By defining a normal vertical pattern repeat and a half drop pattern repeat below it should become obvious what a half drop pattern repeat is and why half drop pattern repeat fabrics require different cutting and fabric calculations.
NORMAL VERTICAL PATTERN REPEAT
This is the most common type of patterned fabric. The pattern is the same on both sides of the fabric roll, and aligns perfectly when side matched. See in the diagram how the fabric matches if you cut the fabric and align the cut panels.
HALF DROP PATTERN REPEAT
A half drop fabric usually has a large pattern. The pattern doesn’t match exactly on each side of the fabric roll, and matches half a drop down instead. See in the diagram how the fabric doesn’t match if you cut the fabric and align the cut panels.
Quick Video Explaination
SO WHAT DO WE DO WITH A HALF DROP PATTERN REPEAT?
A half drop pattern repeat will affect the calculation of the fabric quantity required and the order in which fabric panels are cut and joined to make a curtain or blind.
CALCULATING FABRIC QUANTITY
In our experience an example of how fabric suppliers would describe a half drop pattern repeat fabric would generally be "Pattern Repeat: 64cm Half Drop".
This usually means the whole pattern repeat is 64cm and the fabric is a half drop pattern. If you have any doubt what the whole pattern repeat on the fabric is, you need to either measure it on the fabric or clarify it with the supplier if you don't have the fabric yet.
For your fabric calculations you essentially calculate the fabric quantity as per the tutorial calculations making sure you are using whole pattern repeats. It is crucial you add the one whole pattern repeat to position the pattern on the blind or curtain that is contained in the tutorial calculation. You will now be using half of this whole pattern repeat to position* the pattern and the other half to compensate for the half drop pattern repeat when cutting the fabric drops.
*We recommend only using half a pattern repeat for positioning to save fabric costs. Half drop pattern repeats are generally large and the have a staggered repeat half way down the pattern repeat already, so half a repeat usually works fine for positioning the pattern.
MEASURING AND CUTTING FABRIC
The length of the fabric cut drops (adjusted cut drops) for your curtain or blind will be the same as for a normal patterned fabric (using whole pattern repeats) except for your LAST ODD DROP. The order you cut the drops and how you cut them will also need to be worked out first to compensate for the half drop pattern repeat.
Number each drop that will be required eg: drop1, drop2, drop3 etc
Find the position on the roll to start measuring your cut drop from so the pattern falls on the blind or curtain where you want it. (only use up to half a whole pattern repeat).
Measure and cut all your odd numbered drops and label them. (1,3,5 etc). Measure and cut the LAST ODD drop HALF a whole pattern repeat longer. This will put you at the correct position on the fabric to start measuring and cutting the even drops.
Measure and cut all your even numbered drops and label them.(2,4,6..etc).
You have basically cut half the drops first then shifted along the fabric an extra "half a whole pattern repeat” before then measuring and cutting the remaining drops.
JOINING THE DROPS
After cutting your required number of drops using the principle above, you should find when you come to join your numbered drops odd and even drops align. After joining the drops trim the extra half pattern repeat off the end of the last odd drop.
SEE MORE EXAMPLES OF HOW TO CUT AND COMBINE THE DROPS IN THE RESPONSES TO QUESTIONS BELOW
Extra Help & Comments
As I said i don't think you have a half drop pattern repeat from what you have said so probably wont be cutting odd and even drops.
Heading tape is for the width of the curtain not the length, so the amount of tape will be very different to the length of fabric which is depedent on the number of drops and the length of the curtain.
If you were making curtains with plain fabric the lining and fabric quantities would be similar. But with a patterned fabric you have to cut each drop in terms of pattern repeats so they join correctly. That means very often you have to cut longer lengths of patterned fabric than plain fabric. Hence you may need more patterned fabric than lining.
There are detailed instructions on the website in the tutorials that explain how to calculate lining quantities, plain fabric quantities and patterned fabric quantities. In STEP 3 of the tutorials there is a video that expalins why you need and longer adjusted cut drop for patterned fabric.
We would follow the process in our tutorials it is all laid out and tried and tested. You measure the window first, calculate your quanities, buy all the materials then make.