Most groups should just split it evenly, and most of them do. Here's how to tell when that stops being fair, and what to do about it.


Most groups can just split it evenly. If the bedrooms are broadly comparable and you're there two or three nights, it's really the same difference. Everyone got a good night's sleep, no one had to cram into a closet or settle for a couch. Everyone had their own space.
Split it evenly when:
Let's be honest: what people really care about on a trip with friends isn't the square footage of their room, since they're spending most of the time out and about enjoying the trip. They care about whether the basics are there. A bathroom to themselves, a comfortable bed, and privacy.
There are situations where not everyone has these basics, but some do. This becomes an opportunity to offer the group different price points based on how much comfort they're willing to sacrifice. You might be helping someone on a tight budget and helping someone else get the specific accommodation they want at the same time. The most considerate and optimal thing to do is to present the group with options at different price points.
Do the math when:
Work out what the fair split would be. If it's a meaningful amount of money to someone on the trip, it's worth doing.
Presenting the option for people to take different rooms with different price tags changes the room allocation conversation from ending in an awkward concession, "you can take the nicer room", into an everyone-wins situation. "I get the bigger room, but you save $100" is a strong outcome. I want the space and you want the savings.
This matters most of all.
Before the money is spent, it's just options. The person taking the small room gets to choose it knowingly, which feels completely different from being assigned it and then billed the same as the suite.
Put the room list and the split in the same message as the listing when you share it with the group.
Common space is not as important as sleeping arrangements, and should be weighted at 20% at most, based on what's available. If you're on a trip with friends, hopefully you're spending most of your time out enjoying the trip rather than sitting in the common areas of the Airbnb.
The Airbnb split calculator does this part for you. Fill in what's available in your Airbnb and it works out each person's share, plus a message you can paste straight into the group chat.
Someone arrives late or leaves early. Leave their share alone. The room was held for them either way and nobody else could have used it, so keep their costs the same. If you can find someone to fill the nights they're not there, that's even better. You get to see an additional friend, and it lets you prorate by night: take their total cost, multiply by nights there divided by total trip nights, and the friend who filled in before or after pays the remainder.
If you have a friend who lives near the destination and might be interested in joining for a night or a shorter stretch, this is a great opportunity to invite them for part of the trip. People nearby often don't want to take much time off work or leave their routine to travel so close to home anyway.
Kids. It depends on how much space they took up. A baby or toddler sharing your room shouldn't cost extra. Teenagers with their own rooms should be treated as individual trip members, with the parents paying their share. If a group of kids from different families share a big bunk room, treat that as one person in the calculator above and divide the result by the number of families. That gives you each family's share for that room.
The person who booked. They fronted a few thousand dollars on a personal card and carried the cancellation risk. That's worth acknowledging, but it isn't a discount. It's a reason to divvy up the trip quickly once it ends, in the few days after you get home, and to have everyone else cover more of the grocery and dinner expenses to help even things out. Have everyone add costs to Tabster as they go or on the way home, claim their shares over the next couple of days, and divvy up promptly.
So you've agreed on everyone's accommodations and booked. Now it's time to keep track of all the other trip expenses: groceries, dinners, the boat rental, the person who covered gas twice, and four separate tabs nobody wrote down.
Tabster keeps a running tab for the whole trip. Everyone adds receipts as they go, claims what they actually had, and it settles at the end without a spreadsheet. If that's the part you're dreading, start with how to split expenses on a group trip.