How to Add Seam Allowance & Sew It, Made Simple

Understanding seam allowance is one of the most important beginner sewing skills. It affects the size, fit, and finish of every sewing project, from garments to quilts. This guide explains what seam allowance is, why it matters, standard seam allowance sizes, and how to add seam allowance correctly to sewing patterns. The seam allowance of a sewing pattern is simply the extra room around the stitching lines and the raw edge. This lets you know how far in from the cut raw edge you will be sewing your seam.

What is Seam Allowance?

Seam allowance is a margin between the cut raw edge and the line you will be sewing along. It helps determine the final size and shape of the item and allows for fraying edges. Using the wrong seam allowance will mean your clothing may be too big or too small, so it is important to learn how to add it correctly and sew the correct sized seam.

If you are learning garment construction techniques, explore the full guide to seam types to understand how seam allowances affect different seam finishes and sewing methods.

Maintaining the allowance allows the fabric to fray without interfering with the integrity of the item. Seams may need to be clipped or pressed open for different purposes, and the pattern designer will have factored this into their decisions.

How to Use Seam Allowance

What is a Standard Seam Allowance?

Unfortunately, there is no universally correct amount of average seam allowance. It is important to note that different designers will have different rules, so it is always important to check carefully before you start cutting. The amount will always be specified in the pattern notes at the beginning of patterns.

The actual seam allowance will also vary depending on the project or type of garment. Knit fabric patterns typically have smaller ones than woven fabric projects. In addition, a small item such as a hair bow may have a smaller seam to reduce bulk.

Common Seam Allowance Sizes

How Much Seam Allowance:

• Garment Seams – 1/2 to 5/8 inch (12-15mm)
• Quilting – 1/4″ (6mm) is most common when sewing quilt blocks.
• Leotards and Swimwear – 1/4″ (6mm)

Most designers keep the seam allowance consistent throughout the pattern, but there may be times when it changes for a particular step. This will be clearly outlined in your pattern. For example, in areas over tight curves, it may be reduced.

Different Seam Allowance Rules

Different types of seams may have different seam allowances.

• French seams typically have wider seams to allow for the extra turning step.
• Wider seams also allow you to make adjustments, particularly when sewing a muslin or test garment.
• Collars and cuffs often have narrow seams.

It is not recommended to ever make your seam allowances less than a quarter inch as it will become very difficult to sew. If your pattern requires you to add them, always ensure you have extra fabric to allow for this.

Do All Patterns Include Seam Allowance?

It is common for European designers to add NO seam allowance to their patterns, and you need to manually add it in before sewing. In the ” How to Add Seam Allowance section, I will show you how to do this.

Burda Style Magazine patterns commonly DON’T have seam allowance added. Simplicity, New Look, and Butterick generally DO have them added. My patterns all include the seam allowance, which means you don’t need to alter the pattern. You just cut around the pattern and start sewing.

As a general rule, Treasurie sewing patterns have the following included.:

• Wovens: 1/2 inch (12mm)
• Knits: 3/8 inch (1cm)
• Leotards: 1/4 inch (12mm)
• Doll’s clothing: 1/4 inch (6mm)
• Clutches/Bags: 3/8 inch (1cm)

How to Add Seam Allowance (Step by Step)

If you have a pattern from a designer that needs an allowance added here is what you do. You can add it directly onto your fabric or onto paper first. Use this method to add seams for armholes, shoulders, inseams, and side seams. There are different methods for hems and for placing on the fold, which will be discussed in the next sections.

1. Place Pattern on Paper

Place your pattern pieces on your fabric or paper with enough room between them to add the seam allowance. Decide what amount you would like to add. If you are a beginner and not sure what to add, then refer to my rules above.

2. Mark the New Seam Allowance

Using chalk or a removable pen, add the seam allowance around the pattern using your ruler. If you have a regular ruler, just make a series of dashes and join them up. It can be a bit faster if you use a quilting ruler, as you can draw longer lines at once. Make sure all pieces of material that join together have equal seam allowances.

For curved parts, make a series of small dashes and join them up. You can use a curved ruler or French curve at the end to smooth out the curve.

3. Mark the Allowances on the Fold

Just to trick you, there are a couple of exceptions. Do NOT add seam allowance on a seam that is placed on the fold of the fabric.

How to Add Seam Allowance to Hems

Where you have a hem, you may need to add an extra seam allowance. Your pattern instructions should specify how much hem allowance to add. If it doesn’t, then try adding 3/4 inch (2cm) any hems. This allows you to turn the hem twice. Once at 1/4 inch (6mm), then again at 1/2 inch(12mm).

If you need to add seam allowance to an angled piece such as a sleeve, continuing at the same angle means the hem will become too narrow when folded. You will need to angle the hem outwards, as shown in the diagram.

Seam Allowances Hacks

Marking Hack – Use this easy sewing hack. Add seam allowance by joining 2 pencils together with a rubber band. Use one to mark and the other to guide around the edge.

Distance Hack – New sewers may want to put a piece of colored tape along the seam guide they will be using. You can use painter’s tape or a piece of masking tape if you are worried about it marking your machine. Some people recommend using rubber bands to mark the distance, but in my experience, the rubber bands move around, meaning you may lose accuracy. A piece of tape is not going anywhere!

Stitching the Correct Seam Allowances

Once you have cut your fabric out, you will need to stitch it together using the correct allowance.

• If you look at the plate of your sewing machine, you will see that there are different numbers and lines marked. These are the seam guides.
• When the needle is in its center position of the presser foot, you should align the raw edges of your fabric with the appropriate seam allowance guide. Start stitching, maintaining the same distance.

Best Seam Allowance for Beginners

For beginner sewing projects, 1/2 inch (12mm) is often easiest. It provides enough room for adjustments while still being easy to sew accurately. Smaller seam allowances require more precision.

Seam Allowance FAQs

Seam Allowance – In Conclusion

Now that you are ready to sew, I have an entire article on how to sew a seam. This will show you the correct position and settings and a few tricks and tips on sewing a perfectly straight seam.

Now you have added the seam allowance, the next step is to start cutting your fabric. Here is an article showing you which direction to place the pattern and tricks like cutting on the fold – Cutting Fabric for Sewing

More Sewing for Beginners

• Seam Finishes
• How to Sew a Hem
• How to Use a Seam Ripper
• Basic Machine Stitches