Splitting the bill is a solved problem. Splitting the blame is not.

There are now thirteen apps that will split a restaurant bill for you. Splitwise. Tricount. Settle Up. Most banking apps. The maths of dividing €342.80 into six unequal portions is, technically, a problem we solved before the iPhone existed.

What none of these apps solve — what cannot, in fact, be solved by software — is the friend who orders a third bottle of wine after dinner and then expects everyone to split it.

This essay is for them. And for the rest of you.

The asymmetry

Group spending is asymmetric. Some people order modestly. Some people order a starter, a main, and stop. Some people order, after the bill has been requested, "and a round of digestifs for the table."

The digestif round is the diagnostic moment. There is, in every group of friends, exactly one person whose marginal contribution to the bill is consistently 1.4x the median. This is not a moral failing. It is a calorie-and-curiosity issue. They want to try the thing. They want the night to keep going. Fine.

The problem is not what they order. The problem is the assumption that everyone else is in.

The fix

Here is what works, in our experience:

1. Name it once, then never again.

Early in the trip — first night, second drink — say it lightly: "Let's split everything that's on the table when the bill comes, but the after-dinner round is on whoever orders it." Nobody will object. The norm is now set.

2. The 80/20 rule.

Eighty per cent of group spending should be split evenly. Twenty per cent — the optional add-ons, the cocktails, the late-night taxis to a club one person wants to go to — should be paid by whoever ordered or initiated. Trying to itemise the 80% is where evenings die.

3. The honest reckoning.

Once a trip, ideally over breakfast on the last morning, somebody runs the totals out loud. "Okay, we paid €312 each on shared things. The cocktails on Tuesday were €45 — that was Mark and me. Anyone owe anyone anything else?" This conversation takes five minutes. It dissolves three months of low-grade resentment.

4. The currency of the favour.

If the friend who orders the cocktails also happens to be the friend who books the Airbnb, drives the rental car, and translates the Spanish, the group will, organically, let the cocktails slide. Most groups have an internal accounting that is roughly fair across many small dimensions.

If the cocktail-friend does only the cocktails, the group will, less organically, mention this in the kitchen at midnight.

Why software cannot fix this

Splitwise can divide €18 by 6. It cannot tell you that the €18 round was unwelcome. It cannot rank your friendships by emotional debt. It cannot, in the way a single quiet conversation can, recalibrate the social contract of a group of six adults on holiday.

This is fine. Software does not need to solve everything. We just need to know which problems are ours and which are not.

The one we can help with — the bill itself, the booking, the deposit, the activity payments — is in the app. The rest is yours.