craft

I have problems figuring out finish block sizes when designing a quilt. For example what would the true size of a 5 inch block when sewn in a quilt? Same thing when making a rail fence square blocks. What is the true size of the strips when sewn together into a block. I get so confused.

cjsews

You will have 1/4 inch seam on all sides. So take 1/2 inch off you block. So if you have a 5 inch square it will finish at 4 1/2 inch. When you sew strip sets and cut into blocks, cut the block same width as the height of your sets.

Barb in Louisiana

A pattern for a block will tell you the finished size and then give you instructions to make one and square it up to 1/2 inch more than the size it said you would make. This allows for the 1/4 inch seam on all sides to sew it into your quilt. This is called the unfinished block, because it has not been sewn into the quilt along with other blocks yet.

Once you have sewn all the blocks together, you lose that extra 1/4 inch on all sewn sides. The outside blocks will still have the extra 1/4 inch on any side that hasn't been sewn to another block. If 3 sides have been sewn, then only the remaining side will have the extra 1/4 inch. So, if you were making a 9 inch finished block, it would now measure 9" on the three of the sides, and the other side will measure 9 1/4". If you are looking at a corner block, then two of the sides would measure 9" and the other two will measure 9 1/4".

With something like a rail fence, you just want to make sure you start with squares that are all the same size. You will lose 1/2" when all the blocks are sewn together. The rail fence pattern is a very forgiving pattern and loved by quilters all around the world. So a square that starts out as 6.5", when sewn on all 4 sides, will end up 6 inches.

MadQuilter

A rail fence block is the easiest to figure out assuming you sew a consistent seam. Once all the strips are sewn together and pressed, measure across the strip set. Then cut your blocks to that same measurement so they will be square.

Most of us use a 1/4" seam so each unfinished block is 1/2" bigger than the finished block. In order to make sure, we often square up the block so we have a consistent size.

The 1/4" seam is most important when you have blocks with points such as flying geese blocks. You don't want the points to be chopped off. For most blocks it is important to be consistent.

bearisgray

Generally speaking, most quilters now use a 1/4 inch seam when piecing a block.

I drew up a small example that may (or may not) help understand what happens when pieces are sewn together.

The example I am using is for a 2 x 2 inch "finished size" 4-patch block/unit.

Each of the four pieces is cut 1-1/2 x 1-1/2 inches. (total area "showing" is 9 square inches)

After they are sewn together, the UNFINISHED SIZE unit is measuring 2-1/2 x 2-1/2 inches (total area "showing" is 6-1/4 square inches - 2-1/4 square inches "disappeared" into the seam allowances that are on the back of the block/unit)

After the "UNFINISHED SIZE" unit is sewn into the item, the FINISHED SIZE is what you see on top of the item - and all the seam allowances are on the back of the item.

The unit in this example will (theoretically, anyway) measure 2 x 2 inches.
(Total area "showing" is now only 4 square inches - five inches of the original area is now on the back in the seam allowances.)

Boston1954

Since I have yet to achieve the 1/4 inch seam, these words are really from an advanced learner. I stopped worrying about how big the blocks should be unless I am having a swap with someone. Since I am making all of the blocks and they are "off" a bit they will probably be off by the same amount. If I need the quilt to be larger, I just add another row.

I realize that this does not answer your question, however I hope it helps you to not worry about having things be perfect.