Security deposit interest calculator

Interest on the balance actually held, for the days it was actually held — through top-ups, part-years, and anything already paid out. Prints a statement showing every period.

Why this isn’t one multiplication

The easy version is deposit × rate × whole years, and it is wrong in three ordinary situations. A tenancy almost never ends on an anniversary, so the last period is a part-year. A deposit that was topped up when the rent went up earns on two different balances over two different stretches. And where interest has been paid or credited along the way — which several states require every year — only the remainder is owed at the end.

Each of those splits the term into periods, and the easy version collapses them. Notably, it collapses them in the landlord’s favor every time: whole years rounds the term down, and a single balance is almost always the smaller one. That is the arithmetic this does properly, and prints, so the number can be checked rather than trusted.

What we can’t tell you

Your rate. We have not verified a single state’s statutory interest rule against its statute — not one of the fifty — so this tool asks you for the rate instead of filling one in. That is a deliberate gap, not an oversight. Several states set the rate annually through a banking regulator, a figure copied from an article last year is often already stale, and a confidently wrong rate on a document somebody hands their landlord is worse than no tool at all.

Where to find it: your lease, your state’s security-deposit statute, the statement from the account the deposit sits in, or a local tenant advice service. If you have the rate, everything after it is arithmetic, and the arithmetic is here.

How the days are counted

Actual days over a 365-day year — the convention easiest to check by hand and the one a tenant service or a small-claims court will recognize. Leap days count as days, so a term spanning one earns 366/365 of the annual rate. There are four defensible day-count conventions and they give slightly different answers; saying which one is in use beats picking one quietly.

The rest of the deposit

Interest is usually the last question about a deposit rather than the first. If what you actually need is whether deductions were fair, the deduction calculator prorates charges by an item’s remaining useful life instead of billing a whole carpet. Landlords returning a deposit want the deposit return letter, which itemizes against your state’s deadline. Tenants who never got one want the demand letter, which works out whether the landlord is late and by how many days.

General information, not legal advice. CompliMaint is not a law firm, has not verified any state’s deposit-interest requirement, and takes no view on whether interest is owed in your situation. The rate is whatever you enter; the arithmetic against it is ours. Everything runs on your device.

Questions people ask

How do you calculate interest on a security deposit?
Interest runs on the balance actually held, for the days it was actually held. Simple interest is the deposit multiplied by the annual rate multiplied by days divided by 365. It gets harder when the deposit was topped up part-way through, when interest was paid out during the tenancy, or when the term does not end on an anniversary — each of those splits the term into periods over different balances. This tool does that split and prints the periods so the figure can be checked.
Does my state require interest on a security deposit?
Some states do, some do not, and several set the rate annually. CompliMaint has not verified any state's interest rule against its statute, so this tool does not tell you — it asks you for the rate rather than showing a number nobody checked. The rate is usually in your lease, in your state's security deposit statute, or on the statement from the account the deposit is held in.
Is deposit interest simple or compound?
It depends on the rule that applies. Where a statute says simple or noncompounded, interest accrues on the deposit only. Where it is silent, simple is the conservative reading and the easier one to defend. The difference grows with the rate and the length of the tenancy — over three years at 5% it is a few dollars on a $1,000 deposit, and considerably more on a larger deposit held longer.
What happens to interest if the deposit was increased mid-tenancy?
Each balance earns interest for the period it was held. A deposit of $1,000 topped up to $1,500 after a year earns on $1,000 for that first year and on $1,500 afterwards — not on $1,500 for the whole term, and not on $1,000. Collapsing it to a single figure is the most common error in doing this by hand, and it almost always understates what is owed.