Once you’ve mastered the basics of piecing quilt blocks you may want to alter the size of a block scale them up or down. To enlarge or reduce simple blocks, the quilt maths is relatively easy, if you follow a few basic principles to consider:
- Seam allowances are ¼” throughout and these will not be enlarged or reduced.
- Identify the different shapes that form the quilt block. We will be using simple blocks that include squares, half-square triangles and rectangles.
- Identify the layout or structure of the quilt block. We will be looking at four-patch, a grid of four squares in a 2 x 2 arrangement, two squares across, two squares down; or nine-patch – a grid of nine squares in a 3 x 3 arrangement, three squares across, three squares down.
- Finished Vs Unfinished. Quilt block sizes are usually described as finished e.g. 6” x 6” finished. This measures the quilt block as if it was assembled as part of a quilt. The ¼” seam allowances around the outer edge of the block are not included in that measurement as they would’ve been sewn into the seams. The same block could also be described as 6 ½” x 6 ½” unfinished meaning that from raw edge to raw edge, the pieced quilt block measures 6 ½” square, including seam allowances. You can see the difference in the illustration below; the dashed line indicates the seam allowances.
- When making calculations for enlarging or reducing a quilt block, you need to use the finished measurements. The seam allowances are added on at a later stage.
- These blocks are standard, pieced quilt blocks, not foundation paper pieced.
Enlarging a Nine-Patch Block
Let’s start with a simple nine-patch block. It is made up of nine identically sized squares and we will start with a finished block size measures 6” x 6”. In the diagram you can compare the nine-patch block at 6”, 9” and 12” square.
In the 6” square block, each identical square is 2” x 2” finished and that’s the measurement we will start with.
Enlarging From 6” to 12” Block
To make the block twice as large the original, i.e. to enlarge from a 6” block to a 12” block, the finished size of the individual squares needs to be multiplied by two:
2” x 2 = 4”
Each finished square will now measure 4” along each edge.
Now the seam allowances need to be added on:
4” + ¼”+ ¼”= 4 ½”
So, to make a 12” finished nine-patch block, you will need to cut nine squares, each measuring 4 ½” x 4 ½”; five squares in one colour and four in another. When the block is measured from raw edge to raw edge, its unfinished size (which includes seam allowances) will be 12 ½” x 12 ½”.
Note: When a block is doubled in size, e.g. from a 6” block to a 12” block, the area or space the block takes up is four times the area of the original block!
In percentages, the size increase from 6” block to 12” block is 200%. To work this out, use this simple equation:
Desired enlarged Block size x 100 = resizing percentage
Original Block size
When the measurements for enlarging the 6” block to a 12” block are placed into the equation:
12 x 100 = 200% resizing increase
Enlarging From 6” to 9” Block
If doubling the block is too large an increase, let’s try enlarging from a 6” block to a 9” block. First we can work out the percentage increase as before:
9 x 100 = 150% resizing increase
For the nine-patch block, the original 2” squares in the 6” block need to increase by 150%. You can either work this out mentally or
With this equation:
Length x Percentage Increase = Final length
2” x 150 = 3”
As before, the seam allowances need to be added on:
3” + ¼”+ ¼”= 3½”
Thus, to make a 9” finished nine-patch block, you will need to cut nine squares, each measuring 3 ½” x 3 ½”, five squares in one colour and four in another.
Enlarging a Block with Half Square Triangles
Let’s take a block like a Pinwheel, which is entirely made up of half-square triangles. We are going to double the size of the block from 6” square finished to 12” square finished, an increase of 200%.
As before, we need to identify the shapes and their finished measurements plus the arrangement of the shapes within the block. In Pinwheel, there are eight, identically sized half-square triangles each with the two equal edges measuring 3” in length.
Note: 3” is a finished measurement.
The half-square triangles are joined along the diagonal to make half-square triangle units, which are then arranged in the block with two squares across and two down. I use this method to make half-square triangles, where they start as squares and are trimmed down. The formula to calculate the size of the square to make half-square triangle units is:
Side of finished square + 1”
In the 6” block, the finished square is 3”. To enlarge for a 12” block, this will double:
3” x 2 = 6”
Now an extra 1” is added on which includes the seam allowances plus a little bit extra to make a 7” square from which the half-square triangles are constructed. This creates a generously sized half-square triangle unit that can be trimmed down to make a final finished size 6” (unfinished size 6 ½”). Therefore, to make a 12” pinwheel block, four 7” squares are needed – two in a light colour, two in a darker colour.
Enlarging a Churn Dash Block
This block combines squares, half-square triangles and rectangles in a nine-patch arrangement; three squares across, three squares down. Use the finished dimensions of the original block to calculate the size you wish to enlarge to using the methods from the nine-patch and pinwheel blocks as shown above.
Note: If in doubt, use squared paper and a pencil and draw your blocks out. It’s something I still use frequently to check measurements on designs; especially finished V’s unfinished measurements. I use a square to represent an inch.
If you’d like to try enlarging or reducing blocks to a greater range of sizes or use more complex shapes and block grids, these links may help: