You can get a ball park thought. I go with 70% of the total invited guests (total head count) and I have never had a problem. I have had people come back and say "you were almost spot on with the nuber of people that ended up coming" many times. If it's a toss up between two sizes, one that's a little less and one that's a bit more, I tell them to go with the smaller on and if you realize that as your date gets closer you have a lot more people showing up we can throw a kitchen cake onto the order or change up the tier sizes. People appreciate honesty and it makes them feel like you have their best interests at heart. People almost always decide to go with the slightly bigger option and they feel good about it because it was their choice *and* I get to look like the smart one/good guy because I told them they would need less and they went with the bigger number anyway. 
I have had people come back and say "you were almost spot on with the nuber of people that ended up coming" many times. If it's a toss up between two sizes, one that's a little less and one that's a bit more, I tell them to go with the smaller one....
I've also had people tell me, "We should have gone with your 60% number because that's what we had show up!"
I went the other way, though, on the initial size of the order. Many/Most caterers will permit the bride to increase their order but rarely will permit them to decrease the order. So that was one of my marketing things .... I went ahead and put down the larger number and ALLOWED them to decrease if they wanted. This made me look great to them and they felt like I was working with them more than the other food folks they had talked to.
I then told them:
"It's always more fun to get a smaller invoice than it is to try to come up with extra money if you have to increase it!" 
On the business side of it, a larger order generated a larger deposit, plus I RARELY had them come back and decrease it. Once in a blue moon, a bride would lower the head count, but not often.
Just as an example of this, I delivered two cakes to my husband's office yesterday for their annual open houses. The first was for clients, about 30 people expected, I did about that many servings and they had about one fourth of it left. The second was for bankers, 50 people, and they had about 75% of it left, but they had an open bar at that one that they didn't tell me about. They don't care because they'll be eating cake for breakfast today and they're looking forward to that, but a cake for 50 guests isn't necessarily 50 servings.
If you need to book your caker 6 months to a year ahead of time, you can't possibly know how many will attend, can you?
Nope. 6 months to a year out, all they're doing is securing the date though, right?
Exactly. And similar to indydebi, I would tell my brides, "Nothing we're discussing today is carved in stone. You can modify the number of servings you need up until 2 weeks before the wedding, after you've gotten in all your RSVP's." The only bride who ever modified ended up adding another tier to her cake to add more servings.
I do show them, with dummies, the approximate size of their cake regardless of how many servings they order.
When I meet with brides, I ALWAYS get out the cake dummies. We play with sizes, cake alignment and they know exactly what their cake will look like, sizewise. I've discovered that I really don't like offsetting cake tiers on square cakes but many of my brides love it. We "play" with cake tiers and even exactly how to offset if they are interested.
Plus, having dummies, my brides talk about how much fun and how easy it was to design their cake and I come away from it with a cake order. And, because they might want a certain look, a little bigger than they might have ordered otherwise.
You used to could get them hollowed out so they nested somewhat. They are three inches tall then you could get a one inch ring to add to each one to get it to four inches high. But one set of square and one set of round would surely suffice. And it's just the footprint of two cakes which in some small bakeries and homes might be an issue but it's not that much storage room needed.
Ah yes, I do remember Bakery Crafts offering the hollow ones back in my walmart days- I hated those, but they would be good for space issues. My current supplier only offers 4" solids, and I offer anywhere from 6" to 16" round and the same for square. You could see why space for those would be an issue!
I like the solid ones better too. There's great places to get them uber cheapy cheap cheap. But if space is an issue, you can get 'em hollowed out.
FOOD. One more factor to inquire about. We went to a wedding that had afternoon tea style food. They couldn't get anyone to take cake. Nobody wanted more sweets they wanted a steak. We are going to attend this month a cake only reception, if that is all there is to eat they better have plenty.
Exactly. And similar to indydebi, I would tell my brides, "Nothing we're discussing today is carved in stone. You can modify the number of servings you need up until 2 weeks before the wedding, after you've gotten in all your RSVP's."
Omg, did you have a microphone hidden in my shop? That's almost exactly what I would tell them, too!
great minds, I guess!!
I do show them, with dummies, the approximate size of their cake regardless of how many servings they order.
When I meet with brides, I ALWAYS get out the cake dummies. We play with sizes, cake alignment and they know exactly what their cake will look like, sizewise. I've discovered that I really don't like offsetting cake tiers on square cakes but many of my brides love it. We "play" with cake tiers and even exactly how to offset if they are interested.
Plus, having dummies, my brides talk about how much fun and how easy it was to design their cake and I come away from it with a cake order. And, because they might want a certain look, a little bigger than they might have ordered otherwise.
You could also kill two birds by taking a sharpie and ruler and marking your dummies to show the servings. I have marked all of mine so the bride can see exactly how you can get the number of servings from it. I use Debi's cutting method.
I have my dummies stacked in a closet. Stacking the shapes together(largest of each together, then the next smallest size of each on top of those on up to the smallest size.) biggest to smallest takes up less space too. For odd shaped cakes, I do break out the pans for visual aid. I printed off and assembled the cake slices from here on CC as well for added visuals.
I tell brides how big the pieces are supposed to be, but when they start cutting it at the reception site it's usually totally different. They tend to cut them bigger and thinner so that they look better on the dessert plates.
My own mother did not believe me when I told her I can get 24 slices from an 8 inch cake. I had to cut it in front of her to prove it, and I think, if memory serves me, I actually got 26. Having my dummies marked to show servings proves to the unbelieving bride that if cut correctly, you can get a large number of servings.
I also have the slice of cake from schwammrs' template, (which I love) to show the size of one slice. I added pieces of those flexible foam sheets you use for kids crafts over the "frosted" side of the template to represent a layer of fondant.
I am a visual person so it helps me as well to be confident that as long as the cakes are cut right, everybody should have enough cake.
I tell brides that you can get 24 servings out of a 9x13 pan or 24 cupcakes out of an average cake mix or recipe. Or you can cut the 9x13 in half for two teenagers. That computes in their head I think. I explain there's fillings added so there's more to it than that but just to give them a ballpark they can wrap their head around in a perfect world.
Wilton says an 8" takes 7 cups of batter--extended cake mix gets you that far. An average recipe is about 6-7 cups of batter. Give or take a bit.
I did a wedding where the bride not tell me she had changed the dinner plan from a sit-down, 5-course plated service to a dinner buffet. They had a full dessert bar including ice cream sundaes. Ergo, lots of leftover cake.
Conversely I did another wedding which was a 5-course plated service, which included dessert. Apparently word got around that the wedding cake was yummier than the dessert. Ran out of cake.
In both instances the brides had ordered cake based on 75% of the guests who rsvpd.
Bottom line, standards are good and helpful but bride must also consider the whole reception menu and discuss with the cake pro. We can't help her if we don't know all the facts.
There are so many variables in every situation. After a bride gives me a number I always ask if they have any open invitations (an invite read at church or something put on the bulletin board at work) and have they included the wedding party and their dates? You wouldn't believe how many brides don't think to do that.
Most brides have already heard from the caterer about planning food for 1/2 to 2/3 the number of invited guest. If they are having a grooms cake or other sweets that factors in as well. Time of day, will there be alcohol, amount of food, all these things play into the cake size. But ultimately it is a decision based on budget. Gone are the days of ordering a big cake just for the look.
I show them the size of a cake serving and I explain the best way to cut the cake. And I always tell them if they get nervous about the cake the week of the wedding they can always add a sheet cake for backup.
I always discuss with the bride how many they are actually expecting to show up at the reception. I always tell them they know their family and church members better than me...do they have the kind of family that shows up no matter what and will the church turn out no matter what?! Some say their entire family will be there and church members too - so I tell them it depends on those factors and others as to how many will show up - even with RSVPs!
My cousin's kid got married - he and the new bride were very popular young people that went to a Texas A & M...loads of friends there. 400 invites went out...298 RSVP yes they were coming - 380 showed up. My cousin's wife said the day before the wedding people were calling that they were able to come and were dragging along good ole so and so from school. The venue lady is the one that gave me the count of 380 - I just remember it was SO packed one could barely move and my BIL ended up with chocolate all over his shirt front!
I have also had brides tell me they wanted a certain cake because of the look - omg they HAD to have a 4 or 5 tier cake regardless of who was or was not coming. Each to their own. I give options and scenarios but the bride or her mom is the one who decides the amount of cake - I have changed up and down if necessary. I try to be flexible with my brides. Sometimes they have cake left over and sometimes they don't but I have NEVER had a bride annoyed that she had cake left over!!
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