Serious bookbinders know how damaging spine swell can be. Controlling it starts with the right material choices. Thread is the easiest place to start, but not the simplest to figure out. Why? Because thread and paper both depend on one another to manage spine swell. Choosing the right pair can feel a little bit like working on a puzzle while playing Russian roulette. It’s time to squash the anxiety, because I’m about to teach you an easy way to estimate thread thickness for bookbinding…EVERY time.

Before we jump in, I want to give credit for this technique to fellow bookbinder (and my teacher at the AAB), Peter Geraty. I’m not sure where he picked up this trick, but it has saved me tons of time and frustration. I am so thankful he shared it with us. I hope it helps you, too! I’ve included a link to Peter’s website below if you’d like to check out his work.

Why is thread thickness in bookbinding important?

In sewn bindings, thread is woven through each signature via pierced holes along each spine. These holes, called “sewing stations,” make it possible to secure each signature to the one before it. When thread lays inside the spine of the folded signature, it will naturally increase in thickness. This increase is multiplied by the total number of signatures sewn, which can result in a bloated spine compared to the foredge. We refer to this bloat as “spine swell” and it’s something we try to manage carefully throughout the process.

How much spine swell is acceptable?

We’re adding thread to a book. There’s going to be some spine swell, but how much is too much? According to many experienced bookbinders, the right thread thickness will yield a spine that is about 30% thicker than the foredge of the book. Today’s trick will help you estimate thread based on this desired result.

Bookbinding Tutorial: Estimate thread thickness for a sewn book

Cost Level: No cost

Skill Level: Intermediate

Total Time: 10 minutes or less

MATERIALS NEEDED TO ESTIMATE BOOKBINDING THREAD THICKNESS:

  • The completed (unsewn) text block of the book you’ll be binding
  • One or more spools of bookbinding thread in different thicknesses
  • Metric ruler
  • Ball point pen (Bic or similar)
  • Piece of scrap paper
  • Calculator (optional)

STEPS:

#1.) Measure the thickness of the text block foredge

Set the text block on a flat surface. Compress the foredge with one hand. With a free hand, hold the ruler vertically against the foredge at a 90 degree angle. Capture the thickness (in mm) and write it down on a scrap of paper. Round to the nearest half millimeter.

quick easy way to choose thread thickness to control spine swell in bookbinding

#2.) Count the number of signatures in the unsewn text block

The text block should be unsewn with signatures gathered together in the correct order. Endsheets should also be included, if any. Once you have everything stacked together, count the number of signatures and write it on the piece of scrap paper.

#3.) Find the acceptable amount of swell

Using a calculator or your beautiful brain, multiply the foredge thickness value by 0.3 (30%) and write the number down. We’re going to focus on this number, because it tells us how much swell is acceptable for this book.

Example:

Let’s say your text block foredge width is equal to 10mm.

Multiply: 10 x 0.3 = 3mm.

  • 3mm = the amount of acceptable swell.
  • 10mm + 3mm = 13mm.
  • 13mm = how thick the spine should swell to once sewn.

Since 3mm is the acceptable amount of swell, we’re going to focus on finding a thread that will help us fall within this range.

#4.) Choose a thread thickness to test

If you’re reading this post, you probably have one or two thread thicknesses in mind for the job. If so, make sure these spools of thread are nearby. If you don’t have an idea where to start, just grab a few different spools and we’ll test each one.

Be sure to read my post, How to Choose the Right Bookbinding Thread, to better understand thread properties and why it’s important to use a quality thread for bookbinding.

test different thread thicknesses to know which one will cause the least spine swell using this trick

#5.) Test the thickness of the thread

Wrap the thread around the barrel of a Bic pen (or similar) equal to the number of signatures in your text block.

Example:

If you have 12 signatures in your text block, wrap the barrel of the pen 12 times with the thread.

wrap the barrel of a pen or similar with the thread equal to the number of signatures in the text block

#6.) Compress the thread on the pen and measure the width

When the barrel of the pen is fully wrapped, slide the threads together so there is no space between them. Hold the threads in place with one hand and use a ruler to measure the thickness with your other hand. Write the number down.

quick easy way to choose thread thickness to control spine swell in bookbinding
Measure the thickness of the wrapped thread when compressed together & compare the value to the number you found in Step #3

NOW ASK YOURSELF:

Is this number (from Step #6) larger, smaller or equal to the number you found in Step #3 (acceptable amount of swell)?

  1. If the measurement exceeds the acceptable amount of swell by a lot, you’ll want to go DOWN in thread thickness and run the test again.
  2. If the measurement is equal to (or just slightly over/under) the acceptable amount of swell, you’ve found the right thread thickness to use. YAY!
  3. If the measurement is less than the acceptable amount of swell by a lot, you’ll want to go UP in thread thickness and run the test again.

Video Tutorial: An Easy Way to Estimate Thread Thickness for Bookbinding

Closing thoughts on thread thickness & spine swell in bookbinding

While this trick is very helpful, it’s not an exact science. I always measure the spine of my text block after sewing is complete to understand whether or not the thread gave me an accurate result. Sometimes, the swell is higher than I want (or expect) it to be. This could be due to several reasons, all of which I discuss in my post about spine swell. Have a read to see if you’ve accidentally overlooked one of the other variables that cause spine swell. If so, you’ll know what to account for next time. Have fun!

More bookbinding goodness

✨ What tools do I need to get started bookbinding?

🌟 Looking for a simple way to start making books? 

Try a Complete Book Materials Kit. Each one has everything you need (+ tutorials & videos) to make a beautiful book without all the fuss.

Thank you for taking me along on your book making journey!

Misty