Chuck use 8 SPI for their finer work throughout the saddle. Pedro, trained in traditional French and English saddlery, said, “All the fancier guys I was around usually stitch English style saddles at 9 spi for the ordinary work, like the skirts, pads, etc. And they stitch at 10 spi for the decorative stuff. Of course, all this stuff is based on Vergez Blanchard [French] pricking irons.”

This latter part led me down an exciting rabbit hole about the Paris Inch, which, according to Wikipedia, was used for French pricking irons. It is equal to 1.065 inches, slightly longer than the empirical inch. Looks like the French and English fought about literally every inch .

Chuck indicated that some of the best saddles he has seen from the 19^th century used 10 or even 12 SPI. So, if we’re looking at choosing SPI based on tradition, it seems that there is wide latitude even for items that come under heavy stress.

One group of leatherworkers I didn’t poll was shoemakers. They also have a long tradition, and I’d love to learn more about their traditional SPIs and rationales. I suspect we’d find similar examples.

Update: A shoemaking enthusiast commented below that shoes similarly have finer stitches on higher quality shoes. 10-21 SPI and higher on rare pieces.

Aesthetics

The above sections about tradition and calculated strength tell us we can pick a wide range of SPIs. Excluding other considerations like leather strength and seam length, we can safely choose whichever SPI suits our project. History indicates higher SPIs were used on finer goods, and today’s luxury brands follow this tradition.

This means your customer expectations also factor into aesthetic choices. If they buy higher-end leather items and your project doesn’t match that aesthetic, they’ll feel something is off. They might not be aware specifically of the SPI on their purses, but this detail will contribute to their expectation mismatch.

In the earlier section on thread strength, I described how it increases with thread diameter. You can pick a larger thread to strengthen the seams up to a point. As the thread diameter increases, it takes up more space between the stitch holes until you end up with essentially a rope instead of neat rows of hashes.

If you look at the above samples of SPI and thread combinations, you can see how the thinner thread does not look right on the wider stitch lengths. The thicker thread bunches up on the shorter stitch lengths and doesn’t look right either. Over the years, I’ve tried many different combinations and kept coming back to the same ones. For me, they are a good balance of elegance and effort. Here are my most common combinations:

Time

There’s one final consideration: time. Sewing at 9 SPI is 30% more stitching than at 7 SPI. If you’re doing it for fun or friends and family, knock yourself out. If you’re hand-sewing and selling your items, think long and hard about increasing your stitch count.

The speed of modern sewing machines allows one to sew a 7SPI and a 9SPI seam in virtually the same amount of time. Yes, it’s more stitches, but we are talking seconds unless you have multiple people or are sewing many items.

Many of the older, higher-end saddles Chuck described earlier also had ornate carvings. They weren’t utilitarian, so the extra effort that went into the ornamentation paired with the extra effort of more stitches per inch. Similarly, if you are making more refined work, if you can justify the extra effort, the higher stitch count can further enrich the overall look and feel. 

Final Thoughts

SPI is only one consideration for how well your stitches will hold up. Thread strength, leather strength, and total stitch length, to name a few. How you stitch your seam, i.e., whether you are tensioning and backstitching correctly, etc., can improve your seam strength. Test components when building something where seam strength matters, like bag handles or leashes.

As I mentioned earlier, leather and thread elongate under stress. Both can lead to eventual seam failure. After stretching that strap in the video, I noticed that the seam wiggled more. This could be either the thread lengthening under pressure or that I didn’t put enough tension on the stitch. In the latter case, you can work a seam loose over time if you don’t sew with the correct tension.  The gaps get wider and wider, and eventually, the whole seam fails.

Use common sense if you plan on testing the limits of your seams. For example, I wouldn’t use two thin chevre pieces for a strap and try to secure it to a bag with one measly 1-inch seam at 12 SPI. I might, however, add a webbing filler in that strap and put two 12 SPI parallel two-inch seams where it attaches to the bag.

I think we can conclude that picking the right SPI comes down to tradition, aesthetics, and time. You are free to pick a wide range of SPIs and not worry about seam strength, given suitable leather and proper sewing technique.