A residential lease handles early departure in one of two ways, and they produce very different numbers. The first is an explicit termination clause: give sixty days' notice, pay a stated charge, and the tenancy ends. That converts an open question into a price, which is worth something to both sides. The second is silence: the lease says nothing about leaving early, and the general law of damages fills the gap. A tenant who leaves has then breached, the lease usually terminates or is terminated, and the tenant owes damages measured by the rent that goes unpaid, reduced by what the landlord can and should recover by re-letting.
The doctrine that bounds the second route is mitigation, and California's version shows exactly how the arithmetic works and who has to prove what. Civil Code 1951.2 lets a landlord whose tenant has abandoned recover the unpaid rent earned before termination, plus the amount by which the rent that would have been earned after termination "exceeds the amount of such rental loss that the lessee proves could have been reasonably avoided", plus, subject to conditions, the same measure for the balance of the term, plus any other amount needed to compensate for detriment the breach proximately caused. Two features of that sentence deserve attention. The reduction is real, so a landlord who re-lets quickly cannot collect twice for the same months. And the burden sits on the tenant: it is the lessee who "proves" what could have been avoided, which means a departing tenant who wants the benefit of mitigation should be documenting the market, and ideally supplying replacement applicants, rather than assuming a court will do it for them. Section 1951.2(c) also limits recovery for the period after an award unless the lease says so or the landlord actually re-let and proves it acted reasonably and in good faith. Other states answer these questions differently, including on whether the landlord must try to re-let at all.
A stated fee is not automatically enforceable either, and California is the sharpest illustration. Civil Code 1671(b) makes a liquidated damages clause valid unless the party attacking it shows it was unreasonable when the contract was made. But subdivision (c)(2) pulls residential leases out of that rule and into subdivision (d), under which "a provision in a contract liquidating damages for the breach of the contract is void", with a single exception: the parties may agree on an amount presumed to be the damage "when, from the nature of the case, it would be impracticable or extremely difficult to fix the actual damage." That reverses the usual presumption for a dwelling. Whether a particular clause falls inside it turns on drafting, because there is a real distinction between liquidated damages for a breach and the price of a bargained-for option to terminate, and courts do draw it. A tenant facing a large stated charge in California has a question worth asking rather than a bill worth paying without reading.
One federal statute removes the charge outright, and its details matter more than its headline. The Servicemembers Civil Relief Act, at 50 U.S.C. 3955, lets a covered lessee terminate a residential lease that was signed before entering military service, or signed during service and followed by orders for a permanent change of station or to deploy "for a period of not less than 90 days", or signed on receipt of such orders and then overtaken by a stop movement order. Termination is made by delivering written notice with a copy of the orders, and the statute expressly allows hand delivery, private carrier, mail with return receipt, and electronic means including delivery to a designated electronic address. On a monthly-rent lease the termination takes effect 30 days after the first date the next rental payment is due after the notice is delivered. Then the money rules: rent for the period before that effective date is prorated, and "the lessor may not impose an early termination charge, but any taxes, summonses, or other obligations and liabilities of the lessee in accordance with the terms of the lease, including reasonable charges to the lessee for excess wear, that are due and unpaid at the time of termination of the lease shall be paid by the lessee." Rent paid in advance for any period after the effective date must be refunded within 30 days. And knowingly seizing or holding a terminating servicemember's security deposit or personal effects in order to subject them to a claim for rent accruing after termination is a federal misdemeanor, punishable by a fine, up to a year in prison, or both. The Act also gives the spouse or a dependent a one-year window to terminate after the lessee's death in service, and a one-year window after a catastrophic injury or illness.
Other exits exist and none of them is general. States have enacted statutes letting a survivor of domestic violence end a tenancy early on stated proof, and what proof, how much notice and what liability remains all differ; whether a particular state has such a statute is a question for its landlord-tenant code. A dwelling that is genuinely uninhabitable can, in some circumstances, let a tenant leave and stop paying, but that route has its own preconditions and is covered on the habitability page. Neither this page nor that one states a general rule that a repair problem lets a tenant leave without a charge, because that is a state-law answer and often a fact-specific one.
Finally, the deposit is not the fee. A lease that charges for early termination and also purports to keep the deposit is doing two separate things, and the deposit is still governed by the state's deposit statute, including its itemization and deadline rules. A tenant who pays a break charge should still expect an itemized statement about the deposit.