Split the deposit in the same fair share as the rent
Most flatshares split the security deposit equally and move on — but if the rent isn't split equally (and with unequal rooms it shouldn't be), an equal deposit quietly overcharges whoever pays less rent. The fairest approach is to split the deposit in the same proportion as each person's envy-free rent share, so everyone's deposit matches the room they actually took.
A security deposit is usually one or two months' rent, held against damage and unpaid rent. If Ana pays €420 and Carla pays €250 for their rooms, splitting a €1,000 deposit down the middle asks Carla to put down the same €500 as Ana for a cheaper room — the same imbalance an equal rent split creates. Tying each person's deposit to their room keeps the whole arrangement consistent.
First work out the fair, envy-free rent split with the calculator. Then apply the same shares to the deposit: whoever pays a larger share of the rent puts down a larger share of the deposit. It's one rule for both numbers, so nobody has to argue twice — and it's easy to explain to a landlord or write into the lease.
Rent is €1,000 a month and the landlord asks for a one-month deposit of €1,000. An envy-free rent split prices the three rooms at €420, €330 and €250.
Split the deposit the same way — €420, €330 and €250 — and each person's stake matches their room. If one roommate later moves out, their share is the clear amount to return or hand over to a replacement, with nothing to renegotiate.