Roman Blind Wizard fold calculator for perfect Roman blinds

Roman Blind Wizard

Calculating Folds Below a Pelmet

Category How To 06/12/2020 (GMT)

If you are designing a cascading fold Roman blind with a pelmet over, you might want to calculate the folds of the blind so that they start cascading just below the pelmet.  And if you really want this blind and pelmet combination to look stunning, you will want the distance between the bottom of the pelmet and the bottom of the top fold to be the same as the cascade increments in the blind. 

These calculations are easier with the help of Roman Blind Wizard!

There are three key variables which you need to know before you begin to design this this blind and pelmet combination.  These are the depth of the pelmet board, the depth of the pelmet, and the desired cascade increment (see diagram).

Roman Blind Wizard - Calculating Cascades Below a Pelmet Sketch

Pelmet board

Let’s say the pelmet board is 2.5 cm deep, and that the blind headrail will be fixed directly to the underside of the pelmet board.


Let’s say the pelmet is 16 cm deep.  Of course, this can be varied according to the proportions of the window treatment.

Cascade Increment

This is a design element of the blind which is infinitely variable, but let’s say you require a cascade increment of 3 cm. 


Now the mathematics!

First, decide how many fold sections your blind should have.  Use Roman Blind Wizard to calculate the ideal number of fold sections.  Input the blind width, drop, headrail allowance and cascade increment as normal.  Roman Blind Wizard will calculate and suggest the ideal number of fold sections for your blind.  This will be displayed on the screen – don’t hit “calculate” yet, you don’t need to use up a credit for this part of the calculation!  

Let’s say you decide to have 9 fold sections.  This will result in 4 rods, and therefore you will want to see 5 equal cascade increments below the pelmet (see diagram).  

Now calculate how much of the blind will be hidden behind the pelmet.  If the pelmet is 16 cm deep, and the pelmet board is 2.5 cm deep, then it follows that 13.5 cm of this blind will be hidden behind the pelmet (16 – 2.5 cm).  

Next, calculate how much of the blind will be visible below the pelmet when fully stacked up.  This is simply the sum of all the cascade increments which are visible below the pelmet.  So if the cascade increment is 3 cm, and there are 5 visible cascades (as shown in the diagram), then the amount of stacked-up blind below the pelmet is 5 x 3 = 15 cm

So we can now calculate the required stacked-up depth of this blind as 13.5 + 15 = 28.5 cm (The sum of the portion hidden behind the pelmet, and the cascades falling below the pelmet). 

Now go back to the Roman Blind Wizard input screen.  In the “Headrail allowance” field, enter a larger value, and see what that does to the displayed “Stacked-up depth” value.  You can keep “tweaking” the headrail allowance value until you achieve a stacked-up depth of 28.5 cm.  Enter the remaining data, and now when you hit calculate, the rod positions on the resulting worksheet will give the desired result

If you would like to make your Roman Blind calculations quicker and easier then click here to register for an account with Roman Blind Wizard. New users will receive a free credit to get you started.