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Cosum

How Cosum works

A practical guide to ledgers, payers, splits and payment suggestions.

The model in plain language

Three ideas make every shared expense easier to understand.

Cosum keeps the record of what happened separate from the suggestion for how to settle up. Once that distinction is clear, every screen reads the same way.

1

A participant is a name in a ledger

Add everyone involved by name. They do not need a Cosum account or the app; one person can keep the ledger for the whole group.

An account can claim a name later without changing that participant’s previous shares.

Our flat

3 participants

EEmmaYou
NNoah
MMaya
2

Who paid and who was included are different choices

The payer is the person whose money left their pocket. The split says who benefited from the expense and how much belongs to each person.

If two people paid one bill, add the amount each paid as a separate expense.

Dinner

$120

Payer
Noah
Included
3 people · equally

The payer and the split are saved separately.

3

Who pays whom is a suggestion, not history

Cosum nets everyone’s balance and suggests the fewest payments that close the ledger. The suggested pair may never have exchanged money before.

Marking a suggestion as paid adds a Transfer record. Cosum never moves money.

Payment suggestion

The shortest way to close the balance

Maya

pays

Noah

receives

Suggested amount$48

Frequently asked questions

The questions that usually come up at the first expense.

Short answers to the parts of shared expenses that are easiest to misread.

Does everyone in a ledger need an account?

No. A participant can remain a name in the ledger. One person can add and manage expenses for the whole group, and others may join later if they want.

What if two people paid the same bill?

Add two expenses: one for the amount paid by the first person and one for the amount paid by the second. Each expense has exactly one payer, so the source of the money remains clear.

Why does Cosum suggest a payment between two people who never paid each other?

The suggestion is calculated from everyone’s net balance, not copied from transaction history. It reduces the number of payments needed to close the ledger.

Does Cosum send money when I mark a suggestion as paid?

No. Cosum does not connect to a bank or move money. It records a Transfer after the real-world payment has happened, then recalculates the balances.

Can I start a ledger on my own?

Yes. A one-person ledger skips invitations, payer selection and payment suggestions. You can add participants whenever the expense becomes shared.

Are old expenses shared when I add someone later?

No. Existing expenses keep the people and shares they were saved with. Add the new participant only to the future expenses they are part of.

Still unsure about a specific case?

Tell us what happened in your ledger. We will help without asking you to share private expense details.

Open support