Copied this from a BikeRadar article : How to measure your own sit bones Of course the measure you really want is between the centres of your ischial tuberosities – the pointy lower parts of your pelvic bone on either side.
Many bike dealers have a pad that you can sit on to measure this distance, but you can do it at home too.
Take a piece of aluminium kitchen foil and place it on a carpeted stair.
Sit on the foil, lean forward a bit to approximate your riding position, then lift your feet.
This should leave a good impression of your rear in the foil, and you can measure between the two points of deepest impression to get your sit bone width.
‘Narrow’ sit bone width would be 100mm or less, medium 100-130mm, wide over 130mm.
A saddle’s width is measured from edge to edge across the top, and Specialized recommends a 130mm saddle width for narrow, 143mm for medium and 155mm for wide.
These ﬁgures should translate approximately across other ranges, with all other factors taken into account.
bicycles.stackexchange.com/users/5624/Luigi Villafranca
None of the above techniques worked for me.
I made no impression on the corrugated cardboard and the foil just showed a nice big bum-print after sitting a few different surfaces.
I came up with my own technique that's a bit more trial and error but seemed to work for me.
I got two small erasers, put them on a chair, sat on them and moved them around until they felt like I was sitting on my sit bones.
Then I got up and measured how far apart they were.
bicycles.stackexchange.com/users/24256/eMTB
I know this is an old thread
but it's the top one on Google
and I found it helpful.
I tried all the methods above
and I didn't have a lot of luck
so I wanted to share what I tried.
I made some playdough using the first no cook recipe I found on Google (it was fun and doesn't take long).
I then put it between two pieces of cling film and pressed it to about 2" thick, placed it on a hard surface (my kitchen floor) and put a tea towel over it
so my legs didn't stick to the cling film.
I then sat on it with my back in roughly my riding position.
The impression of my sit bones is really clear, I can see 2 circles (see image) and I measured the distance between the centres.
I tried a few times and it was consistent.
bicycles.stackexchange.com/users/40577/Char
You can do it on any surface which will conform to the shape of your butt, like a piece of memory foam.
Sit on it, with your knees higher than your seat by 4-6 inches, then measure the center of the depressions left.
There are different types of memory foam, and some will have a more durable impression than others.
bicycles.stackexchange.com/users/1259/zenbike
I've used this method which worked well: Fill a large size ziploc bag with icing sugar.
Icing sugar's fine texture means that it binds together when compressed.
This is what we want - it will hold the shape of your buttprint after you get up.
Close the bag making sure to squeeze out the air.
Place on hard chair.
Sit while assuming the degree of forward rotation that you intend to cycle with.
The distance between your sit bones change depending on this rotation. Get up cleanly and measure the distance between the depressions.
bicycles.stackexchange.com/users/5632/Emmanuel
Here's a link to a video that may help you: In this technique all you need is a piece of corrugated cardboard and some chalk.
bicycles.stackexchange.com/users/8001/Tom
A fresh piece of corrugated cardboard on a hard surface such as a coffee table.
Sit down and try to mimic the upper body position you have on your bike.
Your sit bones change position based on how your pelvis is tilted.
Your sit bones should crush the corrugated cardboard slightly and leave two indentations.
Measure center to center as best you can.
bicycles.stackexchange.com/users/3088/802bikeguy.com
There is one more method people here aren't talking about...
Sit on a bike on a trainer, get into riding position, and have someone you don't mind touching you physically measure your sit bones, in proper riding position.
You can feel someone's sit bones on their butt.
They're pretty obvious once you're touching them.
bicycles.stackexchange.com/users/32324/CardMechanic
Some bike shops have gel pads that you sit on to measure the distance between your sit bones. Is it possible to take this measurement on your own at home, or does this really require special-purpose equipment?  [Related question:  Should I use a narrower saddle on a road bike than I would on my hybrid commuter? ]
Is it possible to measure your sit bones at home?
bicycles.stackexchange.com
<saddle><bike-fit><ergonomics>
bicycles.stackexchange.com/questions/9815
bicycles.stackexchange.com/users/3649/amcnabb
To measure the distance between your sit bones at home, get two small erasers, put them on a chair, sit on them and move them around until they feel like they are on your sit bones. Another method is to sit on a bike on a trainer, get into riding position, and have someone you don't mind touching you physically measure your sit bones, in proper riding position. Apparently, ‘narrow’ sit bone width would be 100mm or less, medium 100-130mm, wide over 130mm.  You can measure your sit bones at a bike dealer or at home with a pad that will conform to the shape of your butt (memory foam, cardboard) or using another way of marking the positioning. A narrow sit bone width would be 100mm or less, medium 100-130mm, wide over 130mm. Specialized recommends a 130mm saddle width for narrow, 143mm for medium and 155mm for wide, measured from edge to edge across the top of the saddle. 
You can measure your sit bones at a bike dealer or at home with a pad that will conform to the shape of your butt (memory foam, cardboard) or using another way of marking the positioning. A narrow sit bone width would be 100mm or less, medium 100-130mm, wide over 130mm. Specialized recommends a 130mm saddle width for narrow, 143mm for medium and 155mm for wide, measured from edge to edge across the top of the saddle. 