I recommend you use SketchAndCalc. It calculates the area and perimeter of any irregular shape you draw. And it's free!

Here is a link that might be useful: SketchAndCalc

MiguelD, though I have a guess you'd need to explain what you mean by "crazy paving"... broken slabs of what... tile? concrete? Do you mean "area" of each broken slab?... or of the patio itself. It's hard to understand what you're trying to do and where your calculation problems lie.

By crazy paving I mean broken concrete paving slabs which were originally 2ft by 3ft but now in various pieces, and these are being sold to me by the square yard, I want to know how many square yards to order.
It's just that the shape I want to construct is not a regular one and I was hoping for an easy method for coming up with a figure.

Besides the irregularity of the shape, there is the wild card of exactly how the pieces are going to fit together. Or not, as the case may be.

There may be ideas of how much extra to order. Or maybe order some, with the idea of ordering more when that runs out.

Okay then, tell us about the shape that you want to construct. Do you have a scale drawing? And tell details about the pattern or how you intend to put it together and we'll see if we can help. Mad gallica has a good point; you'll probably have some waste because things won't fit together as planned. Or do you have a plan that accounts for broken pieces not fitting or being fit in a way that doesn't matter. Are these broken 'manufactured' pieces, or just broken concrete from demolition of slabs?

The Sketch and Calc is as easy as it comes.
You can also break the area up into approximate squares and rectangles and do a basic geometry calculation of : L X W = square area
length x width = square area
1 yd = 3 ft
3' x 3' = 9 sq.ft
There is 9 square feet in a square yard.

If you are using irregular random pieces of broken concrete (also known as urbanite ) add in about 15 % to 20 % extra to figure in for fitting purposes if you are going for modestly tight joints.
If you are shooting for wide space between the units then you can add in an extra 10 % of material.
If you are saw cutting to expose the exposed aggregate add in at least 50 % - there is a lot of waste after you trim the units.

Below is a photo of an urbanite ( broken pc's of concrete ) path.
{{gwi:42853}}From Pina Colada

I recommend purchasing a license to SketchAndCalc Pro ($4.99) to utilize the tracing feature.

Pro allows the user to navigate a Google Map to locate an area of land. Providing your soon to be patio is visible from the air, you can trace over the area and calculate the total sq area and perimeter in the measurement system of your choice.

We do lots of landscaping and SketchAndCalc has saved us hours. For site visits we use SketchAndCalc for iPad. Snap a photo and trace it.

If maps aren't possible, you can upload an image to trace. It just takes the pain out of complex area calculations.

Here is a link that might be useful: YouTube video - SketchAndCalc